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Chapter23 - Pregnancy and Development
- 1. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company Chapter 23 PPrreeggnnaannccyy aanndd HHuummaann DDeevveellooppmmeenntt
- 2. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company FFeerrttiilliizzaattiioonn
- 3. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company
- 4. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh How long can sperm remain viable within the female reproductive tract? A. As long as 72 hours B. As long as 6 days C. 24 hours D. 14 hours Copyright © 2012 F.A. Davis Company
- 5. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: B Rationale: The egg is viable for 24 hours, and it takes 72 hours for the egg to reach the uterus. Copyright © 2012 F.A. Davis Company
- 6. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company GGeessttaattiioonn Ranges from conception until birth (266 days) Divided into trimesters: FFiirrsstt ttrriimmeesstteerr: First 12 weeks SSeeccoonndd ttrriimmeesstteerr: Weeks 13 through 24 TThhiirrdd ttrriimmeesstteerr: Week 25 until birth
- 7. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company PPrreennaattaall ddeevveellooppmmeenntt PPrreeeemmbbrryyoonniicc ssttaaggee: First 16 days EEmmbbrryyoonniicc ssttaaggee: Third until eighth week FFeettaall ssttaaggee: Eighth week until birth
- 8. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company PPrreeeemmbbrryyoonniicc ssttaaggee View animation on fertilization and implantation
- 9. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company IImmppllaannttaattiioonn
- 10. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company
- 11. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh The product of conception is at which stage when it implants in the uterus? A. Embryo B. Zygote C. Morula D. Blastocyst Copyright © 2012 F.A. Davis Company
- 12. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: D Rationale: The product of conception is known as an embryo from the third until the eighth week, a zygote at the time of fertilization, and a morula when it first enters the uterine cavity. Copyright © 2012 F.A. Davis Company
- 13. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company EEmmbbrryyoonniicc ssttaaggee Yolk sac Allantois Amnion Chorion
- 14. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh PPllaacceennttaa aanndd uummbbiilliiccaall ccoorrdd Copyright © 2012 F.A. Davis Company Lacunae
- 15. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh FFuunnccttiioonn ooff tthhee ppllaacceennttaa Copyright © 2012 F.A. Davis Company Fetal waste Oxygen, nutrients, & antibodies
- 16. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh FFeettaall cciirrccuullaattiioonn View animation on fetal circulation Copyright © 2012 F.A. Davis Company
- 17. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh What is the function of the foramen ovale? A. Divert blood from the right to the left atrium B. Divert blood from the umbilical vein to the inferior vena cava C. Divert blood from the pulmonary artery to the descending aorta D. Shunt blood from the umbilical artery to the umbilical vein Copyright © 2012 F.A. Davis Company
- 18. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: A. Rationale: The ductus venosus diverts blood from the umbilical vein to the inferior vena cava to bypass the liver. The ductus arteriosus diverts blood from the pulmonary artery to the descending aorta to bypass the lungs. There is no shunt from the umbilical artery to the umbilical vein. Copyright © 2012 F.A. Davis Company
- 19. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company FFeettaall ddeevveellooppmmeenntt 4 weeks
- 20. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company 8 weeks
- 21. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company 12 weeks
- 22. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company 20 weeks
- 23. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company 36 weeks
- 24. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh When is the fetus capable of gas exchange? A.16 weeks gestation B.36 weeks gestation C.28 weeks gestation D.22 weeks gestation Copyright © 2012 F.A. Davis Company
- 25. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: C Rationale: Although the respiratory system is immature at this stage, it is capable of gas exchange. Copyright © 2012 F.A. Davis Company
- 26. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh PPhhyyssiiccaall cchhaannggeess wwiitthh pprreeggnnaannccyy Nausea and vomiting Constipation Heartburn ↑ basal metabolic rate Copyright © 2012 F.A. Davis Company DDiiggeessttiivvee ssyysstteemm
- 27. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh PPhhyyssiiccaall cchhaannggeess wwiitthh pprreeggnnaannccyy ↑ maternal blood volume ↑ cardiac output Hemorrhoids, varicose veins, foot swelling Copyright © 2012 F.A. Davis Company CCiirrccuullaattoorryy ssyysstteemm
- 28. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh PPhhyyssiiccaall cchhaannggeess wwiitthh pprreeggnnaannccyy ↑ ventilation Shortness of breath Copyright © 2012 F.A. Davis Company RReessppiirraattoorryy ssyysstteemm
- 29. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh PPhhyyssiiccaall cchhaannggeess wwiitthh pprreeggnnaannccyy Salt and water retention ↑ urine output Frequent urination (later in pregnancy) Copyright © 2012 F.A. Davis Company UUrriinnaarryy ssyysstteemm
- 30. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh PPhhyyssiiccaall cchhaannggeess wwiitthh pprreeggnnaannccyy IInntteegguummeennttaarryy ssyysstteemm && uutteerruuss Skin stretches over abdomen and breasts Uterus enlarges dramatically Copyright © 2012 F.A. Davis Company
- 31. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Why does maternal cardiac output increase by 30% to 40% by the 27th week? A.Because the fetus needs blood B.Because of the demands of the uterus C.Because of the demands of the placenta D.Because the fetus needs more oxygen Copyright © 2012 F.A. Davis Company
- 32. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: B Rationale: As the uterus expands, it demands a greater blood supply. Copyright © 2012 F.A. Davis Company
- 33. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company CChhiillddbbiirrtthh Probable triggers: Decline in progesterone Release of oxytocin Uterine stretching
- 34. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh SSttaaggee 11:: DDiillaattiioonn ooff cceerrvviixx Copyright © 2012 F.A. Davis Company
- 35. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh SSttaaggee 22:: EExxppuullssiioonn ooff bbaabbyy Copyright © 2012 F.A. Davis Company
- 36. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh SSttaaggee 33:: DDeelliivveerryy ooff ppllaacceennttaa Copyright © 2012 F.A. Davis Company
- 37. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company LLaaccttaattiioonn Progesterone stimulates development of acini Estrogen stimulates growth of ducts
- 38. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Anterior pituitary →Prolactin→ Milk production Posterior pituitary→ Oxytocin →Milk secretion Copyright © 2012 F.A. Davis Company
- 39. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Which hormone is responsible for the production of milk in the mammary glands? A. Estrogen B. Oxytocin C. Prolactin D. Progesterone Copyright © 2012 F.A. Davis Company
- 40. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: C Rationale: Estrogen stimulates the growth of ducts throughout the mammary glands. Progesterone stimulates the development of acini at the ends of the ducts. Oxytocin is responsible for the secretion of milk. Copyright © 2012 F.A. Davis Company
Editor's Notes
- During ejaculation, the male releases as many as 300 million sperm into the female’s vagina. The sperm actively swim toward the fallopian tubes. Only a few hundred sperm make it: the acidity of the vagina destroys many sperm; others fail to make it through the cervical mucus; white blood cells in the uterus destroy still more. Sperm remain viable within the female reproductive tract up to six days. The egg is only viable for 24 hours. Because it takes 72 hours for the egg to reach the uterus, fertilization typically occurs in the distal third of the fallopian tube.
- As hundreds of sperm swarm the egg, the acrosomes on the sperm heads release enzymes that break down the cells and the zona pellucida. Due to the efforts of multiple sperm, a path through the zona pellucida results, allowing a single sperm to penetrate. As soon as a sperm penetrates, the egg undergoes changes that bar any other sperm from entering. The nucleus of the sperm is released into the ovum and its tail degenerates and falls away. The nucleus of the sperm (which has 23 chromosomes) fuses with the nucleus of the egg (which also has 23 chromosomes), creating a single cell with 46 chromosomes. The fertilized egg is now called a zygote. Note: Remind students that this figure shows a “time lapse” view of fertilization. Many sperm assist with fertilization but only one enters the egg.
- The pre-embyonic stage begins when fertilization forms a zygote with 23 chromosomes. Within 24 to 36 hours, the zygote divides by mitosis to form two daughter cells called blastomeres. The mitotic divisions, or cleavage, continue, with cells doubling with each division. Finally, a blackberry-like cluster of 16 cells called a morula results. Three to four days after fertilization, the morula enters the uterine cavity, where it floats for four or five days. As the cell cluster continues to divide, a hollow cavity forms; the cell cluster is now called a blastocyst. The blastocyst consists of an outer layer of cells (the trophoblast) and an inner cell mass. The trophoblast eventually forms the placenta while the inner cell mass becomes the embryo.
- When the blastocyst alights on the endometrium, the plasma membrane cells on the side next to the endometrium disintegrate and the trophoblast cells fuse with, and send shoots into, the underlying endometrium. About that time, the inner cell mass separates from the trophoblast, creating a narrow space called the amniotic cavity.
- The inner cell mass flattens to form the embryonic disc. Some cells on the interior portion of the embryonic disc multiply to form the yolk sac. The rapidly growing endometrium covers the top of the blastocyst, burying it completely. (Show in figure on the left) As shown in the figure on the right, the embryonic disc gives rise to three germ layers: the ectoderm, mesoderm, and endoderm, which produce all the organs and tissues of the body. The ectododerm develops into the epidermis, nervous system, pituitary gland, optic lens, and salivary glands. The mesoderm develops into the skeleton; skeletal, cardiac, and most smooth muscle; cartilage; blood; and kidneys. The endoderm develops into the epithelial lining of the digestive and respiratory tracts, parts of the bladder and urethra, thyroid and parathyroid glands, liver, and thymus.
- The embryonic stage begins once germ layers are formed (16 days after conception); the zygote is now an embryo. During this stage, germ layers differentiate into organs and organ systems and extraembryonic membranes emerge. Amnion: A transparent sac that completely envelops the embryo; it is filled with amniotic fluid, which protects the embryo from trauma and changes in temperature. The fetus “breathes” the fluid and swallows it. The volume remains stable because the fetus regularly urinates into the amniotic sac. Chorion: Surrounds the other membranes. Fingerlike projections penetrate the uterus. Eventually, the chorion forms the fetal side of the placenta. Allantois: Serves as the foundation for the developing umbilical cord. Later, it becomes part of the urinary bladder. Yolk sac: Produces red blood cells until the sixth week, after which this task is taken over by the embryonic liver. The yolk sac also contributes to the formation of the digestive tract.
- About 11 days after conception, the embryo develops a disc-shaped, pancake-like organ called the placenta. The placenta secretes hormones necessary to maintain the pregnancy and supplies the fetus with oxygen and nutrition. The placenta begins to form during implantation when specialized cells in the trophoblastic layer extend into the endometrium. As shown on the slide, these extensions grow into the endometrium like the roots of a tree, forming early chorionic villi. As the villi project deeper into the endometrium, they penetrate uterine blood vessels, causing maternal blood to pool around the villi in sinuses called lacunae. Eventually, blood vessels from the umbilical cord extend into the villi, linking the embryo to the placenta. The umbilical cord contains two umbilical arteries and one umbilical vein. The fetal heart pumps blood into the placenta via the umbilical arteries; the blood returns to the fetus by way of the umbilical vein.
- Beginning the 12th week, the placenta is the fetus’ sole source of nutrition. The mother’s blood furnishes the developing fetus with nutrients, but maternal and fetal blood does not mix. Fetal waste products move from fetal blood in the umbilical arteries to the maternal blood; the maternal veins carry away the waste for disposal. Oxygen, nutrients, and some antibodies pass from the maternal blood —which is pooled in the lacunae around the chorionic villi—to fetal blood in the umbilical veins of the placenta. The placenta also secretes estrogen, progesterone, and human chorionic gonadotropin (HCG) to maintain the pregnancy.
- Because the fetus depends on the placenta for oxygen and nutrients and removal of waste, its circulatory system differs from that of a newborn. Oxygen-rich blood enters the fetus through the vein in the umbilical cord. Most of the blood bypasses the liver by flowing through the ductus venosus into the inferior vena cava (IVC). Placental blood from the umbilical vein mixes with fetal blood from the IVC as it flows to the heart. Blood flows into the right atrium and then most of the blood flows directly into the left atrium through the foramen ovale, bypassing the lungs. The blood that does not flow through the foramen ovale flows into the right ventricle and then into the pulmonary trunk. From there, the blood flows through the ductus arteriosus and into the descending aorta, again bypassing the lungs. Oxygen-depleted, waste-filled blood flows through two umbilical arteries to the placenta. The placenta cleanses the blood—ridding it of carbon dioxide and waste products—reoxygenates it, and returns it to the fetus through the umbilical vein. With neonate’s first breath, ductus arteiosus closes; foramen ovale closes when blood flows into left atrium after circulating through the lungs.
- During first three months, outward appearance of embryo changes rapidly as it develops into fetus. During the last six months, development turns inward as organs grow and mature. Week 4 The brain, spinal cord, and heart begin to develop. The gastrointestinal tract begins to form. The heart begins to beat about day 22. Tiny buds that will become arms and legs are visible.
- Week 8 The embryo is now a fetus. Eyes, ears, nose, lips, tongue, and tooth buds take shape. Head is nearly as large as the rest of the body. Brain waves are detectable. The arms and legs are recognizable. Blood cells and major blood vessels form.
- 12 weeks: The face is well formed. The arms are long and thin. The sex is distinguishable. The liver produces bile. The fetus swallows amniotic fluid and produces urine. 16 weeks: The scalp has hair. The lips begin sucking movements. The skeleton is visible. The heartbeat can be heard with a stethoscope.
- Week 20 Fetus is 8 inches (20 cm) long. A fine hair called lanugo covers the body, which, in turn is covered by a white cheeselike substance called vernix caseosa; both these substances protect the fetus’ skin from amniotic fluid. Fetal movement (quickening) can be felt. Nails appear on fingers and toes. Week 24 The fetus has a startle reflex. Lungs begin producing surfactant. Skin is wrinkled and translucent. Period of rapid weight gain. Week 28 The eyes open and close. The respiratory system, although immature, is capable of gas exchange at 28 weeks. Testes begin to descend into the scrotum. The brain develops rapids. Week 32 The amount of body fat increases rapidly. Rhythmic breathing movements begin although lungs are still immature. The bones are fully formed, although they are still soft.
- Week 36 More subcutaneous fat is deposited. Lanugo has mostly disappeared, although it’s still present on the upper arms and shoulders. The fetus is considered full term at the end of the 37th week. The average full-term infant measures approximately 20” (51cm) long and weights 7 to 7 ½ lbs (3.2 to 3.4 kg).
- Nausea and vomiting (morning sickness) commonly occurs during the first three months; the cause is unknown. Constipation results from decreased intestinal motility. Heartburn occurs later in the pregnancy as the enlarging uterus presses upward on the stomach. The basal metabolic rate rises about 15% during the second half of the pregnancy and the mother’s appetite increases.
- The mother’s blood volume increases by 30% to 50%. Cardiac output increases 30% to 40% by the 27th week as the uterus demands more of the blood supply; at the same time, heart rate also increases. Later in the pregnancy, the uterus exerts pressure on the pelvic blood vessels, interfering with venous return; hemorrhoids, varicose veins, and swelling in the feet may result.
- Ventilation increases about 50% to meet the increased demands for oxygen caused by the developing fetus. Late in the pregnancy, the enlarged uterus pushes against the diaphragm, often causing shortness of breath.
- An increase in aldosterone promotes water and salt retention by the kidneys. The glomerular filtration rate increases to deal with the added burden of disposing of the fetus’ waste; this leads to a slightly elevated urine output. Later in the pregnancy, the enlarged uterus presses on the bladder and reduces its capacity; this leads to increased frequency of urination.
- The skin of the abdomen grows and stretches to accommodate the expanding uterus; this often leads to stretch marks or striae. Skin over the breasts grows to accommodate the breasts, which enlarge in preparation for milk production. The uterus increases from its nonpregnant weight of 0.1 lbs (50 g) to about 2 lbs (900 g) by the end of pregnancy.
- Progesterone inhibits uterine contractions. After 6 months of gestation, progesterone levels declines while estrogen (which stimulates uterine contractions) continues to rise, leading to uterine irritability. Toward the end of pregnancy, the posterior pituitary releases more oxytocin, which stimulates uterine contractions. The uterus also becomes increasingly sensitive to oxytocin, peaking just before the beginning of labor. Oxytocin also causes fetal membranes to release prostaglandins, another substance that stimulates uterine contractions. Uterine stretching increases contractility.
- Labor occurs in three stage: dilation, expulsion, and placental stages. The first stage is the longest: 6 to 18 hours in primiparas, shorter in multiparas. The key features of this stage are cervical effacement (the progressive thinning of the cervical walls) and cervical dilation (the progressive widening of the cervix to allow for passage of the fetus). The fetal membranes usually rupture during dilation, releasing amniotic fluid. When the cervix is fully dilated (approximately 4 inches [10 cm]), the second stage of labor begins.
- The second stage of labor begins with full dilation of the cervix and ends when the baby is born; it lasts 30 to 60 minutes in primiparous women but can be much shorter in multiparous women. Normally, the head of the baby is delivered first. (The first appearance of the top of the head is called crowning.) To facilitate the passage of the head, a surgical incision is sometimes made between the vagina and the anus to enlarge the vaginal opening; this is called an episiotomy. As soon as the head emerges, mucus is cleared from the baby’s mouth and nose so it can begin breathing. The umbilical cord is clamped and cut, and the third stage begins.
- The final stage involves delivery of the afterbirth: the placenta, amnion, and other fetal membranes. After delivery of the baby, the uterus continues to contract, which cause the placenta to separate from the uterine wall; contractions continue, expelling the fetal membranes from the body. The contractions also help seal any blood vessels that are still bleeding.
- Following childbirth, the mammary glands produce and secrete milk (called lactation) to nourish the neonate. High levels of estrogen during pregnancy stimulate the growth of ducts throughout the mammary glands. High levels of progesterone stimulate the development of acini at the ends of the ducts. The production of milk depends on the hormone prolactin; the secretion of milk through the nipple depends on the hormone oxytocin.
- Suckling by the neonate sends nerve impulses to the anterior and posterior pituitary gland. The anterior pituitary secretes prolactin, which initiates the production of milk. (Every time the neonate nurses, the mother’s prolactin levels surge; this boosts milk production for the next feeding.) The posterior pituitary secretes oxytocin. Oxytocin causes the lobules in the breast to contract, forcing milk into the ducts. (This is known as the milk let-down reflex.) During pregnancy, high levels of estrogen block the secretion of prolactin. As soon as the placenta is delivered, the levels of estrogen plummet and the anterior pituitary begins secreting prolactin and milk production begins. A lag of 2 to 3 days occurs between the birth of the baby and the secretion of milk during which the breasts secrete a thin, yellowish fluid called colostrum. Colostrum is rich in protein and immunoglobulins that provide the neonate with passive immunity.