This means that during week 1, you were actually not pregnant yet, but your body was already preparing for this event. Although this week is considered to be the first week of pregnancy, you were actually on your period.
What You’re Seeing: During this week of the first trimester, you can see baby-to-be is developing in a bubble within the gestational sac. The bubble around the embryo is the amniotic cavity filled with amniotic fluid. This liquid environment gives your baby room to grow and develop and to move.
Pregnancy and Fetal Development. Picture of First Trimester (4 Weeks) First Trimester: The Baby at 4 Weeks. Your baby’s brain and spinal cord have begun to form. The heart begins to form. Arm and leg buds appear.