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Once the urine pregnancy test is positive, and the result confirmed by a blood test measuring the hormone HCG, the pregnancy is confirmed. An appointment with the gynecologist then makes it possible to date the pregnancy as precisely as possible, in order to calculate the due date and schedule the various pregnancy medical appointments (ultrasounds, blood tests…).
To estimate the period of pregnancy, two calculation methods coexist: weeks of pregnancy and weeks of amenorrhea. Often summarized SA and SG in pregnancy books, medical reports and forums, mastering these two ways of calculating requires a bit of mental gymnastics, and most importantly a calendar.
A pregnancy lasts on average nine months, i.e. 280 days (+/- 10 days) if we count from the date of last period, and 266 days counting from the date of conception. But to follow the development of the fetus, we prefer the dating of the pregnancy week by week, more precise, since the fetus evolves quickly.
Weeks of pregnancy: the best-known method of calculation
In everyone’s mind, a pregnancy begins when there is fertilization then implantation : the ovum and the spermatozoon meet, and fertilization gives birth to an embryo, which will be implanted in the uterine lining. Ovulation takes place approximately on the 14th day of the menstrual cycle, when it lasts 28 days. Also the method of calculation in weeks of pregnancy begins at ovulation, because little time takes place between ovulation, fertilization and implantation of the embryo.
We count as well 39 weeks pregnant in total.
- How to calculate your week of pregnancy?
To calculate the week of pregnancy (SG), it is necessary to start from the presumed day of ovulation. If this took place on July 18, the delivery date is around April 18 (July 18 + 39 weeks, calculation for a pregnancy in 2018-2019). The 39th week of pregnancy is from April 11 to 18, and the first week of pregnancy is from July 18 to 25.
Weeks of amenorrhea: a more precise calculation method
Since many women have an irregular menstrual cycle, with ovulation not always occurring at the same time from cycle to cycle, another method of dating the pregnancy is used. Rather than starting on the day of presumed ovulation and fertilization, it is based on the first day of the last period, before there has been fertilization, about two weeks before ovulation. We are talking about the method of calculation in weeks of amenorrhea, this last word literally meaning ‘no rules‘. Because, as we know, once the pregnancy begins, there is no longer any period, even if ‘birthday’ periods and other bleeding may occur.
- How to go from calculation in SG to calculation in SA?
To convert her pregnancy date in OS to a date in SA, or weeks of amenorrhea, just add two weeks. It is estimated that a pregnancy has 41 weeks of amenorrhea, or 39 SG.
So if you are 5 SA, you are actually 3 SG.
To sum up mathematically in a simple equation: x SA = y SG + 2, or y SG = x SA – 2.
SA or SG: which is the most used by gynecologists?
Unsurprisingly, the medical profession in need of precision, it is how the pregnancy was dated in weeks of amenorrhea which is preferred by gynecologists, midwives, obstetricians … Better to be comfortable with this method of calculation, because it will often be mentioned, if only on ultrasound or blood test reports. It is also very useful to know your stage of pregnancy, in AS as in SG, to identify the level of the development of the fetus, the symptoms that you might feel or the examinations that will be offered to you.