If pregnant women are more accustomed to talking about weeks of pregnancy, the medical profession – attending physician, gynecologist, midwife – evaluates the progress of the pregnancy in weeks of amenorrhea. This difference, which may seem confusing to expectant mothers, is simply the result of a calculation basis that is not the same: ovulation or the absence of periods.
1. The week of pregnancy (SG): definition and method of calculation
If there is one basic premise that everyone agrees on, it is that a pregnancy is measured in weeks. It is indeed a more precise unit of measurement than the month. Usually used by pregnant women, the calculation in weeks of pregnancy takes the expected date of fertilization as a starting point. Pregnancy then lasts 39 weeks from the day of conception. To know the day of fertilization, you must logically have identified your ovulation period. Because without ovulation, fertilization is not possible. Remember that this takes place approximately 14 days after the start of the last period if the menstrual cycle is 28 days. The date of ovulation plus 39 weeks will then give the expected date of delivery.
If health professionals do not use this calculation method, it is quite simply because they do not consider it sufficiently precise. Some women have irregular menstrual cycles, which makes it more difficult to detect ovulation. In addition, the period of fertilization is longer than that of ovulation. It can oscillate between the 7th day and the 17th day of the cycle.
2. The week of amenorrhea (AS): definition and method of calculation
Favored by the gynecologist to establish the date of the start of pregnancy, the week of amenorrhea begins on the first day of the last period. In medical language, the absence of periods is called ” amenorrhea “. This method of calculation is considered to be more reliable than that based on the date of ovulation. The first day of the last period is always 14 days before the ovulatory peak, whatever the length of the cycle, it is therefore necessary to add two additional weeks to the 39 weeks of pregnancy. For health professionals, pregnancy therefore corresponds to 41 weeks of amenorrhea, or 287 days. This difference in calculation method sometimes explains the difference between the conception date determined by the gynecologist and that considered by the parents. It is therefore useful to understand how the weeks of amenorrhea are calculated, because they are most often mentioned during the pregnancy monitoring : blood tests, ultrasound summary, measurements of fetal development, etc. To simplify, here is a mnemonic to move from one method to another: SA = SG + 2 and SG = SA – 2. If the doctor says 31 weeks of amenorrhea, that means 29 weeks of pregnancy.
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