Fetal Period: Ninth Week to Birth

Trimesters of Pregnancy

Clinically, the gestational period is divided into three trimesters, each lasting 3 months. By the end of the first trimester, one third of the length of the pregnancy, major systems have been developed (see Table 6.1 ). In the second trimester, the fetus grows sufficiently in size so that good anatomical detail can be visualized during ultrasonography . During this period, most major birth defects can be detected using high-resolution real-time ultrasonography . By the beginning of the third trimester, the fetus may survive if born prematurely. The fetus reaches a major developmental landmark at 35 weeks and weighs approximately 2500 g and usually survives if born prematurely.

Measurements and Characteristics of Fetuses

Various measurements and external characteristics are useful for estimating fetal age (see Table 6.1 ). CRL is the method of choice for estimating fetal age until the end of the first trimester because there is very little variability in fetal size during this period. In the second and third trimesters, several structures can be identified and measured ultrasonographically, but the most common measurements are biparietal diameter (diameter of the head between the two parietal eminences), head circumference , abdominal circumference, femur length, and foot length.

Weight is often a useful criterion for estimating age, but there may be a discrepancy between the age and weight, particularly when the mother has had metabolic disturbances such as diabetes mellitus during pregnancy. In these cases, the weight often exceeds values considered normal for the corresponding CRL. Fetal dimensions obtained from ultrasound measurements closely approximate CRL measurements obtained from spontaneously aborted fetuses. Determination of the size of a fetus, especially its head circumference, is helpful to the obstetrician for management of patients.

Nine to Twelve Weeks

At the beginning of the fetal period (ninth week), the head constitutes approximately half of the CRL of the fetus ( Figs. 6.1 and 6.2 A ). Subsequently, growth in body length accelerates rapidly, so that by the end of 12 weeks, the CRL has almost doubled ( Fig. 6.2 B , and see Table 6.1 ). Although growth of the head slows down considerably by this time, the head is still disproportionately large compared with the rest of the body ( Fig. 6.3 ).

At 9 weeks, the face is broad, the eyes are widely separated, the ears are low set, and the eyelids are fused (see Fig. 6.2 B ). By the end of 12 weeks, primary ossification centers appear in the skeleton, especially in the cranium (skull) and long bones. Early in the ninth week, the legs are short, and the thighs are relatively small (see Fig. 6.2 ). By the end of 12 weeks, the upper limbs have almost reached their final relative lengths, but the lower limbs are still not so well developed, and they are slightly shorter than their final relative lengths.

The external genitalia of males and females appear similar until the end of the ninth week. Their mature form is not established until the 12th week. Intestinal coils are clearly visible in the proximal end of the umbilical cord until the middle of the 10th week (see Fig. 6.2 B ). By the 11th week, the intestines have returned to the abdomen (see Fig. 6.3 ).

At 9 weeks, the beginning of the fetal period, the liver is the major site of erythropoiesis (formation of red blood cells). By the end of 12 weeks, this activity has decreased in the liver and has begun in the spleen. Urine formation begins between the 9th and 12th weeks, and urine is discharged through the urethra into the amniotic fluid in the amnionic cavity. The fetus reabsorbs some amniotic fluid after swallowing it. Fetal waste products are transferred to the maternal circulation by passage across the placental membrane (see Chapter 7 , Fig. 7.7 ).

Thirteen to Sixteen Weeks

Growth is very rapid during this period ( Figs. 6.4 and 6.5 , and see Table 6.1 ). By 16 weeks, the head is relatively smaller than the head of a 12-week fetus, and the lower limbs have lengthened ( Fig. 6.6 A ). Limb movements , which 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.

Ossification of the fetal skeleton is active during this period, and the developing bones are clearly visible on ultrasound images by the beginning of the 16th week. 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 , which contain oogonia , or primordial germ cells (see Chapter 12 , Fig. 12.31 ).

The genitalia of male and female fetuses can be recognized by 12 to 14 weeks. By 16 weeks, the eyes face anteriorly rather than anterolaterally. In addition, the external ears are close to their definitive positions on the sides of the head.

Seventeen to Twenty Weeks

Growth slows down during this period, but the fetus still increases its CRL by approximately 50 mm (see Fig. 6.4 , Fig. 6.6 , and Table 6.1 ). Fetal movements (quickening) are commonly felt by the mother. The skin is now covered with a greasy, cheese-like material, the vernix caseosa . It consists of a mixture of dead epidermal cells and a fatty substance from the fetal sebaceous glands. The vernix protects the delicate fetal skin from abrasions, chapping, and hardening that result from exposure to the amniotic fluid. Fetuses are covered with fine, downy hair, lanugo , which helps the vernix to adhere to the skin.

Eyebrows and head hair are visible at 20 weeks. Brown fat forms during this period and is the site of heat production. This specialized adipose tissue is connective tissue that consists chiefly of fat cells; it is found mostly at the root of the neck, posterior to the sternum, and in the perirenal area. The brown fat produces heat by oxidizing fatty acids.

By 18 weeks, the fetal uterus is formed, and canalization of the vagina has begun. Many primordial ovarian follicles containing oogonia are also visible. By 20 weeks, the testes have begun to descend, but they are still located on the posterior abdominal wall, as are the ovaries.

Twenty-One to Twenty-Five Weeks

Substantial weight gain occurs during this period, and the fetus is better proportioned ( Fig. 6.7 ). The skin is usually wrinkled and more translucent, particularly during the early part of this period. The skin is pink to red because blood in the capillaries is visible. At 21 weeks, rapid eye movements begin, and blink−startle responses have been reported at 22 to 23 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 10 ).

Fingernails are present by 24 weeks. Although a 22- to 25-week fetus born prematurely may survive if given intensive care (see Fig. 6.7 ), there is also a chance that it may die because its respiratory system is still immature. The risk for neurodevelopmental disability (e.g., mental deficiency) is high in fetuses born before 26 weeks.

Twenty-Six to Twenty-Nine Weeks

During this period, fetuses usually survive if they are born prematurely and given intensive care ( Fig. 6.8 B and C ). The lungs and pulmonary vasculature have developed sufficiently to provide adequate gas exchange. In addition, the central nervous system has matured to the stage where it can direct rhythmic breathing movements and control body temperature. The highest rate of neonatal mortality occurs in infants classified as low birth weight (≤2500 g) and very low birth weight (≤1500 g).

The eyelids are open at 26 weeks, and lanugo (fine downy hair) and head hair are well developed. Toenails are visible, and considerable subcutaneous fat is under the skin, smoothing out many of the wrinkles. During this period, the quantity of white fat increases to approximately 3.5% of the body weight. The fetal spleen has been an important site of erythropoiesis (formation of red blood cells). This ends at 28 weeks, by which time bone marrow has become the major site of erythropoiesis.

Thirty to Thirty-Four Weeks

The pupillary reflex (change in diameter of the pupil in response to a stimulus caused by light) 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. At this age, the quantity of white fat is approximately 8% of the body weight. Fetuses 32 weeks and older usually survive if born prematurely.

Thirty-Five to Thirty-Eight Weeks

Fetuses born at 35 weeks have a firm grasp and exhibit a spontaneous orientation to light. As term approaches, the nervous system is sufficiently mature to carry out some integrative functions. Most fetuses during this “finishing period” are plump. By 36 weeks, the circumferences of the head and abdomen are approximately equal. After this, the circumference of the abdomen may be greater than that of the head. The foot length of fetuses is usually slightly larger than the femoral length (long bone of the thigh) at 37 weeks and is an alternative parameter for confirmation of fetal age ( Fig. 6.9 ). There is a slowing of growth as the time of birth approaches ( Fig. 6.10 ).

At full term (38 weeks) ( Fig. 6.11 B ), most fetuses usually reach a CRL of 360 mm and weigh approximately 3400 g. The amount of white fat is approximately 16% of the body weight. A fetus adds approximately 14 g of fat per day during these last weeks. The thorax (chest) is prominent, and the breasts often protrude slightly in both sexes. The testes are usually in the scrotum in full-term male neonates; premature male neonates commonly have undescended testes. Although the head is smaller at full term in relation to the rest of the body than it was earlier in fetal life, it is still one of the largest regions of the fetus. In general, male fetuses are longer and weigh more at birth than females.