​Pregnancy is dated from the day of the last period (menses) of a woman. Ovulation takes place approximately two weeks after the last menses, i.e. in the 3rd week of pregnancy’ to be followed by cleavage and subsequent stages of the embryonic development. During the first two weeks of pregnancy there is no developing embryo, but a cohort of ovarian follicles are growing under the influence of FSH in preparation for ovulation of the oocyte which is going to be fertilized.

 

Fertilization takes place in the third week followed by cleavage, morula formation and blastula formation, and implantation in the same week. Thus, the developmental events that take place in the first week of embryonic development are cleavage, morula formation and blastula formation, and implantation. Implantation takes place 6-12 days after ovulation and accordingly may take place in the second week of embryonic development.