In theory, a pregnancy lasts nine months. In reality, however, the normal duration of a pregnancy is a complex concept and varies from one woman to another. When do we talk of a full-term pregnancy? Of premature pregnancy? And of late term? Let’s have a look at the duration of pregnancy.
The duration of pregnancy is highly variable, partly because the date of the beginning of the pregnancy is uncertain, and partly because scientists strive to accurately estimate the average duration of pregnancy in humans. Pregnancy duration is influenced by individual factors, such as the age of the pregnant mother and previous miscarriages.
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Duration of pregnancy: not so precise
The term “normal gestational age” is controversial among scientists. It depends on how it is calculated – it varies from country to country – but it also depends on genetic factors and certain characteristics of the mother.
According to studies, the duration of pregnancy varies from 280 to 290 days from the first day of the last menstrual period (in a regular 28-day cycle), or from 40 weeks of amenorrhea to 41 weeks + 3 days of amenorrhea.
A full-term pregnancy
An information to remember for pregnant women: The duration of a full-term pregnancy is the delivery between 37 weeks of amenorrhea and 41 weeks + 5 days of amenorrhea.
Two things are certain about the duration of pregnancy: the older the woman, the longer the pregnancy, and the duration of subsequent pregnancies are usually as long as the first.
Properly determining the onset of pregnancy
Another important concept is that if you want to know when a pregnancy will end, you must know when it began.
There are two ways to estimate the onset of pregnancy. First, the woman can remember the date of her last menstrual period, or second, the craniocaudal length measured during the first-trimester ultrasound can be used. This measurement provides more accurate information than the time of her last menstrual period, but it is still inaccurate, as the onset of pregnancy may be 5 days more or 5 days less.
It is therefore necessary to have a perspective on the potential date of the end of the pregnancy!
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Pregnancy too short: premature delivery
If the pregnancy lasts less than 37 weeks of amenorrhea, it is called preterm labor, starting from the 25th week. Before the 25th week, the baby is no longer viable, which is called a late miscarriage. Late miscarriages are much rarer than premature miscarriages, which occur before the 15th week of amenorrhea.
Three levels of prematurity have been defined:
- Extremely preterm (less than 28 weeks)
- Very preterm (28 to 32 weeks)
- Moderate to late preterm (32 to 37 weeks)
Prolonged and late pregnancy
We speak of prolonged pregnancy if amenorrhea lasts over 41 weeks, and of late pregnancy, if amenorrhea lasts over 42 weeks.
One week before the due date, which cannot be exceeded, monitoring is set up because there can be complications because of placental senescence (aging of the placenta). If the pregnancy is prolonged, the fetus is closely monitored to exclude the possibility of oligohydramnios (lack of amniotic fluid) or fetal arrhythmias. These measures aim to prevent intrauterine mortality.
After 41 weeks of amenorrhea, the pregnancy is monitored by ultrasound and/or observation, usually every 2-3 days. If any abnormality is detected, labor is induced. Many women give birth naturally during this observation week. At the 42nd week of amenorrhea, even if everything is going well, labor is usually induced.
Most women give birth between 38 and 42 weeks, with an even distribution: 20% at 38 weeks, 20% at 39 weeks, 20% at 40 weeks, 20% at 41 weeks, and 20% at 42 weeks.
