Chapter: Forming New Life PDF

Explain how conception occurs and what causes multiple births. Describe the mechanisms of heredity in normal and abnormal human development. Explain how heredity and environment interact in human development. Describe prenatal development, including environmental influences. Discuss the importan...

Explain how conception occurs and what causes multiple births. Describe the mechanisms of heredity in normal and abnormal human development. Explain how heredity and environment interact in human development. Describe prenatal development, including environmental influences. Discuss the importance of high-quality prenatal care. Conceiving New Life Most people think of development as beginning on the day of birth, when the new child—squalling and thrashing—is introduced to the world. However, development starts earlier. It starts at conception, as sperm and egg meet and an entirely new individual is created from parental genomes. Development continues as the fertilized egg grows and differentiates and edges closer to independent life outside the womb. And it persists in the dance between nature and nurture that shapes the unique individual that is the product of these processes. How fertilization takes place the timing of the fertile window can be highly unpredictable (Wilcox, Dunson, & Baird, 2000) Concurrently, even though conception is more likely during certain parts of the month, it may not always occur during that time. Fertilization, or conception, is the process by which sperm and ovum—the male and female gametes, or sex cells—combine to create a single cell called a zygote, which then duplicates itself again and again by cell division to produce all the cells that make up a baby. But conception is not as simple as it sounds. Several independent events need to coincide to conceive a child, and not all conceptions end in birth. At birth, a girl is believed to have about 2 million immature ova in her two ovaries, each ovum in its own follicle, or small sac. In a sexually mature woman, ovulation —rupture of a mature follicle in either ovary and expulsion of its ovum—occurs about once every 28 days until menopause. The ovum is swept along through one of the fallopian tubes by the cilia, tiny hair cells, toward the uterus, or womb. Sperm are produced in the testicles (testes), or reproductive glands, of a mature male at a rate of several hundred million a day and are ejaculated in the semen at sexual climax. Deposited in the vagina, they try to swim through the cervix, the opening of the uterus, and into the fallopian tubes; but only a tiny fraction make it that far. Fertilization normally occurs while the ovum is passing through the fallopian tube. If fertilization does not occur, the ovum and any sperm cells in the woman’s body die. The sperm are absorbed by the woman’s white blood cells, and the ovum passes through the uterus and exits through the vagina. WHAT CAUSES MULTIPLE BIRTHS? Dizygotic twins , or fraternal twins, are the result of two separate eggs being fertilized by two different sperm to form two unique individuals. Genetically, they are like siblings who inhabit the same womb at the same time, and they can be the same or different sex. Dizygotic twins tend to run in families and are the result of multiple eggs being released at one time. This tendency may have a genetic basis and seems to be passed down from a woman’s mother (Martin & Montgomery, 2002; National Center for Health Statistics [NCHS], 1999). Monozygotic twins are the result of a far different process. They result from the cleaving of one fertilized egg and are generally genetically identical. They can still differ outwardly, however, because people are the result of the interaction between genes and environmental influences. Mechanisms of Heredity The science of genetics is the study of heredity: the genetic transmission of heritable characteristics from parents to offspring. THE GENETIC CODE The “stuff” of heredity is a chemical called deoxyribonucleic acid (DNA). The double-helix structure of a DNA molecule resembles a long, spiraling ladder whose steps are made of pairs of chemical units called bases. The bases— adenine (A), thymine (T), cytosine (C), and guanine (G)—are the “letters” of the genetic code, which cellular machinery “reads.” Chromosomes are coils of DNA that consist of smaller segments called genes, the functional units of heredity. Each gene is located in a definite position on its chromosome and contains thousands of bases. The sequence of bases in a gene tells the cell how to make the proteins that enable it to carry out specific functions. The complete sequence of genes in the human body constitutes the human genome. A useful analogy is to consider the DNA of an individual as a series of books in a library. Until those books are “read” by an enzyme called RNA polymerase, and transcribed into a readable copy of messenger RNA (m-RNA), the knowledge contained within the books is not actualized. And what books will be pulled down from the shelf and read is in part determined by environmental factors that turn genes on and off at different points in development (Champagne & Mashoodh, 2009). In short, DNA holds all the information, but it needs to be read and copied by RNA polymerase to be useful. Environmental factors influence which genes are activated and when. Every cell in the normal human body except the sex cells (sperm and ova) has 23 pairs of chromosomes—46 in all. Through a type of cell division called meiosis, which the sex cells  undergo when they are developing, each sex cell ends up with only 23 chromosomes—one from each pair. When sperm and ovum fuse at conception, they produce a zygote with 46 chromosomes, 23 from the father and 23 from the mother. At conception, then, the single-celled zygote has all the biological information needed to guide its development into a unique individual. Through mitosis, a process by which the non–sex cells divide in half over and over again, the DNA replicates itself, so that each newly formed cell has the same DNA structure as all the others. Mutations are permanent alterations in genetic material. When development is normal, each cell (except the sex cells) continues to have 46 chromosomes identical to those in the original zygote. WHAT DETERMINES SEX? At the moment of conception, the 23 chromosomes from the sperm and the 23 from the ovum form 23 pairs. Twenty-two pairs are autosomes, chromosomes that are not related to sexual expression. The twenty-third pair are sex chromosomes —one from the father and one from the mother—that govern the baby’s sex. Sex chromosomes are either X chromosomes or Y chromosomes. The sex chromosome of every ovum is an X chromosome, but the sperm may contain either an X or a Y chromosome. The Y chromosome contains the gene for maleness, called the SRY gene. When an ovum (X) is fertilized by an X-carrying sperm, the zygote formed is XX, a genetic female. When an ovum (X) is fertilized by a Y-carrying sperm, the resulting zygote is XY, a genetic male. PATTERNS OF GENETIC TRANSMISSION Today we know that the genetic picture in humans is far more complex than Mendel imagined. Although some human traits, such as the presence of facial dimples, are inherited via simple dominant transmission, most human traits fall along a continuous spectrum and result from the actions of many genes in concert. Nonetheless, Mendel’s groundbreaking work laid the foundations for our modern understanding of genetics. Dominant and Recessive Inheritance Do you have dimples? If so, you probably inherited them through dominant inheritance. If your parents have dimples but you do not, recessive inheritance occurred. How do these two types of inheritance work? Genes that can produce alternative expressions of a characteristic (such as the presence or absence of dimples) are called alleles. Let’s take the presence of dimples as an example. Dimples are a dominant trait, so you will have dimples if you receive at least one copy (D) from either parent. If you inherited one allele for dimples from each parent (Figure 4), you are homozygous for this trait and have one or more dimples. If you receive one copy of the dimpling allele (D) and one copy of an allele for lack of dimples (d), you are heterozygous. In both cases, your expressed characteristic is that you have  dimples. The only situation in which you would not have dimples is if you received two recessive copies (d), one from each parent. Relatively few traits are determined in this simple fashion. Most traits result from polygenic inheritance, the interaction of several genes. For example, there is not an “intelligence” gene that determines whether or not you are smart. Rather, a large number of genes work in concert to determine your intellectual potential. Genotypes and Phenotypes: Multifactorial Transmission If you have dimples, that is part of your phenotype, the observable characteristics through which your genotype, or underlying genetic makeup, is expressed. The phenotype is the product of the genotype and any relevant environmental influences. The difference between genotype and phenotype helps explain why a clone (a genetic copy of an individual) or even an identical twin can never be an exact duplicate of another person. Dimples have a strong genetic base; but experience modifies the expression of the genotype for most traits—a phenomenon called multifactorial transmission. Multifactorial transmission illustrates the action of nature and nurture influences and how the mutually and reciprocally affect outcomes. Some physical characteristics (including height and weight) and most psychological characteristics (such as intelligence and musical ability) are products of multifactorial transmission. Many disorders arise when an inherited predisposition (an abnormal variant of a normal gene) interacts with an environmental factor, either before or after birth. Attention- deficit/hyperactivity disorder (ADHD) is one of several behavioral disorders thought to be transmitted multifactorially (Price, Simonoff, Waldman, Asherson, & Plomin, 2001). Epigenesis: Environmental Influence on Gene Expression mounting evidence suggests that gene expression itself is controlled by a third component, a mechanism that regulates the functioning of genes within a cell without affecting the structure of the cell’s DNA. Genes are turned off or on as they are needed by the developing body or when triggered by the environment. This phenomenon is called epigenesis, or epigenetics. In other words, the environment can infl uence when and which genes turn on and off. Epigenesis (meaning “on, or above, the genome”) refers to chemical molecules (or “tags”) attached to a gene that alter the way a cell “reads” the gene’s DNA. If we think of the human genome as a computer, we can visualize this epigenetic framework as the software that tells the DNA when to work. Because every cell in the body inherits the same DNA sequence, the function of the chemical tags is to differentiate various types of body cells, such as brain cells, skin cells, and liver cells. In this way, genes for the types of cells that are needed are turned on, and genes for unneeded cells are left off. One example of epigenesis is genome, or genetic, imprinting. Imprinting is the differential expression of certain genetic traits, depending on whether the trait has been inherited from the mother or the father. In imprinted gene pairs, genetic information inherited from the parent of one sex is activated, but genetic information from the other parent is suppressed.  Imprinting problems also may explain why children who inherit Huntington’s disease from their fathers are far more likely to be affected at an early age than children who inherit it from their mothers (Sapienza, 1990). GENETIC AND CHROMOSOMAL ABNORMALITIES The most prevalent defects are cleft lip or cleft palate, followed by Down syndrome. Other serious malformations involve the eye, the face, the mouth, or the circulatory, gastronomical, or musculoskeletal systems (Centers for Disease Control and Prevention [CDC], 2006b). Not all genetic or chromosomal abnormalities are apparent at birth. Symptoms of Tay-Sachs disease (a fatal degenerative disease of the central nervous system common in Jews of eastern European ancestry) and sickle-cell anemia (a blood disorder most common among African Americans) may not appear until at least age 6 months Dominant or Recessive Inheritance of Defects Most of the time, normal genes are dominant over those carrying abnormal traits, but sometimes the gene for an abnormal trait is dominant. When one parent has one dominant abnormal gene and one recessive normal gene and the other parent has two recessive normal genes, each of their children has a 50-50 chance of inheriting the abnormal gene. Among the 1,800 disorders known to be transmitted by dominant inheritance are achondroplasia (a type of dwarfi sm) and Huntington’s disease. Defects transmitted by dominant inheritance are less likely to be lethal at an early age than those transmitted by recessive inheritance because any affected children would be likely to die before reproducing. Therefore, that gene would not be passed on to the next generation and would soon disappear from the population Recessive defects are expressed only if the child is homozygous for that gene; in other words, a child must inherit a copy of the recessive gene from each parent. Because recessive genes are not expressed if the parent is heterozygous for that trait, it may not always be apparent that a child is at risk for receiving two alleles of a recessive gene Defects transmitted by recessive genes tend to be lethal at an earlier age, in contrast to those transmitted by dominant genes, because recessive genes can be transmitted by heterozygous carriers who do not themselves have the disorder. Thus they are able to reproduce and pass the genes down to the next generation. In incomplete dominance , a trait is not fully expressed. Normally the presence of a dominant/recessive gene pair results in the full expression of the dominant gene and the masking of the recessive gene.  Sex-Linked Inheritance of Defects In sex-linked inheritance (Figure 5), certain recessive disorders affect male and female children differently. This is due to the fact that males are XY and females are XX. In humans, the Y chromosome is smaller and carries far fewer genes than the X chromosome. One outcome of this is that males receive only one copy of any gene that happens to be carried on the sex chromosomes, whereas females receive two copies. So, if a woman has a “bad” copy of a particular gene, she has a backup copy. However, if a male has a “bad” copy of a particular gene, that gene will be expressed. Sex-linked recessive disorders are more common in males than in females. For example, red- green color blindness, hemophilia (a disorder in which blood does not clot when it should), and Duchenne muscular dystrophy (a disorder that results in muscle degeneration and eventually death) are all more common in males, and all result from genes located on the X chromosome. Chromosomal Abnormalities Chromosomal abnormalities typically occur because of errors in cell division, resulting in an extra or missing chromosome. For example, Klinefelter syndrome is caused by an extra female sex chromosome  Down syndrome , the most common chromosomal abnormality, accounts for about 40 percent of all cases of moderate-to-severe mental retardation (Pennington, Moon, Edgin, Stedron, & Nadel, 2003). The condition is also called trisomy-21 because it is characterized in more than 90 percent of cases by an extra 21st chromosome. The most obvious physical characteristic associated with the disorder is a downwardsloping skin fold at the inner corners of the eyes. GENETIC COUNSELING AND TESTING Genetic counseling can help prospective parents like Alicia and Eduardo assess their risk of bearing children with genetic or chromosomal defects. People who have already had a child with a genetic defect, who have a family history of hereditary illness, who suffer from conditions known or suspected to be inherited, or who come from ethnic groups at higher-than-average risk of passing on genes for certain diseases can get information about their likelihood of producing affected children. A genetic counselor takes a family history and gives the prospective parents and any biological children physical examinations. Laboratory investigations of blood, skin, urine, or fingerprints may be performed. Chromosomes from body tissues may be analyzed and photographed, and the photographs enlarged and arranged according to size and structure on a chart called a karyotype. Nature and Nurture: Influences of Heredity and Environment The relative importance of heredity and environment was a major issue among early psychologists and the general public. Today it has become clear that, although certain rare physical disorders are virtually 100 percent inherited, phenotypes for most normal traits, such as those having to do with intelligence and personality, are subject to a complex array of hereditary and environmental forces. Let’s see how scientists study and explain the influences of heredity and environment and how these two forces work together. One approach to the study of heredity and environment is quantitative: it seeks to measure how much heredity and environment influence particular traits. This is the traditional goal of the science of behavioral genetics. Measuring Heritability Behavioral geneticists have developed a means of estimating how much of a trait is due to genetics and how much is the result of environmental influences by using a concept known as heritability. Every trait is a consequence of genes and environment. By looking at groups of people with known genetic relationships, and assessing whether or not they are concordant, meaning the same, on a given trait, behavioral geneticists can estimate the relative infl uence of genes and environment. Heritability is expressed as a percentage ranging from 0.0 to 1.0: the higher the number, the greater the heritability of a trait. A heritability estimate of 1.0 indicates that genes are 100 percent responsible for variances in the trait within the population. A heritability estimate of 0.0 percent would indicate the environment shaped a trait exclusively. Note that heritability does  not refer to the influences that shaped any one particular person because those influences are virtually impossible to separate. Nor does heritability tell us how traits develop. It merely indicates the statistical extent to which genes contribute to a trait at a certain time within a given population. In fact, environmental interventions sometimes can overcome genetically “determined” conditions. For example, a special diet begun soon after birth often can prevent mental retardation in children with the genetic disease phenylketonuria (PKU). HOW HEREDITY AND ENVIRONMENT WORK TOGETHER Instead of looking at genes and experience as operating directly on an organism, they see both as part of a complex developmental system ( Gottlieb, 1991, 1997; Lickliter & Honeycutt, 2003). From conception on, throughout life, a combination of constitutional factors (related to biological and psychological makeup) and social, economic, and cultural factors help shape development. The more advantageous these circumstances and the experiences to which they give rise, the greater is the likelihood of optimum development. Reaction range refers to a range of potential expressions of a hereditary trait. Body size, for example, depends largely on biological processes, which are genetically regulated. Even so, a range of sizes is possible, depending on environmental opportunities and constraints and a person’s behavior. Canalization Some traits have an extremely narrow range of reaction. The metaphor of canalization illustrates how heredity restricts the range of development for some traits. Behaviors that depend largely on maturation seem to appear when a child is ready. Typical babies follow a predictable sequence of motor development: crawling, walking, and running, in that order, at certain approximate ages. This sequence is said to be canalized, in that children will follow this same blueprint irrespective of many variations in the environment. Many highly canalized traits tend to be those necessary for survival. In the case of very important traits such as these, natural selection has designed them to develop in a predictable and reliable way within a variety of environments and a multitude of influences. They are too important to be left to chance. Cognition and personality are not highly canalized. They are more subject to variations in experience: the kinds of families children grow up in, the schools they attend, and the people they encounter. Scientists have begun to recognize that a usual or typical experience, too, can dig canals, or channels for development (Gottlieb, 1991). For example, infants who hear only the sounds peculiar to their native language soon lose the ability to perceive sounds characteristic of other languages. Throughout this book you will find many examples of how socioeconomic status, neighborhood conditions, and educational opportunity can powerfully shape developmental outcomes, from the pace and complexity of language development to the likelihood of early sexual activity and antisocial behavior.  Genotype-environment interaction usually refers to the effects of similar environmental conditions on genetically different individuals, and a discussion of these interactions is a way to conceptualize and talk about the different ways nature and nurture interact. Genotype-Environment Correlation Because genes influence a person’s exposure to particular environments, the environment often reinforces genetic differences (Rutter, 2007). That is, certain genetic and environmental influences tend to act in the same direction. This is called genotype- environment correlation, or genotype-environment covariance, and it works in three ways to strengthen the phenotypic expression of a genotypic tendency (Bergeman & Plomin, 1989; Scarr, 1992; Scarr & McCartney, 1983). The first two ways are common among younger children, the third among older children, adolescents, and adults. Passive correlations: Parents, who provide the genes that predispose a child toward a trait, also tend to provide an environment that encourages the development of that trait. For example, a musical parent is likely to create a home environment in which music is heard regularly, to give a child music lessons, and to take the child to musical events. If the child inherited the parent’s musical talent, the child’s musicality will reflect a combination of genetic and environmental influences. This type of correlation is called passive because the child does not control it. The child has inherited the environment, as well as genes that might make that child particularly well-suited to respond to those particular environmental influences. Reactive, or evocative, correlations: Children with differing genetic makeups evoke different reactions from others. For example, parents who are not musically inclined may make a special effort to provide musical experiences for a child who shows interest and ability in music. This response, in turn, strengthens the child’s genetic inclination toward music. This type of correlation is called reactive because the other people react to the child’s genetic makeup. Active correlations: As children get older and have more freedom to choose their own activities and environments, they actively select or create experiences consistent with their genetic tendencies. A shy child is more likely than an outgoing child to spend time in solitary pursuits. An adolescent with a talent for music will probably seek out musical friends, take music classes, and go to concerts if such opportunities are available. This tendency to seek out environments compatible with one’s genotype is called niche-picking; it helps explain why identical twins reared apart tend to have similar characteristics. What Makes Siblings So Different? The Nonshared Environment Although two children in the same family may bear a striking physical resemblance, siblings can differ greatly in intellect and especially in personality (Plomin & Daniels, 2011). One reason may be genetic differences, which lead children to need different kinds of stimulation or to respond differently to a similar home environment. SOME CHARACTERISTICS INFLUENCED BY HEREDITY AND ENVIRONMENT Physical and Physiological Traits Not only do monozygotic twins generally look alike, but they also are more concordant than dizygotic twins in their risk for such medical disorders as high blood pressure, heart disease, stroke, rheumatoid arthritis, peptic ulcers, and epilepsy (Brass,  Isaacsohn, Merikangas, & Robinette, 1992; Plomin et al., 1994). Life span, too, seems to be influenced by genes (Hjelmborg et al., 2006). Intelligence Heredity exerts a strong influence on general intelligence (as measured by intelligence tests) and, to a lesser extent, on specific abilities such as memory, verbal ability, and spatial ability. Intelligence is a polygenic trait; it is influenced by the additive effects of large numbers of genes working together. Intelligence also depends in part on brain size and structure, which are under strong genetic influence (Toga & Thompson, 2005). Personality and Psychopathology Scientists have identified genes directly linked with specific aspects of personality such as a trait called neuroticism, which may contribute to depression and anxiety (Lesch et al., 1996). Heritability of personality traits appears to be between 40 and 50 percent, and there is little evidence of shared environmental influence (Bouchard, 2004). Temperament, an aspect of personality, is a person’s characteristic way of approaching and reacting to situations. It appears to be largely inborn and is often consistent over the years, though it may respond to special experiences or parental handling (A. Thomas & Chess, 1984; A. Thomas, Chess, & Birch, 1968). Prenatal Development During gestation, the period between conception and birth, an unborn child undergoes dramatic processes of development. The normal range of gestation is between 37 and 41 weeks (Martin, Hamilton, et al., 2009). Gestational age is usually dated from the first day of an expectant mother’s last menstrual cycle. STAGES OF PRENATAL DEVELOPMENT Prenatal development takes place in three stages: germinal, embryonic, and fetal. (Table 4 gives a month-by-month description.) During these three stages of gestation, the original single-celled zygote grows into an embryo and then a fetus. Germinal Stage (Fertilization to 2 Weeks) During the germinal stage, from fertilization to about 2 weeks of gestational age, the zygote divides, becomes more complex, and is implanted in the wall of the uterus. Embryonic Stage (2 to 8 Weeks) During the embryonic stage, from about 2 to 8 weeks, the organs and major body systems—respiratory, digestive, and nervous—develop rapidly. This process is known as organogenesis. This is a critical period, when the embryo is most vulnerable to destructive influences in the prenatal environment. A spontaneous abortion, commonly called a miscarriage, is the expulsion from the uterus of an embryo or fetus that is unable to survive outside the womb. A miscarriage that occurs after 20 weeks of gestation is generally characterized as a stillbirth.  Most miscarriages result from abnormal pregnancies; about 50 to 70 percent involve chromosomal abnormalities (Hogge, 2003). Smoking, drinking alcohol, and drug use increase the risks of miscarriage (American College of Obstetricians and Gynecologists, 2002). Miscarriages are more common in African American, Native American, and Alaskan native women, in both young and older (greater than 35 years of age) mothers, and more likely to occur in pregnancies involving twins or higher order multiples (MacDorman & Kirmeyer, 2009). Fetal Stage (8 Weeks to Birth) The appearance of the first bone cells at about 8 weeks signals the beginning of the fetal stage, the final stage of gestation. During this period, the fetus grows rapidly to about 20 times its previous length, and organs and body systems become more complex. Right up to birth, “finishing touches” such as fingernails, toenails, and eyelids continue to develop. The movements and activity level of fetuses show marked individual differences, and their heart rates vary in regularity and speed. Male fetuses, regardless of size, are more active and tend to move more vigorously than female fetuses throughout gestation (Almli, Ball, & Wheeler, 2001). Thus infant boys’ tendency to be more active than girls may be at least partly inborn (DiPietro, Hodgson, Costigan, Hilton, & Johnson, 1996; DiPietro et al., 2002). ENVIRONMENTAL INFLUENCES: MATERNAL FACTORS A teratogen is an environmental agent, such as a virus, a drug, or radiation, that can interfere with normal prenatal development. However, not all environmental hazards are equally risky for all fetuses. An event, substance, or process may be teratogenic for some fetuses but have little or no effect on others. Sometimes vulnerability may depend on a gene either in the fetus or in the mother. Nutrition and Maternal Weight Pregnant women typically need 300 to 500 additional calories a day, including extra protein. Women of normal weight and body build who gain 16 to 40 pounds are less likely to have birth complications or to bear babies whose weight at birth is dangerously low or overly high. It is estimated that if all women took 5 milligrams of folic acid each day before pregnancy and during the first trimester, an estimated 85 percent of neural-tube defects could be prevented Malnutrition Prenatal malnutrition may have long-range effects. In rural Gambia, in western Africa, people born during the hungry season, when foods from the previous harvest are depleted, are 10 times more likely to die in early adulthood than people born during other parts of the year (Moore et al., 1997). Physical Activity and Strenuous Work Actually, moderate exercise any time during pregnancy does not seem to endanger the fetuses of healthy women (Committee on Obstetric Practice, 2002; Riemann & Kanstrup Hansen, 2000). Regular exercise prevents constipation and improves respiration, circulation, muscle tone, and skin elasticity, all of which contribute to a more comfortable pregnancy and an easier, safer delivery (Committee on Obstetric Practice, 2002). However, strenuous working conditions, occupational fatigue, and long working hours may be  associated with a greater risk of premature birth (Bell, Zimmerman, & Diehr, 2008; Luke et al., 1995). Drug Intake Practically everything an expectant mother takes in makes its way to the uterus. Drugs may cross the placenta, just as oxygen, carbon dioxide, and water do. Vulnerability is greatest in the first few months of gestation, when development is most rapid. In addition, certain antipsychotic drugs used to manage severe psychiatric disorders may have serious potential effects on the fetus, including withdrawal symptoms at birth (AAP Committee on Drugs, 2000). The American Academy of Pediatrics (AAP) Committee on Drugs (2001) recommends that no medication be taken by a pregnant or breastfeeding woman unless it is essential for her health or her child’s (Koren et al., 1998) and that care be taken in choosing the safest drug available. Alcohol Prenatal alcohol exposure is the most common cause of mental retardation and the leading preventable cause of birth defects in the United States. Fetal alcohol syndrome (FAS) is characterized by a combination of retarded growth, face and body malformations, and disorders of the central nervous system. FAS and other less severe alcoholrelated conditions are estimated to occur in nearly 1 in every 100 births (Sokol, Delaney-Black, & Nordstrom, 2003). Nicotine Maternal smoking during pregnancy has been identified as the single most important factor in low birth weight in developed countries (DiFranza, Aligne, & Weitzman, 2004). Women who smoke during pregnancy are more than 1½ times as likely as nonsmokers to bear low-birth- weight babies (weighing less than 5½ pounds at birth). Even light smoking (fewer than five cigarettes a day) is associated with a greater risk of low birth weight (Hoyert, Mathews, et al., 2006; Martin, Hamilton, et al., 2005; Shankaran et al., 2004). Maternal Illnesses Both prospective parents should try to prevent all infections— common colds, flu, urinary tract and vaginal infections, as well as sexually transmitted diseases. If the mother does contract an infection, she should have it treated promptly. Acquired immune deficiency syndrome (AIDS) is a disease caused by the human immunodeficiency virus (HIV), which undermines functioning of the immune system. If an expectant mother has the virus in her blood, perinatal transmission may occur: the virus may cross over to the fetus’s bloodstream through the placenta during pregnancy, labor, or delivery or, after birth, through breast milk. Maternal Anxiety, Stress, and Depression Some tension and worry during pregnancy are normal and do not necessarily increase risks of birth complications, such as low birth weight (Littleton, Breitkopf, & Berenson, 2006). Moderate maternal anxiety may even spur organization of the developing brain. On the other hand, a mother’s self-reported stress and anxiety during pregnancy has been associated with more active and irritable temperament in newborns (DiPietro et al., 2010), inattentiveness during a developmental assessment in 8-month-olds (Huizink, Robles de Medina, Mulder, Visser, & Buitelaar, 2002), and negative emotionality or behavioral disorders in early childhood (Martin, Noyes, Wisenbaker, & Huttunen, 2000; O’Connor, Heron, Golding, Beveridge, & Glover, 2002). Additionally, chronic stress can result in preterm delivery, perhaps through the action of elevated levels of stress hormones (which are implicated in the onset of labor) or the  resulting dampened immune functioning, which makes women more vulnerable to infl ammatory diseases and infection that can also trigger labor (Schetter, 2009). ENVIRONMENTAL INFLUENCES: PATERNAL FACTORS A man’s exposure to lead, marijuana or tobacco smoke, large amounts of alcohol or radiation, DES, pesticides, or high ozone levels may result in abnormal or poor-quality sperm (Sokol et al., 2006; Swan et al., 2003). Men who smoke have an increased likelihood of transmitting genetic abnormalities (AAP Committee on Substance Abuse, 2001). A pregnant woman’s exposure to the father’s secondhand smoke has been linked with low birth weight, infant respiratory infections, sudden infant death, and cancer in childhood and adulthood Monitoring and Promoting Prenatal Development THE NEED FOR PRECONCEPTION CARE Such care should include the following: Physical examinations and the taking of medical and family histories Vaccinations for rubella and hepatitis B Risk screening for genetic disorders and infectious diseases such as STDs Counseling women to avoid smoking and alcohol, maintain a healthy body weight, and take folic acid supplements