Lasting from the ninth week until the end of pregnancy, the period of the fetus is the "growth and finishing" phase. During this longest prenatal period, the organism increases rapidly in size, especially from the ninth to the twentieth week (Moore & Persaud, 1998).
Figure 1: Fetal Development from 8 weeks til 40 weeks. The Third Month:
In the third month, the organs, muscles, and nervous system start to become organized and connected. The brain signals, and in response, the fetus kicks, bends its arms, forms a fist, curls its toes, opens its mouth, and even sucks its thumb. The tiny lungs begin to expand and contract in an early rehearsal of breathing movements. By the twelfth week, the external genitals are well formed, and the sex of the fetus can be detected with ultrasound. Other finishing touches appear, such as fingernails, toenails, tooth buds, and eyelids that open and close. The hearbeat is now stronger and can be heard through a stethoscope (Berk, 2003).Prenatal development is often divided into trimesters, or three equal time periods. At the end of the third month, the first trimester is complete.
The Second Trimester:
By the middle of the second trimester, between 17 and 20 weeks, the new being has grown large enough that the mother can feel its movements. If we could look inside the uterus at this time, we would find the fetus completely covered with a white, cheeselike substance called the vernix. It protects the skin from chapping during the long months spent in the amniotic fluid. A white, downy hair covering called lanugo also appears over the entire body, helping the vernix stick to the skin (Berk, 2003). At the end of the second trimester, many organs are quite well developed. And a major milestone is reached in brain development, in that most neurons are in place; few will be produced after this time. However, glial cells, which support and feed the neurons, continue to increase at a rapid rate throughout the remaining months of pregnancy, as well as after birth (Nowakowski, 1987).
Brain growth means new behavioural capacities. The 20-week-old fetus can be stimulated as well as irritated by sounds. And if a doctor has reason to look inside the uterus with fetoscopy, fetuses try to shield their eyes from the light with their hands, indicating that sight has begun to emerge (Nilsson & Hamberger, 1990). Still, a fetus born at this time cannot survive. Its lungs are too immature, and the brain cannot yet control breathing and body temperature (Berk, 2003).
The Third Trimester:
During the final trimester, a fetus born early has a chance for survival. The point at which the baby can first survive is called the age of viability. It occurs sometime between 22 and 26 weeks (Moore & Persaud, 1998). If born between the seventh and eighth months, a baby would still have trouble breathing , and oxygen assistance would be necessary. Although the respiratory centre of the brain is now mature, tiny air sacs in the lungs are not yet ready to inflate and exchange carbon dioxide for oxygen (Berk, 2003).
The brain continues to make great strides during the last 3 months. The cerebral cortex, the seat of human intelligence, enlarges. As neurological organization improves, the fetus spends more time awake. At 20 weeks, heart rate variability reveals no periods of alertness. But by 28 weeks, fetuses are awake about 11 percent of the time, a figure that rises to 16 percent just before birth (DiPietro et al, 1996).
Figure 2: Image of a third trimester baby
The third trimester brings greater responsiveness to external stimulation. Around 24 weeks, fetuses can first feel pain, so after this time painkillers should be used in any surgical procedures (Royal College of Obstetricians and Gynecologists, 1997). By 25 weeks, fetuses react to nearby sounds with body movements and in the last weeks of pregnancy, they learn to prefer the tone and rhythm of their mother's voice (Berk, 2003).
During the final 3 months, the fetus gains more than 2300 grams (5 pounds) and grows 18 centimetres (7 inches). As it fills the uterus, it gradually moves less often. In addition, brain development, which permits the organism to inhibit behaviour, may contribute to a decline in physical activity (DiPietro et al, 1996).
In the eighth month, a layer of fat is added under the skin to assist with temperature regulation. The fetus also receives antibodies from the mother's blood to protect against illnesses, since the newborn's own immune system will not work well until several months after birth. In the last weeks, most fetuses assume an upside-down position, partly because of the shape of the uterus and because of gravity: the head is heavier than the feet. Growth slows, and birth is about to take place (Berk, 2003).
