women s health issues
Stages of Development of the Fetus
Development of the Fetus and Placenta
At the end of the 8th week after fertilization (10 weeks of pregnancy), the embryo is considered a fetus. During this stage, the structures that have already formed grow and develop. The following are markers during pregnancy: By 12 weeks of pregnancy: The fetus fills the entire uterus. By about 14 weeks: The sex can be identified. By about 16 to 20 weeks: Typically, the pregnant woman can feel the fetus moving. Women who have been pregnant before typically feel movements about 2 weeks earlier than women who are pregnant for the first time. By about 24 weeks: The fetus has a chance of survival outside the uterus. The lungs continue to mature until near the time of delivery. The brain accumulates new cells throughout pregnancy and the first year of life after birth. As the placenta develops, it extends tiny hairlike projections (villi) into the wall of the uterus. The projections branch and rebranch in a complicated treelike arrangement. This arrangement greatly increases the area of contact between the wall of the uterus and the placenta, so that more nutrients and waste materials can be exchanged. The placenta is fully formed by 18 to 20 weeks but continues to grow throughout pregnancy. At delivery, it weighs about 1 pound. 