Duration of Pregnancy

Normal Duration

Pregnancy - how long does it last?

The duration of pregnancy is conventionally established as forty weeks. This time frame is however approximate, so much so that the births that took place between the thirty-seventh and the forty-first week are still normal.

Conversely, when the duration of pregnancy is less than 37 (complete) weeks, it is referred to as premature or preterm birth; similarly, births occurring from the forty-second week onwards are considered post-term or post-mature.

How to calculate it?

How to calculate the duration of pregnancy

The duration of a "standard" pregnancy is therefore 280 days (or 10 lunar months), but how to calculate it?

If the count starts from the first day from the last menstruation, the average duration is just 280 days; this relationship is all the more truthful the more regular are the menstrual cycles (which normally last 28 days), and vice versa.

Always assuming a certain regularity of the cycles, the duration of pregnancy can be calculated starting from the presumed time of ovulation, bearing in mind that this always occurs 14 days before the last flow and, on average, 14 days after the start of the last menstruation ( in the latter case the variability is greater and depends on the regularity of the cycle). This date roughly coincides with that of fertilization, because we know that the mature egg cell - after its release - can survive for a maximum of 24-48 hours, after which - if not fertilized - it falls apart. Therefore, if calculated from the day of the presumed fertilization, the average duration of pregnancy is 265 days.

Obviously, it is possible to know with certainty the exact day of fertilization in the event that a single and isolated sexual relationship has taken place after the last menstruation and that this has been followed by amenorrhea (absence of menstrual flow).
In technical terms, we speak of gestational age when the duration of pregnancy is calculated from the first day of the last menstruation, and of conceptional age when the count coincides with the moment of presumed fertilization.

Finally, let us underline Naegele's rule, which calculates the presumed date of birth with a good approximation on the basis of a simple mathematical operation:

Start date of last menstruation plus one year plus seven days minus three months.

In some cases, for example, due to the strong menstrual irregularities of the woman or because the fertilization took place during a period of physiological, pathological or iatrogenic amenorrhea, the duration of the pregnancy is rather difficult to calculate. The gestational age is then calculated with an excellent approximation by means of an ultrasound check.

A human pregnancy lasts on average 280 days, that is 9 months and 10 days, that is 40 weeks, counting from the first day of the last menstruation.

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