At 12 weeks, a fetus may not be big enough to have mommy-to-be in maternity clothes, but the growth and development that occur during the first trimester is the most dramatic of the entire pregnancy, according to the University of Virginia Health System. By the end of 12 weeks, a fetus has all major organs and systems in place, complete with arms, legs, genitals and a beating heart.
A healthy 12-week-old fetus is approximately 3 to 3.5 inches long and weighs about 1 oz., according to the National Women's Health Information Center. The head comprises almost half of the baby's size.
A fetus begins to develop a more normal appearance during week 12. The ears transition nearer to final position, according to the American Pregnancy Association, and the eyes move closer together. Fingernails also begin to develop about this time, and muscles and bones start to grow and develop.
The arms of the fetus are longer than the legs, and the hands are more prominent than the feet. Twenty buds that represent future teeth appear during the 12th week, and all internal parts have now formed, although they are not fully developed.
The genitals of a fetus are developed by week 12, but the sex usually can't be detected by ultrasound until later in the second trimester. The kidneys begin to function during this week, and the fetus can now make a fist and suck his thumb.
Babies develop at different rates, and the American Pregnancy Association advises that information about developmental stages of a fetus should serve only as a general guide.