64 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
Figure 7–1 Endovaginal scan of a 9-week fetus with a crown–
rump length of 41.7 mm (calipers). Chorionic cavity (CC) has low-level echoes normally, while the amniotic cavity (AC) is echo-free.
1
CC AC
Table 7–1 Criteria for Estimating Fertilization Age during the Fetal Period AGE
(weeks) CROWN–RUMP
LENGTH (mm)* FOOT LENGTH
(mm)* FETAL
WEIGHT (g)† MAIN EXTERNAL CHARACTERISTICS Previable Fetus
9 50 7 8 Eyelids are closing or have closed. Head is rounded. External
genitalia are still not distinguishable as male or female.
Intestinal herniation is present.
10 61 9 14 Intestine is in the abdomen. Early fingernail development.
12 87 14 45 Sex is distinguishable externally. Well-defined neck.
14 120 20 110 Head is erect. Lower limbs are well developed. Early toenail
development.
16 140 27 200 Auricles of the ears stand out from the head.
18 160 33 320 Vernix caseosa covers the skin. Fetal movement (quickening) is
felt by the mother.
20 190 39 460 Head and body hair (lanugo) are visible.
Viable Fetus‡
22 210 45 630 Skin is wrinkled and red.
24 230 50 820 Fingernails are present. Lean body.
26 250 55 1000 Eyes are partially open. Eyelashes are present.
28 270 59 1300 Eyes are open. Most fetuses have scalp hair. Skin is slightly
wrinkled.
30 280 63 1700 Toenails are present. Body is filling out. Testes are descending.
32 300 68 2100 Fingernails extend to fingertips. Skin is smooth.
36 340 79 2900 Body is usually plump. Lanugo is almost absent. Toenails extend
to the toe tips. Flexed limb; firm grasp.
38 360 83 3400 Prominent chest; breasts protrude. Testes in the scrotum or
palpable in the inguinal canals. Fingernails extend beyond fingertips.
*These measurements are averages, and dimensional variations increase with age.
†These weights refer to fetuses that have been fixed for approximately 2 weeks in 10% formalin. Fresh specimens usually weigh approximately 5% less.
‡There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable or beyond which survival is ensured, but experience has shown that it is uncommon for an infant to survive if its weight is less than 500 g or if its fertilization age or developmental age is less than 22 weeks.
C H A P T E R 7 FETAL PERIOD: THE NINTH WEEk TO BIRTH 64.e1
(Courtesy E. A. Lyons, MD, Professor of Radiology, and Obstetrics and Gynecology, and Anatomy, University of Manitoba, Health Sciences Centre, Winnipeg, Manitoba, Canada.)
C H A P T E R 7 FETAL PERIOD: THE NINTH WEEk TO BIRTH 65
first occur at the end of the embryonic period, become coordinated by the 14th week, but they are too slight to be felt by the mother. However, these movements are visible during ultrasonographic examinations.
By the beginning of the 16th week, the developing bones are clearly visible on ultrasound images. Slow eye movements occur at 14 weeks. Scalp hair patterning is also determined during this period. By 16 weeks, the ovaries are differentiated and contain primordial ovarian follicles that have oogonia (primordial germ cells). The eyes face anteriorly rather than anterolaterally.
Seventeen to Twenty Weeks
Growth slows down during this period, but the fetus still increases its CRL by approximately 50 mm (see Fig. 7-3 and Fig. 7-5; see Table 7-1). Fetal movements—
quickening—are commonly felt by the mother. The skin is now covered with a greasy material—vernix caseosa. It consists of dead epidermal cells and a fatty secretion from the fetal sebaceous glands. The vernix caseosa protects the delicate fetal skin from abrasions, chapping, and harden- ing that could result from exposure to the amniotic fluid.
Fetuses are usually completely covered with fine, downy hair—lanugo—that helps to hold the vernix on the skin.
Eyebrows and head hair are also visible. Brown fat forms during weeks 17 through 20 and is the site of heat production, particularly in the neonate. This specialized adipose tissue, found chiefly at the neck, posterior to the sternum, produces heat by oxidizing fatty acids.
By 18 weeks, the fetal uterus is formed and canaliza- tion of the vagina has begun. By 20 weeks, the testes have begun to descend, but they are still located on the poste- rior abdominal wall.
Twenty-One to Twenty-Five Weeks
Substantial weight gain occurs during this period, and the fetus is better proportioned. The skin is usually wrinkled and more translucent. The skin is pink to red because blood is visible in the capillaries. At 21 weeks, rapid eye movements begin, and blink-startle responses have been reported at 22 to 23 weeks. Fingernails are present by 24 weeks. Also by 24 weeks, the secretory epithelial cells (type II pneumocytes) in the interalveolar walls of the lung have begun to secrete surfactant, a surface-active lipid that maintains the patency of the developing alveoli of the lungs (see Chapter 11). Although a 22- to 25-week fetus born prematurely may survive initially if given intensive care support; however, the fetus may die because its respiratory system is still immature. Fetuses born coils are clearly visible in the proximal end of the umbili-
cal cord until the middle of the 10th week. By the 11th week, the intestines have returned to the abdomen (Fig.
7-2). Urine formation begins between the 9th and 12th weeks, and urine is discharged through the urethra into the amniotic fluid. The fetus reabsorbs some of this fluid after swallowing it. Fetal waste products in blood are transferred to the maternal circulation by passing across the placental membrane (see Chapter 8).
Thirteen to Sixteen Weeks
Growth is very rapid during this period (Fig. 7-3 and Fig.
7-4; see Table 7-1). By 16 weeks, the head is relatively small compared with that of the 12-week fetus, and the lower limbs have lengthened. Limb movements, which
Table 7–2 Comparison of Gestational Time Units
REFERENCE POINT
CALENDAR LUNAR
DAYS WEEKS MONTHS MONTHS
Fertilization 266 38 8.75 9.5
Last normal menstrual period 280 40 9.25 10
Figure 7–2 An 11-week fetus that was spontaneously aborted (×1.5). Its chorionic and amniotic sacs have been removed. Note that the head is relatively large.
C H A P T E R 7 FETAL PERIOD: THE NINTH WEEk TO BIRTH 65.e1 (Courtesy Jean Hay, late, Asso ciate Professor, University of Mani-
toba, Winnipeg, Manitoba, Canada.)
66 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
before 26 weeks of gestation have a high risk of neuro- developmental (functional) disability.
Twenty-Six to Twenty-Nine Weeks
During this period, fetuses usually survive if born prema- turely and given intensive care because the lungs have developed sufficiently to provide adequate gas exchange.
Figure 7–3 Diagram drawn to scale illustrating the changes in size of human fetuses.
C H A P T E R 7 FETAL PERIOD: THE NINTH WEEk TO BIRTH 66.e1 (Courtesy Jean Hay, late, Associate Professor, University of Mani-
toba, Winnipeg, Manitoba, Canada.)
C H A P T E R 7 FETAL PERIOD: THE NINTH WEEk TO BIRTH 67
Figure 7–5 A, A 17-week fetus (actual size). Fetuses at this age are unable to survive if born prematurely, mainly because the respiratory system is immature. B, Magnetic resonance imaging scan of an 18-week-old normal fetus (20 weeks’ gestational age). (A, From Moore KL, Persaud TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed. Philadelphia, Saunders, 2000.)
A B
Figure 7–6 A healthy male neonate at 36 weeks’ gestational age.
Thirty to Thirty-Eight Weeks
The pupillary light reflex of the eyes can be elicited at 30 weeks. Usually, by the end of this period, the skin is pink and smooth and the upper and lower limbs have a chubby appearance. Fetuses born at 32 weeks usually survive.
Fetuses at 35 weeks have a firm grasp and exhibit a spontaneous orientation to light. As term approaches (37–38 weeks), the nervous system is sufficiently mature to carry out some integrative functions. Most fetuses during this “finishing period” are plump (Fig. 7-6). At 36 weeks, the circumferences of the head and abdomen are approximately equal. Growth slows as the time of birth approaches (Fig. 7-7). Most fetuses weigh approximately 3400 g at term (Fig. 7-8). A fetus adds approximately 14 g of fat daily during the last weeks of gestation. The chest is prominent, and the breasts protrude slightly in both sexes.
Expected Date of Delivery
The expected date of delivery of a fetus is 266 days, or 38 weeks, after fertilization (i.e., 280 days or 40 weeks after the LNMP) (see Table 7-2). Approximately 12% of babies are born 1 to 2 weeks after the expected time of birth.