Physiology of pregnancy

Pregnancy commences with the fertilization of an ovum, and terminates thirty-eight weeks later with the commencement of parturition. Once a month, approximately fourteen days before commencement of menstruation, an ovum is liberated from a mature graafian follicle. Fertilization usually occurs just before or soon after the ovum enters a fallopian tube. The zygote (fertilized ovum) takes approximately four days to pass down the tube, and on reaching the uterus it has divided several times to form the blastocyst. The blastocyst remains free in the uterus a further two to three days before being embedded in the endometrium (endothelial lining of the uterus). After ovulation the graafian follicle develops into the corpus luteum, a specialized endocrine structure.