Prenatal Development: Germinal and Embryonic Stages

Germinal Stage

• Spans from conception to implantation, approximately two weeks
• The zygote begins to divide within 36 hours of conception
• Dividing cells nourished by the yolk of the original egg prior to implantation
• One-third of pregnancies result in miscarriage, typically due to development abnormalities

Embryonic Stage

• Begins with implantation (3rd week) through the 8th week
• Major organ systems differentiate during this stage
• Embryonic disk forms three germ layers
• The ectoderm (outer layer) develops into the nervous system, sensory organs, nails, hair, teeth, and the outer layer of skin
• The endoderm (inner layer) forms the digestive and respiratory systems, the liver, and the pancreas
• The mesoderm (middle layer) develops into the excretory, reproductive, and circulatory systems, muscles, skeleton, and inner layer of skin
• The neural tube develops into the brain and spinal cord
• Head and blood vessels begin to form in the 3rd week
• Arm and leg buds appear in the 4th week
• Eyes, ears, nose, and mouth begin to take shape
• By the 8th week, limbs elongate, and facial features become distinct
• Teeth buds form, and the kidneys and liver function
• The neural tube produces 400 million neurons per day by the end of the first month
• Neurons migrate to different brain structure areas, and form cerebral hemispheres by the 5th week
• Cells in the nervous system begin to release neurotransmitters in the 2nd month
• Sex organs form between 5 to 6 weeks
• Internal and external genitals resemble female structures at this stage
• Sex organs begin to differentiate based on genetic code in the 7th week
• The Y chromosome causes testes to differentiate, and the absence of a Y chromosome causes ovaries to differentiate
• Distinct external genital structures appear by 4 months
• Testes produce androgens, leading to male hormonal influences
• Testosterone differentiates the male duct system (Wolffian)
• Dihydrotestosterone (DHT) triggers the development of male external genital organs
• Müllerian inhibiting substance (MIS) prevents Müllerian ducts from developing into the female duct system
• Small amounts of androgens are produced, playing a role in secondary sexual characteristics and sex drive of females
• Wolffian ducts degenerate while Müllerian ducts develop into Fallopian tubes, the uterus, and the inner part of the vagina

Amniotic Sac and Placenta

• Amniotic sac is surrounded by a clear membrane and contains amniotic fluid to protect the embryo/fetus in the uterus
• The placenta is a mass of tissue that exchanges nutrients and wastes between embryo/fetus and mother
• The umbilical cord connects the fetus to the placenta.
• The placenta acts as a filter between the mother's and the embryo/fetus' bloodstream
• Oxygen and nutrients reach the child via the placenta
• Carbon dioxide and waste pass to the mother through the placenta, alongside germs and drugs

Fetal Stage

• The third month until birth
• Major organ systems, fingers, and toes are formed by the end of the first trimester
• Eyes can be distinguished, and the fetus' sex can be visually determined
• The fetus opens and shuts eyes, sucks its thumb, and alternates between sleep and wakefulness by the end of the second trimester
• The fetus responds to light and sound
• The survival rate is about 50% if born at 22 to 25 weeks and is related to the quality of care
• Heart and lungs are increasingly capable of sustaining independent life by the end of the third trimester
• The fetus gains weight and length, and turns upside-down in the uterus during the 7th month
• The survival rate is nearly 90% if born at the end of the 7th month
• First movements begin around the 4th or 5th month
• The fetus moves vigorously, turns somersaults, begins slow squirming movements and sharp jabbing and kicking movements by 29-30 weeks
• Fetal movements become restricted as the fetus grows
• Prenatal activity predicts activity levels after birth

Maternal Nutrition

• Malnutrition, especially in the last trimester, is linked to low-birth-weight, prematurity, stunted growth, retardation of brain development, cognitive deficiencies, behavioral problems, and cardiovascular disease
• Effects of fetal malnutrition can sometimes be overcome by a supportive caregiving environment
• Maternal obesity is linked to a higher risk of stillbirth
• Pregnant women need protein, vitamins A, B, C, D, and E, iron, trace minerals like zinc and cobalt, folic acid, calcium, and calories
• Most women who eat a well-rounded diet do not need supplements, but doctors recommend them to be safe
• Mothers who are too slim risk preterm deliveries and low-birth-weight babies
• The amount of recommended weight gain depends on pre-pregnancy weight
• BMI below 18.5: gain 28 to 40 pounds
• Normal weight: gain 25 to 35 pounds
• BMI between 25 and 29.9: gain 15 to 25 pounds
• BMI over 30: gain 11 to 20 pounds

Environmental Influences and Risks

• Teratogens are environmental agents that can harm the embryo or fetus
• Includes drugs ingested by the mother, heavy metals, excessive hormones, radiation, and pathogens
• Critical periods of vulnerability correspond to times when organs are developing
• Syphilis can cause miscarriage, stillbirth, or congenital syphilis, which is diagnosed by blood tests early in pregnancy
• The baby will probably not contract syphilis if the infected mother is treated before the 4th month of pregnancy
• A baby has a 40-70% chance of being infected in utero or developing congenital syphilis if the infected mother is not treated
• HIV/AIDS, a human immunodeficiency virus/acquired immunodeficiency syndrome, disables the body's immune system and is lethal unless treated
• About 25% of babies born to HIV/AIDS-infected mothers become infected, many during childbirth, and HIV is also found in breast milk
• Influenza (Flu) is a respiratory infection that may be related to fetal brain abnormalities, autism, and schizophrenia
• Rubella (German measles) may cause birth defects (abnormalities detected during pregnancy)
• Pre-Eclampsia (Toxemia) may cause premature or undersized babies and is a cause of pregnancy-related maternal death
• Rh Incompatibility involves a transfer of maternal antibodies that may cause brain damage or death
• Accutane can cause numerous abnormalities during the first trimester for acne
• Thalidomide leads to missing or stunted limbs
• Tetracycline, an antibiotic, may lead to yellowed teeth and bone abnormalities
• Progestin can masculinize the external sex organs of a female embryo
• High doses of vitamins A and D are associated with central nervous system damage, small head size, and heart defects
• Maternal addiction to Heroin and Methadone is linked to low-birth-weight, prematurity, and toxemia and the baby may be born addicted
• Marijuana (Cannabis) use risks low-birth-weight, immature development of the nervous system, and predisposes offspring to dependence on opiates, even in adulthood
• Cocaine use increases the risk of stillbirth, low-birth-weight, and birth defects
• Alcohol passing through the placenta poses risks for death of fetus and neonate, malformations, and growth deficiencies
• Fetal Alcohol Spectrum Disorders includes Fetal Alcohol Syndrome (FAS), physical and psychological defects, and Fetal Alcohol Effect (FAE)
• Inconsistent findings with Caffeine, but some studies show an increased risk of miscarriage and low-birth-weight
• Nicotine and carbon monoxide from Cigarettes pass through the placenta, with nicotine stimulating the fetus and carbon monoxide is toxic, decreasing oxygen to the fetus
• Cigarettes can lead to smaller babies, stillbirth or infant death soon after birth, short attention spans, hyperactivity, lower cognitive scores, and poor grades, as well as men who smoke are more likely to produce abnormal sperm
• Heavy metals (lead, mercury, cadmium, selenium, zinc) threaten the baby's cognitive development
• Radiation can cause defects in a number of organs (including eyes), central nervous system, skeleton, and an increased risk of intellectual disabilities
• Maternal stress is linked to the secretion of stress hormones
• Adrenaline stimulates the mother's heart and respiratory rates
• Corticosteroids decrease the growth of the placenta
• Extreme anxiety has been linked to low birth weight, prematurity, and miscarriage