The length of pregnancy: is it really 9 months?

The length of pregnancy: is it really 9 months?

One of the first things a woman learns when she is pregnant is to count time by weeks . He soon discovers that the average gestation lasts a little more than 9 months, exactly 9 months and a week. But why? And above all, why is it important to calculate the length of pregnancy well?

When the pregnancy starts

The reason is that historically the duration of pregnancy was measured in lunar months or menstrual periods, usually 28 days, and therefore the duration was 10 lunar months, that is, 280 days or 40 weeks.

However, in reality, pregnancy does not start just after the start date of the last menstrual period, but generally 14 days later, when the egg that each woman produces in the middle of her menstrual cycle is fertilized by sperm .

From that moment the embryo exists as such. Thus, strictly speaking, the embryo or fetus is two weeks less than the duration of pregnancy, so that the actual time from conception to deliveryis 38 weeks. On a practical level, and despite knowing this difference for many years, for practical reasons we continue to count the pregnancy since the last period.

The importance of "gestational age"

From a medical point of view, it is very important to know the time of pregnancy, which we technically call “gestational age”. The pregnancy is considered to have come to term when the fetus is ready to be born without needing help. In medical terms, pregnancy, and therefore delivery, is "full term" between 37 and 42 weeks of pregnancy. This strip of more than a month gives us an idea of ​​how wide normality is in the duration of pregnancy.

Thus, a delivery can occur between eight and a half months to nine and a half months and we will consider it normal. Below 37 weeks a pregnancy or preterm delivery is considered, while after week 42 there is talk of a prolonged pregnancy. Both variations are relevant.

Premature delivery

When a baby is born less than 37 weeks old, it may have trouble surviving without help beyond the warmth of the mother and breast milk. They are premature babies.

This limit is important because it determines the time of pregnancy in which if labor begins, doctors can recommend measures to try to avoid it, and if it is not possible to stop it, labor should always occur with a trained pediatrician.

Of course, a pregnancy should not be terminated for medical reasons if there is no important reason that justifies it.

Prolonged gestation

When the pregnancy lasts more than 42 weeks , there is a certain risk that the placenta , which is near its final stage of life, will cause breathing and feeding problems in the fetus.

Although the placenta is normally programmed to stop working beyond the normal length of pregnancy, it sometimes "ages" before its time. In general, from 42 weeksthere is a small risk that the placenta will fail, and for this reason it is common to induce labor to avoid risks to the mother and fetus. It is important to remember that the definition of "prolonged pregnancy" is relative according to the characteristics of each pregnancy.

For example, in high-risk pregnancies, it is common to reduce the cutoff time for inducing labor to one to two weeks before 42, that is, 40 or 41 weeks.

What is the most correct way to calculate the duration of a pregnancy

Precisely because the dates are relevant to know if the pregnancy is between 37 and 42 weeks, it is important to know its real duration. The best way to calculate it is the first trimester ultrasound , which has a much higher precision than the date of the last period. Accuracy is higher for two reasons.

First, although it seems difficult to go wrong, the reality is that a proportion of women may have small errors in the date of the last period. The second and most important, because ovulation does not always occur 14 days after the first day of bleeding, and in fact it can range between 7 and 21 days.

These factors combined can sometimes cause a pregnancy to have a real duration of up to 10 days different from what is calculated by the last rule. It has been proven that correcting the duration of pregnancy, and above all, the estimated date of delivery, through ultrasound , avoids wrong decisions.

Thus, the correct thing to do is to correct the estimated date of delivery, even if it is only for one day, since in this way labor inductions, possible unnecessary interventions, inconveniences for mothers and health costs are avoided.

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