Week of amenorrhea, week of pregnancy: what's the difference?


Your pregnancy is confirmed, the calculations to know the term and the theoretical date of conception of your baby begin to be done in your head and in those of the health professionals who follow you. We are talking about weeks of amenorrhea and weeks of pregnancy. But what is it exactly? We will explain everything to you.

A pregnancy lasts nine months, but it is rather counted in weeks because this method of calculation is more precise to determine the principal stages of development of the fetus and to schedule the medical examinations to be envisaged. We speak of either weeks of pregnancy (SG) or weeks of amenorrhea (SA).

When does pregnancy begin?

For there to be a pregnancy, there must be fertilization, which is the meeting between a sperm and an egg. This takes place during the woman's ovulation period, when the egg is expelled from one of the two ovaries and travels down to the uterus. Ovulation lasts between 12 and 24 hours. If, on its way, the egg is fertilized by a sperm, then an embryo is formed which is the origin of the future baby. The day of conception of the baby is considered to be the first day of pregnancy.


Weeks of pregnancy: what are we talking about?

The day the baby was conceived is the first day of the first week of pregnancy. In total, a normal pregnancy (excluding premature births) takes place over 39 weeks from the day of conception. The calculation in weeks of pregnancy (SG) therefore starts at the time of the woman's ovulation, i.e. around the 14th day of the menstrual cycle (the first day of the menstrual cycle begins with the first day of menstruation), if the cycle lasts 28 days. It is believed that ovulation, fertilization and implantation of the embryo (when the embryo enters the uterus) follow each other rapidly in time.

Example of a calculation in weeks of pregnancy (SG)
Concretely, a pregnant woman who estimates the presumed day of her ovulation as August 15 should normally give birth 39 weeks later, that is to say around May 15. Since the baby was conceived on August 15, the first week of pregnancy is from August 15 to 22 and the last will be from May 8 to 15 (for a calculation in 2020/2021).

Weeks of amenorrhea: what are we talking about?

The word “amenorrhea” means “no periods”. When we talk about weeks of amenorrhea in a pregnant woman, we mean the time during which she has stopped having her period, one of the first signs of pregnancy.
Not all women have regular 28-day cycles. Some have longer cycles, others shorter … Others do not know exactly when in the cycle they are ovulating. This is why doctors and midwives prefer to use the date of the first day of the last period to date the pregnancy rather than the day of conception (so theoretically the day of ovulation). Regardless of the date of the presumed ovulation, it usually occurs 14 days after the first day of your period. Once the pregnancy is confirmed by a blood test, the doctor or midwife who follows the pregnant woman will determine the start of her pregnancy based on the first day of her last period. Theoretically, the conception of the baby took place 14 days after this date. The first week of amenorrhea therefore begins on the first day of the last period, two weeks before the date of conception. According to the National College of French Gynecologists and Obstetricians (CNGOF), although the length of gestation varies from woman to woman, it lasts an average of 41 weeks.

Example of a calculation in weeks of amenorrhea (SA)
In a pregnant woman for whom the last period started on August 1 (and whose cycle of 28 days is regular), the onset of pregnancy is August 14. The first week of amenorrhea is August 1 to 7 and the last will be May 7 to 14 (for a calculation in 2020/2021), with a delivery scheduled for May 14.

It's simple, to convert a pregnancy date in OS to a pregnancy date in SA, you have to add two weeks because it is estimated that a pregnancy lasts 41 weeks of amenorrhea, so 39 weeks of pregnancy. For example, if you are in your 20th week of pregnancy, you are in your 22nd week of amenorrhea, or 5 months of pregnancy.

How to determine as precisely as possible the onset of pregnancy (GD)?

The date of onset of pregnancy is determined during the dating ultrasound, the first pregnancy ultrasound. This takes place during the first trimester, between 11 and 13 weeks of amenorrhea (or between 9 and 11 weeks of pregnancy).
The health professional who performs the exam assesses the gestational age of the fetus and can date the pregnancy more precisely than the calculation based on the date of the start of the last period requested of the mother-to-be. To determine the gestational age, the practitioner measures the length between the head and the buttocks of the embryo and compares the result to a reference curve. It is thanks to this that he will date the onset of pregnancy (GD) officially on the ultrasound report, and can mention the number of SA and the number of OS.

There you are, now the DG, SA and SG mentions have no secrets for you. You can include them without fuss on your calendar or your diary!

To read also on aufeminin
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Pregnancy examinations: what follow-up for pregnant women?
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Video by Catherine nowak