Scientists have been baffled for centuries about how the zygote, a single cell, becomes transformed into an intricately complex organism with many different types of cells, tissues and organs. Birth defects have been very crucial in providing at least some answers regarding human development and function.

Following fertilization in the upper part of the fallopian tube, the zygote continues to divide; the mass of dividing cells is now called the blastocyst. After floating free for about two days, the blastocyst attaches itself to and begins to burrow into the spongy lining of the uterus. The embryo is now dependent on the mother's blood supply for its nourishment and development. The implantation into the uterus is complete by the end of the second week.



By the third week, the preliminary neural tissue continues to develop into the neural tube that gives rise to the brain and spinal cord during later stages of development. The cardiovascular system is one of the first functional systems to be developed. It links the embryo with specialized tissue, the placenta that provides a channel for exchange of nutrients and waste materials between the mother and the embryo. In addition, it produces hormones that maintain pregnancy and prevent menstruation.