Читать книгу Infants and Children in Context - Tara L. Kuther - Страница 144

During the fetal period, from the ninth week to birth, the organism, called a fetus, grows rapidly, and its organs become more complex and begin to function. Now all parts of the fetus’s body can move spontaneously, the legs kick, and the fetus can suck its thumb (an involuntary reflex). By the end of the 12th week, the upper limbs have almost reached their final relative lengths, but the lower limbs are slightly shorter than their final relative lengths (Sadler, 2018).

By the 14th week, limb movements are coordinated, but they will be too slight to be felt by the mother until about 17 to 20 weeks. The heartbeat gets stronger. Eyelids, eyebrows, fingernails, toenails, and tooth buds form. The first hair to appear is lanugo, a fine down-like hair that covers the fetus’s body; it is gradually replaced by human hair. The skin is covered with a greasy material called the vernix caseosa, which protects the fetal skin from abrasions, chapping, and hardening that can occur with exposure to amniotic fluid (Moore & Persaud, 2016). At 21 weeks, rapid eye movements begin, signifying an important time of growth and development for the fetal brain. The brain begins to become more responsive. For example, startle responses have been reported at 22 to 23 weeks in response to sudden vibrations and noises (Hepper, 2015). During weeks 21 to 25, the fetus gains substantial weight, and its body proportions become more like those of a newborn infant. Growth of the fetal body begins to catch up to the head, yet the head remains disproportionately larger than the body at birth.

During the last 3 months of pregnancy, the fetal body grows substantially in weight and length; specifically, it typically gains over 5 pounds and grows 7 inches. At about 28 weeks after conception, brain development grows in leaps and bounds. The cerebral cortex develops convolutions and furrows, taking on the brain’s characteristic wrinkly appearance (Andescavage et al., 2016). The fetal brain wave pattern shifts to include occasional bursts of activity, similar to the sleep-wake cycles of newborns. By 30 weeks, the pupils of the eyes dilate in response to light. At 35 weeks, the fetus has a firm hand grasp and spontaneously orients itself toward light.

Although the expected date of delivery is 266 days or 38 weeks from conception (40 weeks from the mother’s last menstrual period), about 1 in every 10 American births is premature (Centers for Disease Control and Prevention, 2017e). The age of viability—the age at which advanced medical care permits a preterm newborn to survive outside the womb—begins at about 22 weeks after conception (Sadler, 2018). Infants born before 22 weeks rarely survive more than a few days, because their brain and lungs have not begun to function. Although a 23-week fetus born prematurely may survive in intensive care, its immature respiratory system places it at risk; only about one-third of infants born at 23 weeks’ gestation survive (Stoll et al., 2015). At about 26 weeks, the lungs become capable of breathing air and the premature infant stands a better chance of surviving if given intensive care. About 80% of infants born at 25 weeks survive, and 94% of those born at 27 weeks also survive. Premature birth has a variety of causes, including many environmental factors.

Our discussion thus far has emphasized fetal development; however, expectant mothers also experience radical physical changes during pregnancy. In addition to changes in body weight and shape, pregnancy is accompanied by changes in brain structure and function, as discussed in the Lives in Context feature.