Last menstruation

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Catherine Le Nevez
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We talk about conception we take into consideration the date of last menstruation that preceded a pregnancy because it represents the main information that allows you to date the pregnancy itself.



In this article

  • Last menstruation
  • Why is the onset of pregnancy calculated from the last menstruation?
Read also: the dictionary of pregnancy

Last menstruation

In most cases it is not possible to identify which was the fecundating coitus which started the gestation. With an approximation of two weeks, therefore, the last menstruation is chosen as date of the onset of pregnancy, to which 40 weeks - plus or minus 280 days - must be added to obtain the presumed date of delivery.



Only the first trimester ultrasound can possibly correct this information, if the woman has irregular menstrual cycles: based on the characteristics of the embryo, the ultrasound examination can restore gestation, specifying the date of birth.

Read also: How conception occurs

Why is the onset of pregnancy calculated from the last menstruation?

It is very difficult to trace the exact day of conception, since it is sperm it takes a few days to fertilize the egg and the same fertilized egg takes 6-7 days to nest inside the endometrium, the mucous membrane that covers the inner part of the uterus.



In addition, every menstrual cycle, even moles more regular subjects, can involve variations and temporal changes that delay or anticipate the stages of conception. For this reason, the convention suggests dating the onset of gestation from the first day of the last menstruation.

When is a cycle considered regular?

The "perfect" cycle is the one that repeats itself every 28 days. However, the menstrual cycle is also considered to be regular which, based on the different characteristics of the subject, is repeated with a certain frequency over a period of 21-36 days.

Sources: ISS; MSD

Questions and answers

How to understand when conception took place?

Due to the unpredictable nature of the menstrual cycle, it is conventionally chosen to calculate the onset of pregnancy from the first day of the last menstruation



What is meant by last menstruation?

The last manifestation of the menstrual cycle before termination due to pregnancy.

How long does a standard pregnancy last?

From the day of the last menstruation, about 40 weeks are started (more or less 280 days)

Read also: The calculation of conception: how to do it

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