Fetal development standards at 39 weeks of pregnancy 39 weeks of pregnancy is the third trimester of pregnancy. At this time, the fetus has fully developed, the nutrients supplied by the placenta no longer increase, and the physical burden on the pregnant woman becomes heavier. At this time, the average weight of the fetus is about 3.3 kg and the body length is about 50 cm. The following are standards for fetal development at 39 weeks of pregnancy. The first natural section: Changes in head circumference Fetal head circumference at 39 weeks of pregnancy is approximately