Transition from the embryonic period to the fetal period occurs at 7 weeks after fertilization, corresponding to 9 weeks after onset of the last menses. At this time, the fetus approximates 24 mm in length, most organ systems have developed, and the fetus enters a period of growth and maturation. These phases are outlined in Figure 7-2.
The uterus usually is just palpable above the symphysis pubis. Fetal growth is rapid, and the fetal crown-rump length is 5 to 6 cm (Fig. 7-7). Centers of ossiication have appeared in most fetal bones, and the fingers and toes have become diferentiated. Skin and nails develop, and scattered rudiments of hair appear. he external genitalia are beginning to show deinitive signs of male or female gender. he fetus begins to make spontaneous movements.
Fetal growth slows at this time. The crown-rump length is 12 cm, and the fetal weight approximates 150 g (Hadlock, 1991). Practically speaing, the sonographic crown-rump length is not measured beyond 13 weeks, which corresponds to approximately 8.4 cm. Instead, biparietal diameter, head circumference, abdominal circumference, and femur length are measured. Fetal weight in the second and third trimesters is estimated from a combination of these measurements (Chap. 10, p. 184).
Eye movements begin at 16 to 18 weeks, coinciding with midbrain maturation. By 18 weeks in the female fetus, the uterus is formed and vaginal canalization begins. By 20 weeks in the male, testicles start to descend.
FIGURE 7-7 This image of a 12-week, 3-day embryo depicts measurement of the crown-rump length. The fetal profile, cranium, and a hand and foot are also visible in this image.
This is the midpoint of pregnancy as estimated from the LMP. The fetus now weighs somewhat more than 300 g, and weight increases substantially in a linear manner. From this point onward, the fetus moves approximately every minute and is active 10 to 30 percent of the day (DiPietro, 2005). Brown fat forms, and the fetal skin becomes less transparent. Downy lanugo covers its entire body, and some scalp hair can be seen. Cochlear function develops between 22 and 25 weeks, and its maturation continues for 6 months after delivery.
The fetus now weighs almost 700 g (Duryea, 2014). he skin is characteristically wrinkled, and fat deposition begins. The head is still comparatively large, and eyebrows and eyelashes are usually recognizable. By 24 weeks, the secretory type II pneumocytes have initiated surfactant secretion (Chap. 32, p. 607). The canalicular period of lung development, during which the bronchi and bronchioles enlarge and alveolar ducts develop, is nearly completed. Despite this, a fetus born at this time will attempt to breathe, but many will die because the terminal sacs, required for gas exchange, have not yet formed. The overall survival rate at 24 weeks is barely above 50 percent, and only approximately 30 percent survive without severe morbidity (Rysavy, 2015). By 26 weeks, the eyes open. Nociceptors are present over all the body, and the neural pain system is developed (Kadic, 2012). The fetal liver and spleen are important sites for hemopoiesis.
The crown-rump length approximates 25 cm, and the fetus weighs about 1100 g. The thin skin is red and covered with vernix caseosa. The pupillary membrane has just disappeared from the eyes. Isolated eye blinking peaks at 28 weeks. The bone marrow becomes the major site of hemopoiesis. he otherwise normal neonate born at this age has a 90-percent chance of survival.without physical or neurological impairment.