After getting a positive pregnancy test result, the question that naturally arises is: what is the due date? Establishing a precise date, however, is more complicated than you might think. Here is how the due date is calculated.
How long is a pregnancy? In general, specialists estimate that pregnancy lasts 280 days or 40 weeks.
However, some studies now indicate that a normal pregnancy is actually a little longer. For a first pregnancy, about half of women would give birth after 40 weeks and 5 days, or 285 days. In the case of a second pregnancy, we are talking about 40 weeks and 3 days.
Some women can also have a longer pregnancy. The same study reveals that a quarter of women pregnant with a first baby gives birth after 41 weeks and 2 days of pregnancy. In theory, however, no woman would exceed 300 days, that is, 42 weeks and 6 days.
In fact, the doctor will generally offer the pregnant woman to induce labor between 41 and 42 weeks of pregnancy, because the risks of continuing the pregnancy would then be greater than those of inducing labor.
In general, health professionals speak of pregnancy in the number of weeks, not months, since this is more precise. Pregnancy weeks are also divided into 3 trimesters:
1: from fertilization to 14 weeks
2: from 15 weeks to 28 weeks
3: from 29 weeks to birth
How to calculate the due date?
During pregnancy monitoring, the healthcare professional will try to determine the expected date of delivery. It is generally estimated that it will be 40 weeks, or 280 days, after the first day of the last period.
The doctor or midwife therefore calculates the first 2 weeks following the last menstruation as the number of weeks of pregnancy, even if the mother was not yet pregnant. So, when we talk about “20 weeks pregnant,” we mean the full 20 weeks that have passed since the first day of the last menstrual period.
Misinterpretation of Naegele’s rule?
Before, most doctors and midwives used Naegele’s rule to calculate the due date. It was stated by a Dutch physician in 1744.
In its original form, this rule mentions adding 7 days to the last period and then counting 9 months. However, it does not specify whether to start on the first or the last day. The rule was therefore interpreted differently from doctor to doctor until the turn of the 20th century. This is when the American medical books chose to add 7 days to the first day of the last period.
Also, counting 9 months is problematic since the months do not all have the same number of days, which in theory varies the length of pregnancy. For example, for a woman whose last period was in May, the pregnancy would last 283 days, while she would have only 280 days for a woman whose last period started in June.
Is the due date reliable?
Although this way of calculating the due date has been used for over 200 years, it is criticized by many health professionals.
This rule is based on the idea that a menstrual cycle lasts 28 days. Yet many women have irregular cycles. In fact, 30% of women say they have a cycle of at least 30 days.
The rule assumes that ovulation occurs on exactly day 14 of a 28-day cycle, which most women do not. In fact, in the general population, ovulation tends to occur after the middle of the menstrual cycle. In women who have a 28-day cycle, only 10% ovulate on day 14 and only 30% of these women are fertile on day 10 to 17 of the cycle.
Many women do not remember the date of their last period. According to a 1985 study, 79% of women are sure when their last period was, that is, they can tell what week it was.
The rule does not take into account that some embryos may take longer to implant in the lining of the uterus, which lengthens the pregnancy.
If the last bleeding is light, it may not be the last period, but rather breakthrough bleeding or bleeding caused by the embryo implanting in the uterus.
It is difficult to determine the date of conception based on the history of the cycle since the number of waves of hormones leading to the release of the egg varies from woman to woman. In addition, sperm can remain in the female genital tract for 5 to 7 days.