The age of an embryo or fetus can be measured from both the fertilization date, and the date of the last menstrual period. Usually in the embryonic period, the fertilization date is used, while, in the fetal period and in obstetrical practice, the date of the last menstruation is used.

The fusion of the male sperm and female oocyte leads to fertilization and formation of the zygote (or fertilized embryo). As the zygote traverses the maternal tubal and uterine cavity, the initial stages of embryonic development have been completed. During the first week, the first zygote's (mitotic) divisions produces a series of blastomeres and, because of further cellular divisions (cleavage), the so called morula. A morula contains about 10-30 cells. The morula stage is the final stage prior to formation of a fluid-filled cavity called the blastocele cavity. Once the cavitation is...