Nursing Care During Normal Pregnancy and Care of the Developing Fetus PDF

Nursing Care During Normal Pregnancy and Care of the Developing Fetus FETAL DEVELOPMENT JULIUS T. FLORECE, RN, MAN Instructor  Fetal Development  Zygote – the cells that result from fertilization of the ovum by the sperm c...

Nursing Care During Normal Pregnancy and Care of the Developing Fetus FETAL DEVELOPMENT JULIUS T. FLORECE, RN, MAN Instructor  Fetal Development  Zygote – the cells that result from fertilization of the ovum by the sperm cell, the fertilized ovum from conception to two weeks.  Blastomere – Mitotic division of the zygote gives rise to daughter cells called blastomeres.  Morula – The solid ball of cells formed by 16 or more blastomeres.  Blastocyst – after the morula reaches the uterus, it is termed as blastocyst.  Embryo – extends from the 7th day to 7th week post conception. The zygote is considered an embryo after the appearance of villi.  Fetus – from 8th week until term.  Conceptus – refers to all the products of conception. ORIGIN AND DEVELOPMENT OF ORGAN SYSTEMS Stem Cells  During the first 4 days of life, zygote cells are termed totipotent stem cells, or cells so undifferentiated they have the potential to grow into any cell in the human body.  In another 4 days, as the structure implants and becomes an embryo, cells begin to show differentiation, or lose their ability to become any body cell. Instead, they are slated to become specific body cells, such as nerve, brain, or skin cells and are termed pluripotent stem cells.  In yet another few days, the cells grow so specific they are termed multipotent, or are so specific they cannot be deterred from growing into a particular body organ such as spleen or liver or brain (Chen et al., 2017). Zygote Growth As soon as conception has taken place, development proceeds in a cephalocaudal (head-to-tail) direction; that is, head development occurs first and is followed by development of the middle, and finally, the lower body parts. This pattern of development continues after birth as shown by the way infants are able to lift up their heads approximately 1 year before they are able to walk. Primary Germ Layers  At the time of implantation, the blastocyst already has differentiated to a point at which three separate layers of these cells are present: the ectoderm , the endoderm, and the mesoderm  Knowing which structures arise from each germ layer is helpful to know because coexisting congenital disorders found in newborns usually arise from the same germ layer.  For example, a fistula between the trachea and the esophagus (both of which arise from the endoderm layer) is a common birth anomaly. In contrast, it is rare to see a newborn with a malformation of the heart (which arises from the mesoderm) and also a malformation of the lower urinary tract (which arises from the endoderm).  One reason rubella infection is so serious in pregnancy is because this virus is capable of infecting all three germ layers so can cause congenital anomalies in a myriad of body systems.  All organ systems are complete, at least in a rudimentary form, at 8 weeks gestation (the end of the embryonic period). During this early time of organogenesis (organ formation), the growing structure is most vulnerable to invasion by teratogens (i.e., any factor that affects the fertilized ovum, embryo, or fetus adversely, such as a teratogenic medicine; an infection such as toxoplasmosis; cigarette smoking; or alcohol ingestion). Cardiovascular System  The cardiovascular system is one of the first systems to become functional in intrauterine life.  Simple blood cells joined to the walls of the yolk sac progress to become a network of blood vessels and a single heart tube, which forms as early as the 16th day of life and beats as early as the 24th day.  The septum that divides the heart into chambers develops during the sixth or seventh week; heart valves develop in the seventh week.  The heartbeat may be heard with a Doppler instrument as early as the 10th to 12th week of pregnancy.  An electrocardiogram (ECG) may be recorded on a fetus as early as the 11th week, although the accuracy of such ECGs is in doubt until about the 20th week of pregnancy, when conduction is more regulated.  The heart rate of a fetus is affected by oxygen level, activity, and circulating blood volume, just as in adulthood. After the 28th week of pregnancy, when the sympathetic nervous system has matured, the heart rate stabilizes or begins to show a consistent beat of 110 to 160 beats/min. Fetal Circulation  The unique structures in fetal circulation not found in adult circulation are the following: 1. Foramen Ovale – connects the left and right atrium, bypassing fetal lungs. Obliterated after birth to become fossa ovalis. 2. Umbilical vein – brings oxygenated blood coming from the placenta to the heart and liver, becomes ligamentum teres after birth. 3. Umbilical arteries – carry unoxygenated blood from the fetus to the placenta, become umbilical ligaments after birth. 4. Ductus venosus – carry oxygenated blood from umbilical vein to inferior vena cava, bypassing fetal liver, becomes ligamentum venosum after birth. 5. Ductus Arteriosus – Carry oxygenated blood from pulmonary artery to aorta, bypassing fetal lungs, becomes ligamentum arteriosum after birth. Fetal Circulation  Fetal circulation differs from extrauterine circulation because the fetus derives oxygen and excretes carbon dioxide not from gas exchange in the lungs but from exchange in the placenta. Blood arriving at the fetus from the placenta is highly oxygenated.  This blood enters the fetus through the umbilical vein (which is still called a vein even though it carries oxygenated blood because the direction of the blood is toward the fetal heart). Specialized structures present in the fetus then shunt blood flow to first supply the most important organs of the body: the liver, heart, kidneys, and brain.  Blood flows from the umbilical vein to the ductus venosus, an accessory vessel that discharges oxygenated blood into the fetal liver, then connects to the fetal inferior vena cava so oxygenated blood is directed to the right side of the heart. Because there is no need for the bulk of blood to pass through the lungs, the bulk of this blood is shunted as it enters the right atrium into the left atrium through an opening in the atrial septum, called the foramen ovale.  From the left atrium, it follows the course of adult circulation into the left ventricle, then into the aorta, and out to body parts.  A small amount of blood that returns to the heart via the vena cava does leave the right atrium by the adult circulatory route; that is, through the tricuspid valve into the right ventricle, and then into the pulmonary artery and lungs to service the lung tissue. However, the larger portion of even this blood is shunted away from the lungs through an additional structure, the ductus arteriosus , directly into the descending aorta.  As the majority of blood cells in the aorta become deoxygenated, blood is transported from the descending aorta through the umbilical arteries (which are called arteries because they carry blood away from the fetal heart) back through the umbilical cord to the placental villi, where new oxygen exchange takes place. Oxygenated blood enters the umbilical vein from the placenta ––––> enters the ductus venosus ––––> passess through the inferior vena cava enters the right atrium ––––> passess through left ventricle ––––> flows to ascending aorta to supply nourishment to the brain and upper extremities ––––> enters superior vena cava ––––> goes to right atrium –– ––> enters right ventricle ––––> enters pulmonary artery with some blood going to the lungs to supply oxygen and nourishment ––––> flows to ductus arteriosus ––––> enters descending aorta (some blood going to the lower extremities) ––––> enters hypogastric arteries ––––> goes back to the placenta.  At birth, an infant’s oxygen saturation level is 95% to 100% and pulse rate is 80 to 140 beats/min.  Because there is a great deal of mixing of blood in the fetus, the oxygen saturation level of fetal blood reaches only about 80%. In light of this, the fetal heart has to beat rapidly (110 to 160 beats/min) to supply needed oxygen to cells. Even with this low blood oxygen saturation level, however, carbon dioxide does not accumulate in the fetal system because it rapidly diffuses into maternal blood across a favorable placental pressure gradient. Fetal Hemoglobin  Fetal hemoglobin differs from adult hemoglobin in several ways. It has a different composition (two alpha and two gamma chains, compared with two alpha and two beta chains of adult hemoglobin).  It is also more concentrated and has greater oxygen affinity, two features that increase its efficiency. Because hemoglobin is more concentrated, a newborn’s hemoglobin level is about 17.1 g/100 ml, compared with a normal adult level of 11 g/100 ml; a newborn’s hematocrit is about 53%, compared with a normal adult level of 45%.  Fetal blood volume is about 78ml/kg if immediate cord clamping is done.  Fibrinogen and clotting factors, especially those dependent to vitamin K, are decreased at birth. Without prophylactic Vitamin K administration, the newborn may suffer from bleeding. Platelet counts are the same as adult levels and thrombin time is prolonged. Respiratory System  At the third week of intrauterine life, the respiratory and digestive tracts exist as a single tube. Like all body tubes, initially this forms as a solid structure, which then canalizes (i.e., hollows out).  By the end of the fourth week, a septum begins to divide the esophagus from the trachea. At the same time, lung buds appear on the trachea.  Until the seventh week of life, the diaphragm does not completely divide the thoracic cavity from the abdomen.  This causes lung buds to extend down into the abdomen, re-entering the chest only as the chest’s longitudinal dimension increases and the diaphragm becomes complete (at the end of the seventh week).  If the diaphragm fails to close completely, the stomach, spleen, liver, or intestines may be pulled up into the thoracic cavity. This causes the child to be born with intestine present in the chest (i.e., diaphragmatic hernia), compromising the lungs and perhaps displacing the heart (Gowen, 2019). Other important respiratory developmental milestones include:  Spontaneous respiratory practice movements begin as early as 3 months gestation and continue throughout pregnancy.  Specific lung fluid with a low surface tension and low viscosity forms in alveoli to aid in expansion of the alveoli at birth; it is rapidly absorbed shortly after birth.  Surfactant , a phospholipid substance, is formed and excreted by the alveolar cells of the lungs beginning at about the 24th week of pregnancy. This decreases alveolar surface tension on expiration, preventing alveolar collapse and improving the infant’s ability to maintain respirations in the outside environment at birth (Sweet et al., 2019).  Surfactant has two components: lecithin (L) and sphingomyelin (S).  Early in the formation of surfactant, sphingomyelin is the chief component. At about 35 weeks, there is a surge in the production of lecithin, which then becomes the chief component by a ratio of 2:1. As a fetus practices breathing movements, surfactant mixes with amniotic fluid.  Using an amniocentesis technique, an analysis of the lecithin/ sphingomyelin (L/S) ratio in surfactant (whether lecithin or sphingomyelin is the dominant component) is a primary test of fetal maturity.  Respiratory distress syndrome, a severe breathing disorder, can develop if there is a lack of surfactant or it has not changed to its mature form at birth.  Any interference with the blood supply to the fetus, such as occurs with placental insufficiency or maternal hypertension, appears to raise steroid levels in the fetus and enhance surfactant development.  Synthetically increasing steroid levels in the fetus (e.g., the administration of betamethasone to the mother late in pregnancy) can also hurry alveolar maturation and surfactant production without interfering with permanent lung function prior to a preterm birth (Cole et al., 2019).  Amniocentesis is done to remove amniotic fluid and cells from the uterus for testing or treatment. Amniotic fluid surrounds and protects a baby during pregnancy.  Amniocentesis can be done for a number of reasons: Genetic testing. Genetic amniocentesis involves taking a sample of amniotic fluid and testing the DNA from the cells for diagnosis of certain conditions, such as Down syndrome. This might follow another screening test that showed a high risk of the condition. Diagnosis of fetal infection. Occasionally, amniocentesis is used to look for infection or other illness in the baby. Treatment. Amniocentesis might be done to drain amniotic fluid from the uterus if too much has built up — a condition called polyhydramnios. Fetal lung testing. If delivery is planned sooner than 39 weeks, amniotic fluid might be tested to help find out whether a baby's lungs are mature enough for birth. This is rarely done.  Nervous System  Like the circulatory system, the nervous system begins to develop extremely early in pregnancy.  A neural plate (a thickened portion of the ectoderm) is apparent by the third week of gestation.  The top portion differentiates into the neural tube, which will form the central nervous system (brain and spinal cord), and the neural crest, which will develop into the peripheral nervous system.  All parts of the brain (cerebrum, cerebellum, pons, and medulla oblongata) form in utero, although none are completely mature at birth. Brain growth continues at high levels until 5 or 6 years of age.  Brain waves can be detected on an electroencephalogram (EEG) by the eighth week.  The eye and inner ear develop as projections of the original neural tube.  24 weeks, the ear is capable of responding to sound and the eyes exhibit a pupillary reaction, indicating sight is present.   The neurologic system seems particularly prone to insult during the SPINA BIFIDA - is a early weeks of the embryonic period birth defect that and can result in neural tube occurs when the disorders, such as a meningocele (i.e., spine and spinal cord don't form properly. herniation of the meninges), especially if there is lack of folic acid (which is contained in green leafy vegetables and pregnancy vitamins) (Halstead & Seay, 2021). All during pregnancy and at birth, the system is vulnerable to damage if anoxia should occur. ANENCEPHALY - s a serious birth defect in which a baby is born without parts of the brain and skull Endocrine System The function of endocrine organs begins along with neurosystem development.  The fetal pancreas produces insulin needed by the fetus (insulin is one of the few substances that does not cross the placenta from the mother to the fetus).  The thyroid and parathyroid glands play vital roles in fetal metabolic function and calcium balance.  The fetal adrenal glands supply a precursor necessary for estrogen synthesis by the placenta. Fetal Liver  Because fetal red blood cells have shorter lifespan that the adult, the fetus produce more bilirubin. Most of the bilirubin formed by the fetus is excreted via the placenta and conjugated in the mother’s liver. The liver is not mature enough at birth. In fact two of the main health threat to the newborn after birth is hyperbilirubinemia (excessive breakdown products from destroyed red blood cells) and hypoglycemia (low blood sugar) that can occur in the first 24 hours after birth because, although active, liver function is still immature..  The fetal liver begins to store glycogen as early as 9 weeks. Near term, its glycogen storage is two to three times higher than those in adult liver. This decreases rapidly after delivery. Most of the cholesterol in the fetus is formed in the liver.  The fetal liver cannot synthesize coagulation factors because of lack of vitamin K which is synthesized by the bacteria that normally inhabit the adult intestine. Since the fetal intestine is sterile at birth, it cannot synthesize vitamin K to all newborns. Pancreas The pancreas originates from the foregut and is formed between the fifth and eight weeks. The islet of Langerhans develop during the twelfth week, it produce insulin beginning 20th week. The fetus produces and utilizes its own insulin. No maternal insulin enters the fetal circulation. But excess glucose from diabetic mother is transferred to the fetus. Fetal hyperglycemia stimulates increase fetal insulin production and increased glucose storage resulting in macrosomic infants. The hyperinsulinemia inhibits lung maturation, thus, infants of diabetic mothers are at risk for respiratory distress after birth.  A fetus larger than 4000 to 4500 grams (or 9 to 10 pounds) is considered macrosomic  19.2 pound baby Digestive System  Part of the yolk sac develops into the primitive gut.  5 to 6 weeks: 1. The foregut begin to give rise to the pharynx, part of the lower respiratory tract, esophagus, first half of duodenum, liver, pancreas and gallbladder. 2. The midgut give rise to the distal half of the duodenum, jejunum, ileum, the cecum and appendix and the proximal half of the colon. 3. The hindgut develops into the distal half of the colon, rectum, parts of the anal canal, urinary bladder and urethra. The most common malformation of the digestive tract are anorectal abnormalities such as imperforate anus.  6 weeks: the intestines enter the base of the umbilical cord because the abdomen is still too small to accommodate all of it. It remains there until the 10th week when the fetal abdomen has enlarged enough to accommodate all the intestinal mass. If the intestines fail to return to the abdominal cavity so that it protrudes to the umbilicus, this is a malformation called, omphalocele.  Omphalocele is a rare abdominal wall defect where abdominal organs protrude through a hole into the base of the umbilical cord.  Gastroschisis (pronounced gas-troh-skee-sis) is a birth defect where there is a hole in the abdominal wall beside the belly button.  12 weeks : the fetus swallows amniotic fluid and propel the unabsorbed substances to the lower colon. Thus meconium is present in the bowel as early as 12 weeks. Meconium is apparently excreted in early fetal life. Excretion of meconium late in pregnancy suggests fetal anoxia and respiratory distress. Meconium is black or green and sticky in nature. It derives its color from bile pigments. White meconium suggests biliary obstruction. Although the fetus exhibits swallowing as early as 12 weeks, swallowing and sucking reflexes are not mature until the fetus is about 32 weeks or weighs 1500 grams.  26 weeks: the fetus begins to store brown fat to be utilized as a source of heat in the first few hours after birth.  36 weeks: the gastrointestinal tract secretes enzymes necessary for digestion of carbohydrates and protein except for secretion of amylipase (for complex starches) which matures only at 3 months after birth. Lipase, enzyme for fats, is not available in many newborns. Thus, the newborn infant has poor ability in digesting fats. Little saliva is produced.  Meconium , a collection of cellular wastes, bile, fats, mucoproteins, mucopolysaccharides, and portions of the vernix caseosa (i.e., the lubricating substance that forms on the fetal skin), accumulates in the intestines as early as the 16th week.  An important neonatal nursing responsibility is recording that a newborn has passed meconium as this rules out a stricture (noncanalization) of the anus (Ezomike, et. Al., 2019).  The ability of the gastrointestinal tract to secrete enzymes essential for carbohydrate and protein digestion is mature at 36 weeks. However, amylase, an enzyme found in saliva and necessary for digestion of complex starches, does not mature until 3 months after birth. Many newborns have also not yet developed lipase, an enzyme needed for fat digestion (a reason breast milk is the best food for newborns because its digestion does not depend on these enzymes). Musculoskeletal System During the first 2 weeks of fetal life, cartilage prototypes provide position and support to the fetus. Ossification of this cartilage into bone begins at about the 12th week, continues all through fetal life and into adulthood. Carpals, tarsals, and sternal bones generally do not ossify until birth is imminent. A fetus can be seen to move on ultrasonography as early as the 11th week, although the mother usually does not feel this movement ( quickening ) until almost 20 weeks of gestation. *Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. Reproductive System  A child’s sex is determined at the moment of conception by a spermatozoon carrying an X or a Y chromosome and can be ascertained as early as 8 weeks by chromosomal analysis or analysis of fetal cells in the mother’s bloodstream.  At about the sixth week after implantation, the gonads (i.e., ovaries or testes) form. If testes form, testosterone is secreted, apparently influencing the sexually neutral genital duct to form other male organs (i.e., maturity of the wolffian, or mesonephric, duct).  In the absence of testosterone secretion, female organs will form (i.e., maturation of the müllerian, or paramesonephric, duct). This is an important phenomenon, because if a woman should unintentionally take an androgen or an androgen-like substance during this stage of pregnancy, a child who is chromosomally female could appear more male than female at birth. If deficient testosterone is secreted by the testes, both the müllerian (female) duct and the wolffian (male) duct could develop (i.e., pseudohermaphroditism, or intersex) (Kutney, et al., 2016).  The testes first form in the abdominal cavity and do not descend into the scrotal sac until the 34th to 38th week of intrauterine life. Because of this, many male preterm infants are born with undescended testes. These boys need a follow up to be certain their testes do descend when they reach what would have been the 34th to 38th week of gestational age, because testicular descent does not always occur as readily in extrauterine life as it would have in utero. Testes that do not descend (cryptorchidism) require surgery as they are associated with poor sperm production and possibly testicular cancer later in life (Bartz, et al., 20118. Urinary System  Although rudimentary kidneys are present as early as the end of the fourth week of intrauterine life, the presence of kidneys does not appear to be essential for life before birth because the placenta clears the fetus of waste products. Urine, however, is formed by the 12th week and is excreted into the amniotic fluid by the 16th week of gestation.  At term, fetal urine is being excreted at a rate of up to 500 ml/day. An amount of amniotic fluid less than usual (oligohydramnios) suggests fetal kidneys are not secreting adequate urine and that there is a kidney, ureter, or bladder disorder (Kumar, 2018).  The complex structure of the kidneys gradually develops during intrauterine life and continues to mature for months afterward. The loop of Henle, for example, is not fully differentiated until the child is born. Glomerular filtration and concentration of urine in the newborn are still not efficient, because the ability to concentrate urine is still not mature at birth.  Early in the embryonic stage of urinary system development, the bladder extends as high as the umbilical region and there is an open lumen between the urinary bladder and the umbilicus. If this fails to close, (termed a patent urachus), this is revealed at birth by the persistent drainage of a clear, acid–pH fluid (urine) from the umbilicus (Zmora et al, 2021). Integumentary System  The skin of a fetus appears thin and almost translucent until subcutaneous fat begins to be deposited underneath it at about 36 weeks. Skin is covered by soft downy hairs (lanugo) that serve as insulation to preserve warmth in utero, as well as a cream cheese–like substance, vernix caseosa, which is important for lubrication and for keeping the skin from macerating in utero. Both lanugo and vernix are still present at birth. Immune System  The fetal liver begins to produce B lymphocytes at 9 weeks and the fetal thymus begins to produce T lymphocytes at 14 weeks.  Immunoglobulin (Ig) G maternal antibodies cross the placenta into the fetus as early as the 20th week and certainly by the 24th week of intrauterine life to give a fetus temporary passive immunity against diseases for which the mother has antibodies.  These often include:  poliomyelitis  rubella (German measles)  Rubeola (regular measles)  diphtheria  tetanus  infectious parotitis (mumps)  hepatitis B  pertussis (whooping cough).  Infants born before this antibody transfer has taken place have no natural immunity and so need more than the usual protection against infectious disease in the newborn period.  A fetus only becomes capable of active antibody production late in pregnancy. Generally, it is not necessary for a fetus to produce antibodies because they need to be manufactured only to counteract an invading antigen, and antigens rarely invade the intrauterine space.  Because IgA and IgM antibodies (the types which develop to actively counteract infection) cannot cross the placenta, their presence in a newborn is proof that the fetus has been exposed to an infection. A ctive Immunity. A ctive immunity is more common in our bodies than passive immunity. O ur individual immune systems build up active immunity instinctively as we’re exposed to new bacteria and strange pathogens. A ctive immunity happens in response to breathing new air, eating new food, and touching new things. People with with average immune systems don’t get sick every time something new enters their body because active immunity immunity is constantly working to neutralize foreign agents. agents. Pas s ive Immunity. Any contributions not made by the body are considered passive immunity. These are less common, but they are incredibly important because they let let our bodies take a proactive defense against dangerous dangerous illnesses and diseases.  Examples of Passive Immunity  Placenta. Pregnant women give their babies nutrition and defense against illness through placentas and blood circulation. With blood, maternal antibodies and other immunity defenses travel to the unborn child. Although the baby is mostly safe from bacteria and illness before birth, immediately after leaving its mother’s body the baby is susceptible to them.  Breastmilk. Breastmilk offers maternal antibodies, too. Specifically, the colostrum produced by mothers immediately after birth helps pass along immunity. Colostrum has extremely high levels of antibodies that help protect the intestines and other important systems.  Immunity from the mother’s system prepares the child for whatever they come into contact with before they can build up their own immune system.  Vaccines are another common form of passive immunity. When you receive a vaccine, you are given a tiny dose of pathogens that your body is likely to defeat. After killing the foreign substances, your body builds up a temporary defense. For a period of time that varies by vaccine, your immune system is well-equipped to battle the same pathogens.  Milestones of Fetal Growth and Development  When fetal milestones occur can be confusing because the life of the fetus is typically measured from the time of ovulation or fertilization (ovulation age), but the length of a pregnancy is more commonly measured from the first day of the last menstrual period (gestational age).  Because ovulation and fertilization take place about 2 weeks after the last menstrual period, the ovulation age of the fetus is always 2 weeks less than the length of the pregnancy or the gestational age.  Both ovulation and gestational age are typically reported in lunar months (4-week periods) or in trimesters (3-month periods) rather than in weeks.  In lunar months, a total pregnancy is 10 months (40 weeks, or 280 days) long; a fetus grows in utero for 9.5 lunar months or three full trimesters (38 weeks, or 266 days). The following discussion of fetal developmental milestones is based on gestational weeks. MILESTONES IN FETAL DEVELOPMENT End of Fourth Gestational Week (One Lunar Months)  The length of the embryo is about 0.75 cm; weight is about 400 mg.  Body is C-shaped  The rudimentary heart appears as a prominent bulge on the anterior surface. Heart begins to beat as early as 14th day.  Cartilage formation.  Arms and legs are bud-like structures; rudimentary eyes, ears, and nose are discernible.  Primary lung buds appear  Well-marked midbrain flexure. The spinal cord is formed and fused at the midpoint.  The head is large in proportion and represents about one third of the entire structure. MILESTONES IN FETAL DEVELOPMENT End of Eighth Gestational Week (Two Lunar Months)  The length of the fetus is about 2.5 cm (1 in.); weight is about 20 g.  Organogenesis is complete – testis and ovaries distinct.  The heart, with a septum and valves, beats rhythmically.  Facial features are definitely discernible; arms and legs have developed.  External genitalia are forming, but sex is not yet distinguishable by simple observation.  The abdomen bulges forward because the fetal intestine is growing so rapidly.  A sonogram shows a gestational sac, which is diagnostic of pregnancy. MILESTONES IN FETAL DEVELOPMENT End of 12th Gestational Week (First Trimester) (Three Lunar Months)  The length of the fetus is 7 to 8 cm; weight is about 45 g.  Nail beds are forming on fingers and toes.  Spontaneous movements are possible, although they are usually too faint to be felt by the mother.  Some reflexes, such as the Babinski reflex, are present.  Bone ossification centers begin to form.  Tooth buds are present.  Sex is distinguishable on outward appearance.  Urine secretion begins but may not yet be evident in amniotic fluid.  The heartbeat is audible through Doppler technology. MILESTONES IN FETAL DEVELOPMENT End of 16th Gestational Week  The length of the fetus is 10 to 17 cm; weight is 55 to 120 g.  Fetal heart sounds are audible by an ordinary stethoscope.  Lanugo is well formed.  Both the liver and pancreas are functioning.  The fetus actively swallows amniotic fluid, demonstrating an intact but uncoordinated swallowing reflex; urine is present in amniotic fluid.  Sex can be determined by ultrasonography. MILESTONES IN FETAL DEVELOPMENT End of 20th Gestational Week  The length of the fetus is 25 cm; weight is 223 g.  Spontaneous fetal movements can be sensed by the mother.  Antibody production is possible.  Hair, including eyebrows, forms on the head; vernix caseosa begins to cover the skin.  Meconium is present in the upper intestine.  Brown fat, a special fat that aides in temperature regulation, begins to form behind the kidneys, sternum, and posterior neck.  Passive antibody transfer from mother to fetus begins.  Definite sleeping and activity patterns are distinguishable as the fetus develops biorhythms that will guide sleep/wake patterns throughout life. MILESTONES IN FETAL DEVELOPMENT End of 24th Gestational Week (Second Trimester)  The length of the fetus is 28 to 36 cm; weight is 550 g.  Meconium is present as far as the rectum.  Active production of lung surfactant begins.  Eyelids, previously fused since the 12th week, now open; pupils react to light.  Hearing can be demonstrated by response to sudden sound.  When fetuses reach 24 weeks, or 500–600 g, they have achieved a practical low-end age of viability if they are cared for after birth in a modern intensive care nursery. MILESTONES IN FETAL DEVELOPMENT End of 28th Gestational Week  The length of the fetus is 35 to 38 cm; weight is 1,200 g.  Lung alveoli are almost mature; surfactant can be demonstrated in amniotic fluid.  Testes begin to descend into the scrotal sac from the lower abdominal cavity.  The blood vessels of the retina are formed but thin and extremely susceptible to damage from high oxygen concentrations (an important consideration when caring for preterm infants who need oxygen).  MILESTONES IN FETAL DEVELOPMENT End of 32nd Gestational Week  The length of the fetus is 38 to 43 cm; weight is 1,600 g.  Subcutaneous fat begins to be deposited (the former stringy, “little old man” appearance is lost).  Fetus responds by movement to sounds outside the mother’s body.  An active Moro reflex is present.  Iron stores, which provide iron for the time during which the neonate will ingest only breast milk after birth, are beginning to be built.  Fingernails reach the end of fingertips. MILESTONES IN FETAL DEVELOPMENT End of 36th Gestational Week  The length of the fetus is 42 to 48 cm; weight is 1,800 to 2,700 g (5 to 6 lb).  Body stores of glycogen, iron, carbohydrate, and calcium are deposited.  Additional amounts of subcutaneous fat are deposited.  Sole of the foot has only one or two crisscross creases, compared with a full crisscross pattern evident at term.  Amount of lanugo begins to diminish.  Most fetuses turn into a vertex (head down) presentation during this month. MILESTONES IN FETAL DEVELOPMENT End of 40th Gestational Week (Third Trimester)  The length of the fetus is 48 to 52 cm (crown to rump, 35 to 37 cm); weight is 3,000 g (7 to 7.5 lb).  Fetus kicks actively, sometimes hard enough to cause the mother considerable discomfort.  Fetal hemoglobin begins its conversion to adult hemoglobin.  Vernix caseosa is fully formed.  Fingernails extend over the fingertips.  Creases on the soles of the feet cover at least two thirds of the surface.