Human embryology is the science dealing with the development of the embryo and fetus from fertilization to birth. Development begins with fertilization, the process by which the male gamete, the sperm, and the female gamete, the oocyte, unite to give rise to a zygote. The zygote undergoes a series of mitotic divisions, increasing the number of cells; a 16-to-32-cell morula is formed, which is transformed into a blastocyst. The blastocyst has two parts, the inner cell mass or embryo proper, and the outer cell mass or trophoblast (future placenta). Teratology is the science dealing with congenital malformations. Approximately 2 to 3% of all living newborns show at least one recognizable congenital malformation. During a cer­tain critical period during pregnancy, embryos are more susceptible to agents or factors causing abnormal development than at other times. Damage to the embryo during the first 3 weeks of embryogenesis (the early period, before the onset of organogenesis) is unlikely to result in defective development, because it either kills the embryo or is compensated for by the power­ful regulatory properties of the early embryo. The period of maximal susceptibility to abnormal development occurs between weeks 3 and 8, when most of the major organs and bodily regions are first established.