Week 4: Our baby this week

The most dramatic and vulnerable period in your baby's development begins this week and continues for the next six. During this time, all of your baby's organs will begin to develop and function. While your baby is in this "embryonic period," she's particularly susceptible to anything that might interfere with her development. The primitive placenta is made up of two layers at this point. Also present now are the amniotic sac, which houses your baby, the amniotic fluid, which cushions her as she grows, and the yolk sac, which produces your baby's red blood cells.