The embryonic period lasts from the third week after conception through the eighth week.

This period and the early fetal period are the most important time for fetal development. Many birth defects occur during these weeks. Effects of infections, medications, illegal drugs, and alcohol are of critical importance during this time. Control of maternal diabetes is also critical now.

Week Three

Rapid cell division continues and the embryo develops from the embryonic disc into three layers. All tissues and organs will eventually develop from these three layers. During this time, human chorionic gonadotropin (hCG) is being produced. HCG is released by the tissues into the mother's blood and gets filtered into the urine. Pregnancy tests detect a form of hCG, beta-hCG, in the blood or the urine.

The disc further develops with a complex series of folds to create sides to the embryo that will become the head and the feet.

By the end of the third week after conception, or only one week after a menstrual period is missed, the neural tube closes. The neural tube is the precursor to the spinal cord.

Problems with closure of the neural tube lead to neural tube defects, otherwise known as spina bifida. Folic acid supplements help prevent neural tube defects, but they must be taken before the mother finds out that she is pregnant to prevent this birth defect. Also at this time, blood vessels that help carry nutrition to the embryo are formed.

Week Four

During the fourth week, growth is rapid and the embryo triples in size. It also becomes cylindrical due to early formation of the head, heart, and gut. By the end of the fourth week, the limb buds (beginnings of arms and legs) begin to form. The eyes and ears are starting to form. There is even a tail!

Week Five

The brain grows quickly in the fifth week and is much larger than the rest of the embryo. The hands now begin to form. The arms develop a few days earlier than the legs. At this point, the embryo is curled up-in the fetal position.

Weeks Six through Eight

In the sixth week, the major joints of the arm are created.

In the seventh week, the fingers are formed and are webbed. The toes begin to form during the eighth week, and the tail disappears. The head is so large now it makes up 50% of the whole body!

During the latter half of the embryonic period, the embryo is clearly visible by ultrasound, but detail of all the organs is not apparent until the second trimester. However, all of the organs have started developing.

The fetus is now as big as the width of a palm.