The date of the beginning of pregnancy is the day when the egg was fertilized by the spermatozoon. You may know this day, especially if you have very regular menstrual cycles and you know your date of ovulation. In this case, simply count 266 days from the date of the beginning of pregnancy to know your expected date of birth. If necessary, the doctor can also determine the date of the beginning of pregnancy with enough precision thanks to the evolution of the fetus (visible on the ultrasound).
This is the most commonly used method of calculation because most women are more familiar with their last menstrual day than with fertilization. To calculate the date of birth, the principle is to add 287 days to the date of the beginning of your last period. These 287 days correspond to 41 weeks of amenorrhea.
Specialists estimate that a pregnancy lasts 280 days or 40 weeks. But some studies indicate that a normal pregnancy would actually be a little longer. For a first pregnancy, half of the women give birth after 40 weeks and 5 days, that is to say 285 days. For a second pregnancy, it is 40 weeks and 3 days.
Some women have a longer pregnancy. The same study reveals that a quarter of pregnant women with a first baby give birth after 41 weeks and 2 days of pregnancy. In theory, no woman would exceed 300 days, that is, 42 weeks and 6 days. In fact, the doctor will usually offer the pregnant woman to give birth between 41 and 42 weeks of pregnancy, because the risks of continuing the pregnancy would be greater than the delivery.
To estimate the progress of your pregnancy, there are two main methods: the calculation of the week of pregnancy or the calculation of the week of amenorrhea. Per week of pregnancy, we mean "effective pregnancy week": that is, the calculation starts from the day of fertilization. The weeks of amenorrhea count the number of weeks without menstruation, so since the day of the beginning of your last period.
This difference between the calculation methods results in a lag of two weeks in the result obtained. In total, a "normal" pregnancy of 9 months is equivalent to 39 weeks of pregnancy or 41 weeks of amenorrhea. Practical data to determine your delivery date!
To calculate your week of pregnancy, it is necessary to count the weeks passed since the supposed day of the fertilization. If you have very regular 28-day cycles, your ovulation normally takes place on the 14th day after the start of your period. Your fertility period then extends between the 10th day and the 15th day of the cycle (four days before ovulation and one day after).
You can refine the result depending on the date of your intercourse, but in practice, determining the exact date of fertilization is impossible for most women because they do not know the day of their ovulation. That's why doctors usually prefer to calculate weeks of amenorrhea: just count the weeks since the start date of your last period. This date is much easier to know than your ovulation!
The majority of doctors and midwives use the Naegele rule to calculate the expected date of delivery. Basically, this rule was to add 7 days to the first day of the last menstrual period and then to 9 months.
This was problematic, however, since not all months had the same number of days, which theoretically varied the duration of the pregnancy. For example, for a woman whose last menstruation took place in May, the pregnancy lasted 283 days, while she had only 280 days for a woman whose last menstrual period began in June. Now, professionals will say instead that the expected date of delivery will be 40 weeks after the first day of the last menstrual period.
The doctor or midwife calculates the first 2 weeks after the last menstrual period in the number of weeks of pregnancy, even if the mother was not yet pregnant. Thus, when we talk about "20 weeks of pregnancy", we refer to the 20 complete weeks that have elapsed since the first day of the last menstrual period.
An ultrasound done in early pregnancy would be more accurate than the date of the last menstrual period to determine the expected date of delivery. This is especially true if the ultrasound is performed between 8 and 16 weeks of pregnancy. The technician then measures the height of the baby, from the top of his head to the buttocks. As babies develop very similarly in early pregnancy, it is then possible to make a connection between this measure and the duration of the pregnancy. The accuracy of this measurement may however vary from ultrasound to ultrasound.
If the date calculated using Naegele's rule differs from that obtained by ultrasound by more than 7 days, the physician may choose to change the expected date of delivery to reflect the outcome of the first trimester ultrasound. If the ultrasound is performed between 12 and 20 weeks, the difference should be more than 10 days for the doctor to consider changing the date.