Pregnancy Weekly Guide
[Week 4 Baby] The embryo is still as tiny as a powder grain.
The embryo is very tiny, measuring just under 0.1mm. And there is a tinier heart beating inside.
Once the embryo is situated in the uterus, there will be a "yolk," which supplies nutrition to the embryo during the initial stage of pregnancy, and an "amniotic sac," which will protect the baby until the delivery. While the baby is still very small (0.2mm), organs such as the mouth, spinal cord, heart, and arteries are forming quickly.
The heart begins to beat as early as week 4. During this period of rapid growth, the fetus is very susceptible to toxic substances that can cause birth defects -- double-check before you take any medication!