A woman's pregnancy begins with the implantation of the fertilized egg in her uterus and, under normal conditions, ends about nine months later with the birth of a baby. However, for certain purposes of medical calculation the beginning of pregnancy is not dated from the time of implantation, but from the first day of the last menstruation. (The reason for this is that a woman is not aware of an implantation, but usually can recall the date of her last menstruation.) According to this timetable, the woman will give birth 280 days (40 weeks) later. Using the same basis for calculation, the date of delivery can also be predicted by this rule of thumb: "Months minus 3, days plus 7." For example: First day of last menstruation: December 10, or, in numerals, 12/10.
"Months minus 3": 12-3 = 9
"Days plus 7": 10+7=17
Date of delivery: 9/17 or September 17.
Before a woman can deliver a healthy baby, a fascinating and extremely complex process of growth has to take place within her body. Even today, the exact mechanisms of this process are not completely understood. Nevertheless, its main phases can be described. The following paragraphs summarize how the fertilized egg develops through various stages into a viable and independent human being and how this development is experienced by the pregnant woman.