Some people believe that the pregnancy is the amount of time elapsed from the moment of conception. This view is correct, but in this case we are talking about the embryonic period. As a rule, it lasts 38 weeks, but the baby can be born a bit sooner or a bit later.
In modern medicine is considered to be not the embryonic period of pregnancy, and obstetric. Its peculiarity is that it begins to count down, starting from the first day of the last menstrual period. When setting pregnant on account of the doctor specifies in an exchange map this date and focused on her during the calculation period.
Ovulation occurs around mid cycle. That is why obstetric pregnancy about 2 weeks more embryonic. Thus, conception occurs only at 2-3 week obstetric period. In the first week of pregnancy, the egg is just starting to Mature and ready for fertilization.
Usually, pregnancy lasts exactly 40 weeks obstetrics. But the birth of a child at 38-42 week too is considered normal. Formally, the pregnancy may be considered premature or post-Mature, but actually the birth occurs in a term. The reason for this discrepancy is that the calculation of the obstetric period is pretty average. It coincides with embryonic only if conception occurs in the middle of a 28 day cycle before pregnancy was different regularity.
In addition to standard obstetric calculation period on the calendar, the doctor can set it, taking into account the result of ultrasonic diagnostics. This is possible only if the fetus is developing according to the time of pregnancy.
If a woman suspects that conception could not occur in the middle of the cycle, as is usually the case, and shortly before his graduation, or at the very beginning, it should tell your gynecologist. A specialist will be able to adjust their calculations, which will allow him to more accurately determine a preliminary delivery date.