Fetal Development Basics & Pregnancy. At 4 weeks, the cells of the fetus are specializing into a nervous system, hair, organs, and skin. At 12 weeks, the fetus is around 2 _ inches in size and makes movement on its own. At 20 weeks, the nervous systems functions, sex can be determined, and you will feel your baby move.At 28 weeks, the fetus weighs over 2 pounds and if born prematurely, would most likely be able to survive. At 32 weeks, the baby weighs 4 pounds and fat layers begin to develop under its skin.our doctor will use your gestational age to track your baby's growth and development and possibly to schedule tests throughout your pregnancy. Normal gestation can range from about 37 to 41 weeks, so estimated due dates are accurate for only about one out of every 20 pregnancies.