Anatomy: Fetus in Utero A thin-walled sac surrounds the fetus during pregnancy. The sac is filled with amniotic fluid, which protects the fetus from injury. First Trimester A healthy first trimester is crucial to the normal development of your baby. You may not be showing much on the outside, but inside, all the major body organs and systems of the fetus are forming. Second Trimester During this trimester, the weight of your growing baby will multiply more than seven times—and you will start showing more. Third Trimester The third trimester marks the home stretch for your pregnancy. You may feel more uncomfortable now as you continue to gain weight. You also may have false labor contractions (called Braxton-Hicks contractions). Overview of Multiple Pregnancy Multiple pregnancy is a pregnancy with two or more fetuses. In the United States, the multiple birth rate is rising. Common Tests During Pregnancy Certain tests and screenings may be scheduled during your pregnancy. Your health care provider will advise you on which tests you'll need.