During the first trimester, the fertilized egg implants in the uterus and develops major organs. By the 9th week it is called a fetus. In the second trimester, the uterus grows in size and movements from the fetus can be felt around 18-22 weeks. The fetus is about 10 inches long and 1.5 pounds by this time. In the third trimester, the fetus' organs continue maturing and it grows larger, settling head-down in preparation for delivery between 37-42 weeks of pregnancy.