Twenty-six to Twenty-nine Weeks
At the end of the seventh month, the crown rump length is about 270 mm (vertex to heels length is from 35–36 cm). The weight is about 1300 g (46 oz). The eyes open during this period. The body is well covered with lanugo. The testes should have begun descent into the scrotum.
As birth approaches, the fetus rotates to a vertex, or upside-down position, owing chiefly to the shape of the uterus and partly because the head is the heaviest part of the body.
The fetus has a good chance of survival if born prematurely now, but the mortality rate is high. The respiratory system of the fetus is sufficiently mature by this time that she or he might survive if born premature. Its metabolism cannot yet maintain a constant body temperature, and the respiratory muscles have not matured enough to provide a regular respiratory rate. The premature infant may survive, however, if placed in an incubator with its breathing maintained by a respirator. Nevertheless, survival is difficult and full term fetuses have the best chances.
The fetus at this age may suck its thumb, hiccup, and cry. It can taste sweet or sour, and respond to stimuli, including pain, light, and sound.
Placental function begins to diminish, as does the volume of amniotic fluid as fetus fills the uterus.
Thirty to Thirty-eight Weeks
The fetus can see and hear. Most systems are well developed. The lungs may still be immature, but premature fetuses 32 weeks and older usually survive. Fetuses born at 36 weeks of gestation have excellent chances for survival.
Weight increases considerably during the second half of pregnancy, especially during the last 2 _ months, when the fetus gains half of the full term weight (approximately 3200 grams). At 32 weeks, the fetus is about 30 cm (12 in) long and weighs about 2.25 kg (5 lb). Growth, especially of the brain, is great in this period.
There is a slowing of growth as the time of birth approaches. At the end of the 9th month, the head has the largest circumference of all parts of the body, though this is nearly equal to that of the abdomen. After the ninth month, the circumference of the abdomen exceeds that of the head. The diameters of the body parts are important considerations with regard to passage through the birth canal. At birth the fetus weighs 3000–3500 grams, has a crown-rump length of about 36 centimeters, and a crown-heel length of about 50 centimeters.
Final preparations are being made for birth, which can safely take place any time after the eighth month because the lungs are mature. More confined, and possibly engaged in the pelvis, the fetus may seem less active. The fetus usually assumes an upside-down position as birth approaches; this position is partly the result of the shape of the uterus and partly because the head outweighs the feet.
By full term (38 weeks after fertilization or 40 weeks after LMP), fetuses are fully developed, or "full-term." About 5 cm (2 in) and 1 kg (2.5 lb) are added to the length and weight. Male fetuses generally grow faster than females, and they generally weigh more at birth, about 3400 gm (7.5 pounds; range about 6.5–8 pounds). The crown-rump length is about 360 millimeters or 14 in (total length from crown to heel is about 50 centimeters, or 20 inches). The chest is prominent and both sexes have protruding breasts. At birth the head is about one-fourth the crown-heel length and has the greatest circumference.
Most fetuses are plump and have smooth skin resulting from the accumulation of subcutaneous fat. The skin is pinkish-blue, even on fetuses of dark-skinned parents because melanocytes do not produce melanin until the skin is exposed to sunlight. Lanugo hair is sparse and generally occurs on the head and back.
The chest is prominent, and the mammary area protrudes in both males and females. The umbilicus is almost in the middle of the body. The external genitalia are somewhat swollen.
Pregnancy, Labor, and Delivery
Pregnancy
Pregnancy is a sequence of events that includes fertilization, implantation, embryonic growth, and fetal growth that terminates in birth. The period of gestation is the time of pregnancy, or the time the zygote, embryo, or fetus spends in prenatal development in the female reproductive tract. Obstetrics (obstetrix = midwife) is the specialized branch of medicine concerned with pregnancy, labor, and the period of time immediately following delivery. The total human gestational period is ordinarily 266 days or about 280 days from the beginning of the last menstrual period to birth, or parturition. Most fetuses are born within 10–15 days before or after this due date.
Gestation, and Trimesters
For convenience, the gestation period is usually regarded as three integrated trimesters, each of 3 months duration. The first trimester, the period of embryonic and early fetal development, is a critical period for development because during this time the rudiments of all the major organ systems develop. The process of organ formation is called organogenesis. Many important and complex developmental events occur during the first trimester, such as cleavage, implantation, placentation, and embryogenesis. By the end of the first trimester, the fetus is almost 75 m (3 in) long and weighs about 14 g (0.5 oz). Only about 40% of embryos survive the first trimester. For this reason, pregnant women should avoid drugs or other disruptive stresses during the first trimester, in the hope of preventing a developmental abnormality, or anomaly. During the second trimester, the organs and organ systems complete most of their development, the body proportions change, and the fetus begins to acquire distinctively human characteristics. The third trimester is a period of rapid fetal growth. Most of the major organ systems become fully functional early in the third trimester, and an infant born 1–2 months prematurely may survive.
The uterus occupies most of the pelvic cavity by about the end of the third month of gestation. The uterus extends ever higher into the abdominal cavity as the fetus continues to grow. Toward the end of a full-term pregnancy, the uterus occupies nearly the entire abdominal cavity, rising above the costal margin almost to the xiphoid process of the sternum. The uterus will expand from 7.5 cm (3 in) to 30 cm (12 in) long and will contain approximately 5 liters of fluid. The uterus and its contents weigh approximately 10 kg (22 lb). This remarkable expansion occurs through enlargement and elongation of existing smooth muscle fibers. The expansion causes displacement of the maternal intestines, liver, and stomach upward, elevation of the diaphragm, widening of the thoracic cavity, and compression of the ureters and urinary bladder. Furthermore, the breasts enlarge in anticipation of lactation, and the areolae around the nipples become darkly pigmented.
Besides the anatomical changes associated with pregnancy, there are also physiological changes induced by pregnancy. Cardiac output rises by 30–40% by the twenty-seventh week attributable to increased maternal blood flow to the placenta and increased metabolism. Pulse rate increases by about 15 beats per minute. Blood volume increases up to 30–50%, mostly during the latter half of pregnancy. In the supine position, the enlarged uterus may compress the aorta, causing diminished blood flow to the uterus. Hormonal changes associated with pregnancy and compression of the inferior vena cava can cause varicose veins. Pulmonary function changes during pregnancy in that functional residual capacity decreases and tidal volume increases; the latter is attributable to hypocapnia and compensated respiratory alkalosis. There is a general decrease in gastrointestinal motility that can produce constipation and a delay in gastric emptying time. Pressure on the urinary bladder by the enlarging uterus can cause urinary symptoms, such as frequency, urgency, and stress incontinence.
Labor and Delivery
The delivery or expulsion of the fetus, a process known as parturition, or birth, is accompanied by a sequence of physiological and physical events called labor.
The onset of labor apparently relates to a complex interaction of many factors. Just before birth, the myometrium layer of the uterus contracts rhythmically and forcefully. Placental and ovarian hormones seem to play a role in these contractions. Labor cannot occur until the inhibitory effects of progesterone on uterine contractions are diminished. At the end of gestation, there is a sufficient level of estrogens in the mother’s blood to overcome the inhibiting effects of progesterone and labor commences. Perhaps some factor released by the placenta, fetus, or mother rather suddenly overcomes the inhibiting effects of progesterone so that estrogens can exert their effect. The contractions in the myometrium during labor are stimulated by (1) oxytocin, a polypeptide hormone produced in the hypothalamus and secreted by the posterior pituitary, and (2) prostaglandins , a class of fatty acids produced within the uterus itself. Labor can indeed be induced artificially by injections of oxytocin or by the insertion of prostaglandins into the vagina as a suppository. The hormone relaxin, produced by the corpus luteum, may also assist in labor and parturition. Relaxin softens the symphysis pubis in preparation for parturition and probably also softens the cervix in preparation for dilation. Relaxin, however, may not affect the uterus, but rather progesterone and estradiol may be responsible for this effect. Further research is necessary to understand the physiological effects of these hormones.
Uterine contractions, like peristaltic contractions, occur in waves; they begin at the fundus of the uterus and sweeps downward toward the cervix to expel the fetus. True labor begins when the pains corresponding to uterine contractions occur at regular intervals and intensify as the interval between contractions shortens. Another sign of true labor in some women is localization of pain in the back that is intensified by walking. A reliable indication of true labor is dilation of the cervix and a "show," or discharge of blood-containing mucus that accumulates in the cervical canal and vagina during labor. As parturition approaches, the force and frequency of the contractions increase, changing the position of the fetus and moving it toward the cervical canal. In false labor, abdominal pain is felt at irregular intervals, the pain does not intensify and is not changed significantly by walking, and there is a lack of cervical dilations and cervical "show."
Three to five percent of newborns are born breech. In a breech birth, the fetus has not rotated and the buttocks or legs are the presenting part; that is, these parts enter the vaginal canal before the fetal head. The main concern of a breech birth is the increased time and difficulty of the expulsion stage of parturition. Risks to the infant are relatively higher in breech births because the umbilical cord may become constricted, thus ending placental circulation. The cervix may dilate sufficiently to pass the legs and body but not the head, normally the widest part of the fetus. Entrapment of the fetal head compresses the umbilical cord, prolongs delivery, and subjects the fetus to severe distress and potential harm. Attempts to rotate the fetus by using forceps may injure the infant. If the fetus cannot be repositioned manually or delivered breech, a cesarean section must be performed. A cesarean section is delivery of the fetus through an incision into the abdominal wall and the uterus.
Dystocia, or difficult labor, may result from various deformities of the female pelvis that may be congenital or acquired from disease, fractures, or poor posture. Malposition of the fetus, malpresentation of the fetus, and premature rupture of the fetal membranes are among the conditions associated with difficult labor.
If the vaginal canal is too small to allow passage of the fetus and there is acute danger of perineal tearing, the passageway may be temporarily enlarged by performing an incision, or episiotomy, through the perineal musculature. After delivery the episiotomy can be repaired with sutures, a much simpler procedure than trying to control bleeding and repairing tissue damage associated with a potentially extensive perineal tear. If either dystocia or prolonged labor are present or if unexpected complications arise during the dilation or expulsion stages, it may be necessary to deliver the baby by a cesarean section. In a c-section, a low, horizontal incision is made through the abdominal wall and uterus, through which the baby and placenta are removed. Cesarean sections are performed during 15–25% of the deliveries in the United States. Efforts are being undertaken to reduce the frequency of episiotomies and cesarean sections.
Premature labor is true labor that begins before the fetus has completed normal development. The chances of newborn survival directly relate to body weight at delivery. Newborns that weigh less than 400 g (14 oz) will not survive even with massive supportive efforts, primarily because the respiratory, cardiovascular, and urinary systems cannot support life without the aid of maternal systems. Consequently, the separation between spontaneous abortion and immature delivery is usually 500 g (17.6 oz), the normal weight near the end of the second trimester.
Infants delivered before completing 7 months of gestation (weight less than 1 kg) have less than a 50% chance of survival. Most survivors suffer severe developmental abnormalities. A premature delivery produces a newborn that weighs more than 1 kg (35.2 oz); its chances of survival are fair to excellent, depending on individual circumstances.
Stages of Labor
Labor is usually divided into three stages—dilation, expulsion, and placental.
Dilation Stage
The stage of dilation is the time from the onset of true labor to complete dilation of the cervix. During this stage there are regular contractions of the uterus, usually a rupturing of the amniotic sac ("bag of waters"), and complete dilation of the cervix to a diameter of approximately 10 cm; the fetus begins to slide down the cervical canal. The amniotic sac is ruptured artificially if it does not rupture spontaneously. The dilation stage may last eight to twenty-four hours, depending on whether it is occurring in the first or a subsequent pregnancy. During this period the labor contractions occur at intervals of once every 10–30 minutes.
Expulsion Stage
The stage of expulsion is the time from complete cervical dilation to birth, or delivery (parturition). This period may require thirty minutes to 2 hours in a first pregnancy, but only a few minutes in subsequent pregnancies. The expulsion stage begins as the cervix dilates completely, pushed open by the descending fetus. Forceful uterine contractions and abdominal compressions expel the fetus from the uterus and through the vagina.
A pudendal nerve block may be administered during the early part of the expulsion stage to ease the trauma of delivery for the mother and for procedures such as episiotomy. The pudendal nerve provides the primary innervation to the skin and muscles of the perineum. In the transvaginal approach, the needle is inserted through the lateral vaginal wall to a point medial to the ischial spine. The anesthesia produces loss of the anal reflex, relaxation of the muscles of the floor of the pelvis, and loss of sensation to the vulva and lower third of the vagina.
Placental Stage
The placental stage is the period between delivery and expulsion of the placenta, or "afterbirth." Expulsion of the placenta usually occurs within an hour after delivery, and generally within ten to fifteen minutes after parturition. Powerful uterine contractions tear the connections between the endometrium and the placenta and constrict uterine blood vessels that were torn during delivery to reduce the possibility of hemorrhage. Blood loss accompanying disruption of the placenta in a normal delivery does not exceed 350 ml but may be as much as 500-600 ml, but the loss can be tolerated without difficulty because the maternal blood volume increased during pregnancy.
Figures for Second Submission
The table is arranged so that graphics (identified in column III) can be associated with the appropriate text topic (identified in column II).
descriptions for histology photomicrographs
Blastocyst Formation (Blastulation)
Gastrulation (Formation of Germ Layers)