Human pregnancy lasts an average of 36 weeks from the time of conception to birth. The stages of prenatal development are generally classified into two time periods, the embryonic and fetal periods. The embryonic period ends at around week eight, starting the beginning stages of the fetal period. An embryo becomes a fetus when all the major organ precursors are intact and the unborn offspring starts to physically resemble the adult parent. During the second and third trimester, significant changes take place and the fetus grows in weight and size over a hundred times from the start of conception.
Weeks eight to 17
Week eight starts off the transition from embryo to fetus. Major developments in the body start to take place, with organ formation and a more advanced circulatory system. According to the University of Maryland Medical Center, during these weeks the face becomes well formed, the genitals appear well differentiated, tooth buds start to appear, red blood cells start to form in the liver, the baby begins to make active movements and begins to make sucking movements with the mouth. During this time the fetus also forms fine hairs on the head, called lanugo. In early fetal development, mothers may have weekly ultrasounds to monitor new developments.
Weeks 18 to 27
During these weeks the fetus starts to gain considerable size and weight, finishing off week 27 at around 14 inches long and weighing around 2 1/2 pounds. According to the American Pregnancy Association, the fine protective hair, lanugo, now covers the entire body. The skin becomes less transparent and starts to develop melanin, the body's natural skin color. The liver and pancreas are developing rapidly as the fetus starts to form its own independent immune system. In the incidence of early labor, the baby would stand a healthy chance of survival after 26 weeks. During these weeks, the brain develops rapidly and the nervous system starts to engage in the natural sleep-wake cycle. Amniotic fluid starts to form in the lungs during this time frame.
Weeks 28 to 36
Weeks 28 through 36 overlap with with the third and final trimester of prenatal development. During this time, the fetus will triple in weight, finishing off at around 7 to 8 pounds. The baby starts to make significant movements in the womb as muscles strengthen, limbs extend and bones start to harden. MayoClinic.com states that during this time, the baby's eyes start to open, the skin starts to smooth out, hair on the head becomes more course and filled in, the clitoris or testicles become more pronounced, breathing begins and the baby starts to detect light through the mother's stomach. During the final weeks of development before birth, the baby's lanugo will start to disappear, fingernails and toenails will grow, breast buds start to appear on both sexes and baby fat accumulates.
