How are the weeks of pregnancy calculated?

How are the weeks of pregnancy calculated?

Published: 07.11.2023
Updated: 06.12.2023

Pregnancy lasts about 40 weeks, i.e. 270 to 280 days (10 lunar months, 9 months), and is divided into trimesters. The first trimester lasts 13 weeks, the second also 13 and the last 14 weeks. The length of pregnancy is calculated from the first day of the last menstrual period, despite the fact that fertilization actually took place approximately 3 weeks after the last period. Therefore, in the first 2 weeks of pregnancy, a woman is not pregnant yet.

The reason why pregnancy is counted from the first day of the last menstrual period is that it is not possible to determine the exact day of fusion of reproductive cells (female egg and male sperm). When a woman finds out with a pregnancy test that she is pregnant, she is approximately 3 to 4 weeks pregnant (fetal age of the fetus). The "baby" has just settled in the uterus and the production of the hCG hormone has begun.

The first day of the last menstrual period is thus considered the first day of pregnancy, and the doctor determines the expected date of birth based on it. It can change according to the following examinations (e.g. CRL measurement of the fetus, i.e. the length of the baby from the crown to the coccyx). You can find out the estimated date yourself - just add 280 days to the first day of your last period. From the moment the egg is fertilized, a normal pregnancy lasts 38 weeks (266 days). The most accurate determination of the week of pregnancy is an ultrasound examination, according to the stage of the child's development.

You can calculate the date of birth and the week of pregnancy according to the date of the last menstruation, from an ultrasound examination, according to the date of conception (if you know it exactly) or according to the first movements of the baby. In the case of calculation from the date of the last menstruation, you calculate the weeks simply - every 7 days is one week of pregnancy. Add 7 days to your last period and subtract 3 months to get the probable due date. If you know the exact date of conception, add 266 days (38 weeks) to this date and get the probable date of birth, or use the calculation: date of conception - 7 days - 3 months + 1 year. The most imprecise is the calculation based on fetal movements. Usually around the 20th week, the mother-to-be notices tickling, bubbling or tingling, which are the baby's movements and based on which the due date can be calculated.

How to calculate weeks of pregnancy trimester

The weeks of pregnancy are recorded in the pregnancy book. Attention, the weeks are written one week behind, so the doctor writes the first week of pregnancy as the zero week of pregnancy. 5tt + 3 days means that it is the fourth day of the 6th week, 12tt + 5 that it is the 6th day of the 13th week of pregnancy, etc.

In connection with the weeks of pregnancy, we can talk about the gestational and fetal age of the fetus, which are 2 different data. The gestational age is calculated in the same way as the weeks of pregnancy in the pregnancy book, i.e. from the date of the last menstrual period. The fetal age of the fetus indicates the actual age of the embryo and is calculated from the date of conception. In case you don't know its exact date, this is just a guide.

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