Pregnancy lasts 40 weeks, or 280 days, from the last day of menstruation. Most women have a menstrual cycle of 28 days. However, the normal range extends from 21 to 35 days in adult women.
Between the 11th and 21st days of each menstrual cycle, the female body undergoes the process of ovulation, which can potentially lead to pregnancy. During ovulation, the egg released by the ovary moves down the fallopian tube towards the uterus. At the same time, the endometrium (mucosal surface) of the uterus is preparing to accept and support a future pregnancy: it thickens and its blood supply increases.
The uterus will remain in such readiness for about 14 more days. If the egg does not meet the sperm, it continues on its way to the uterus alone. In the absence of a fertilized egg, the uterus gets rid of the extra layer of endometrium along with the egg, and menstruation begins.
If the egg and sperm meet and fuse, pregnancy occurs.
Even though the egg has already been fertilized, it will take several more days before a home pregnancy test can show a positive result. This depends on the hormone hCG (human chorionic gonadotropin) produced by the fertilized egg.
The hCG hormone tells the ovary that fertilization has occurred and maintains blood flow to the uterine lining so that the fertilized egg continues to develop.
The presence of the hCG hormone in the urine or blood directly indicates that pregnancy has begun. This can be confirmed at the very initial stage by urine tests (home pregnancy test) and blood tests (from a vein, laboratory test).
After establishing the fact of pregnancy, you yourself, and even more so your doctor, need to determine its duration.