Fetus
From ChildWiki.com
An unborn baby is referred to as a fetus after the first 8 weeks of gestation. Before this time, it is referred to as an embryo. The second trimester begins after 12 weeks, about one month into the fetal period. Once the baby reaches the fetal period, it is considered a fetus until the time of birth. The fetal state and the embryonic stage describe the total of prenatal development
By the start of the fetal period the beginnings of most of the major organs have already grown in. That means that the fetus is in much less danger of damage than the embryo. Once in the fetal stage exposure to toxins is more likely to create birth defects than death.
The fetus develops very quickly at the beginning, and reaches important milestones in its development for the entire gestation. The following description of the timing of the growth of a fetus is approximate, and can vary. In week 9-12 the eyelids of the fetus close, and they stay closed until around week 28. Also in this period genitals can be differentiated, and the baby can make a fist.
In weeks 13-16 the skin is still nearly transparent. Hair, called lanugo, develops on the head of the baby. This is the period where the baby first begins to be active and move. In weeks 17-19 the baby can hear and make movements strong enough the mother can occasionally feel them. In week 20 eyebrows and eyelashes develop, as well as fingernails and toenails. At this point the fetal heartbeat can be heard with a stethoscope.
In weeks 21-23 bone marrow begins to make blood cells, the lower airways begin to develop, and the baby begins to store fat. By week 24 all the pieces of the eye are formed, and the baby has developed hand reflexes and the startle reflex. Footprints and fingerprints begin to form at this stage. Weeks 25-28 involve an increased development of the brain, and the nervous system has control of some body functions. The respiratory system has developed enough that gas exchange is possible.
In weeks 29-32 breathing movements occur, even though the lungs are not fully developed. The baby will store more fat, and begins to store iron, calcium and phosphorus as well. By week 36 the baby's nails will reach the end of the fingertips. In weeks 47-40 the nails extend beyond the fingertips, head hair is coarse, and small breast buds are now present.
