At 4 weeks of pregnancy, your baby is still in the very earliest stage of development and is called an embryo. Around this time, the fertilized egg has successfully implanted into the lining of the uterus, which is essential for the pregnancy to continue. The embryo is incredibly tiny—about the size of a poppy seed—but major events are already unfolding. Cells are rapidly dividing and organizing into layers that will later form all the baby’s organs and tissues. The amniotic sac and yolk sac beg