gynecology and obstetrics
Conception and Prenatal Development
Embryo
Around day 10, 3 germ layers (ectoderm, mesoderm, endoderm) are usually distinct in the embryo. Then the primitive streak, which becomes the neural tube, begins to develop. Around day 16, the cephalad portion of the mesoderm thickens, forming a central channel that develops into the heart and great vessels. The heart begins to pump plasma around day 20, and on the next day, fetal red blood cells (RBCs), which are immature and nucleated, appear. Fetal RBCs are soon replaced by mature RBCs, and blood vessels develop throughout the embryo. Eventually, the umbilical artery and vein develop, connecting the embryonic vessels with the placenta. Most organs form between 21 and 57 days after fertilization (between 5 and 10 weeks gestation); however, the central nervous system continues to develop throughout pregnancy. Susceptibility to malformations induced by teratogens is highest when organs are forming. 