Week 5: Hormone levels increase

The fifth week of pregnancy, or the third week after conception, the levels of HCG hormone produced by the blastocyst quickly increase. This signals your ovaries to stop releasing eggs and produce more estrogen and progesterone. Increased levels of these hormones stop your menstrual period, often the first sign of pregnancy, and fuel the growth of the placenta.

The embryo is now made of three layers. The top layer — the ectoderm — will give rise to your baby's outermost layer of skin, central and peripheral nervous systems, eyes, and inner ears.

Your baby's heart and a primitive circulatory system will form in the middle layer of cells — the mesoderm. This layer of cells will also serve as the foundation for your baby's bones, ligaments, kidneys and much of the reproductive system.

The inner layer of cells — the endoderm — is where your baby's lungs and intestines will develop.