Week 5: Hormone levels rise

The fifth week of pregnancy, or the third week after conception, the level of the pregnancy hormone human chorionic gonadotropin, also called HCG, quickly rises. That tells the ovaries to stop releasing eggs and make more of the hormones estrogen and progesterone. Higher levels of estrogen and progesterone stop the menstrual period and fuel growth of the placenta.

The embryo is now made of three layers. The top layer, called the ectoderm, will become the outer layer of skin, central and peripheral nervous systems, eyes, and inner ears.

The heart and a primitive circulatory system will form in the middle layer of cells, called the mesoderm. This layer of cells also will serve as the foundation for the bones, ligaments, kidneys and much of the reproductive system.

The inner layer of cells, called the endoderm, is where the lungs and intestines will develop.