The embryonic period comprises the first 8 weeks of pregnancy.
It is divided into a preembryonic phase (from the 1rst to the 3rd week), in which the three germinal layers arise, and into the embryonic phase proper (from the 4th to 8th week), in which the embryonic organ anlagen arise.
Development takes place thanks to the genetic program and environmental factors that are precisely tuned to each other.
During the embryonic period the risk of congenital abnormalities is the greatest. Before, spontaneous miscarriages mostly occur. Later, the frequency of abnormalities and their effects are smaller.
The most important teratogenic factors are infectious diseases, chemical substances, medications, and ionizing radiation.