The obstetric rule: what it is and what it is used for

Who I am
Catherine Le Nevez
Author and references
Fonte: Alamy

In this article

  • what it is and how the weeks of pregnancy are counted
  • how the obstetric slide works

What it is and how weeks of pregnancy are counted

What is it 

"The obstetric rule is a tool - also known as a pregnancy wheel or wheel - that is used for count the weeks of pregnancy and estimate the estimated date of delivery»Explains, gynecologist of the Pediatric Polyclinic of our city.

It is therefore used to:

  • count the weeks of pregnancy
  • estimate the expected date of delivery

How to count the weeks of pregnancy

Generally, a woman who is expecting a baby is asked: "How many months are you?". But it would be more correct to inquire about the number of weeks of gestation.

Conventionally, in fact, theearly pregnancy it is set for the first day of the woman's last menstruation. This is because it is very difficult to know the precise moment of ovulation and conception.

In all, the duration of pregnancy is fixed at 280 days (around 40 weeks), but a pregnancy is considered to term as early as 38 weeks and can last up to 42, when delivery is usually induced.

How does the obstetric rule work and what is the re-adjustment

How the obstetric slide works

It consists of two overlapping paper wheels: in the lower part there is a calendar with all the days of the year; in the upper one, the number of weeks of pregnancy and two arrows, one at the beginning and one at the end of the numbers.

Read also: Weeks of Pregnancy Calculation

«It is enough to make the first arrow coincide with the day of the last menstruation to have the complete picture of the gestation and the presumed date of delivery. In some models, in correspondence of certain weeks it is also written what are the exams to be done (for example, the morphological ultrasound) ".

Although it is a valid and widely used tool, also because it is convenient to keep in your pocket, it is starting to get a little dated: "We are slowly replacing it with the various dedicated apps available on smartphones".

Read also: Pregnancy: 8 apps to download on your smartphone

The redaction

As mentioned, the calculation of the weeks of pregnancy is not scientifically perfect and this is because it is difficult to estimate the exact moment of conception.

For this reason it may happen that, within the first trimester, the gynecologist makes a reduction of the presumed date of birth. An operation that is done with an ultrasound, measuring the length of the baby from the head to the coccyx (CRL). In fact, certain measures correspond to a specific gestational period.

Updated on 06.03.2022

  • weeks of pregnancy
  • weeks 1-13
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