Fetal Development

The fetal period extends from the ninth week to birth.⁶,⁸ During the 9th to 12th weeks, fetal head growth slows, whereas body length growth is greatly accelerated. By the 11th week, the intestines in the proximal portion of the cord have returned to the abdomen. The primary ossifica­tion centers are present in the skull and long bones, and maturation of the fetal external genitalia is established by the 12th week. During the fetal period, the liver is the major site of red blood cell formation (i.e., erythropoiesis); at 12 weeks, this activity has decreased, and erythropoiesis

begins in the spleen. Urine begins to form during the 9th to 12th weeks and is excreted into the amniotic fluid.⁸

The 13th through 16th weeks are notable for ossifica­tion of the skeleton, scalp hair patterning, and differenti­ation of the ovaries in female fetuses. By the 17th through 20th week, growth has slowed. The fetal skin is covered with a fine hair called lanugo and a white, cheeselike ma­terial called vernix caseosa. Eyebrows and head hair are vis­ible. In male fetuses, the testes begin to descend, and in female fetuses, the uterus is formed. Brown fat also forms during this period. Brown fat is a specialized type of adi­pose tissue that produces heat by oxidizing fatty acids. Brown fat is similar to white fat but has larger and more numerous mitochondria, which provide its brown color. Brown fat is found near the heart and blood vessels that supply the brain and kidneys and is thought to play a role in maintaining the temperature of these organs during ex­posure to environmental changes that occur after birth.

During the 21st through 25th weeks, significant fetal weight gain occurs. The type II alveolar cells of the lung begin to secrete surfactant (see Chapter 29). The pulmo­nary system becomes more mature and able to support res­piration during the 26th through 29th weeks. Breathing movements are present as a result of central nervous sys­tem (CNS) maturation. At this age, a fetus can survive if born prematurely and given intensive care. There also is an increasing amount of subcutaneous fat, with white fat making up 3.5% of body weight.⁶

The 30th through 34th weeks are significant for an in­creasing amount of white fat (8% of body weight), which gives the fetal limbs an almost chubby appearance.⁶ During the 35th week, grasp and the pupillary light reflex are pres­ent. If a normal-weight fetus is born during this period, it is premature by "date" as opposed to premature by "weight."⁶

Expected time of birth is 266 days, or 38 weeks after fer­tilization, or 40 weeks after the last menstrual period (LMP).⁶ At this time, the neurologic, cardiovascular, and pulmonary systems are developed enough for the infant to make the transition to extrauterine life. The survival of the newborn depends on this adaptation after the placenta is removed.

Fetal Growth and Weight Gain. Development during the fetal period is primarily concerned with rapid growth and differentiation of tissues, organs, and systems. Fetal weight gain is linear from 20 weeks' gestation through 38 weeks' gestation. In the last half of pregnancy, the fetus gains 85% of birth weight. After 38 weeks' gestation, the rate of growth declines, probably related to the constraint of uterine size and decreased placental function. After birth, weight gain again increases, similar to intrauterine rates. Birth weight can be affected by a variety of factors, including maternal nutrition, genetic factors, chronic maternal diseases, pla­cental abnormalities, sex, socioeconomic factors, multiple births, chromosomal abnormalities, and infectious diseases.