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The obstetric slide rule is a tool used by obstetricians and gynecologists to date pregnancy and estimate the most likely day of delivery.

When using the obstetric ruler, the gynecologist takes the beginning of the last menstruation as a reference and takes into account an average gestational duration of 280 days, that is 40 weeks.

The result that is obtained provides a dating of the week of pregnancy in which the woman is and, consequently, establishes the day near which the birth of the child could occur.

To date the pregnancy, doctors usually refer to the day of the beginning of the last menstruation. If gestation proceeds normally, without complications, it has an average duration of 280 days, equal to 40 weeks starting from the day taken as reference. However, a physiological pregnancy can end peacefully even between 37 and 42 weeks.

If your menstrual cycles are 28-30 days and the pregnancy goes smoothly, delivery can occur at any time, between two weeks before and two weeks after the expected date. For a better approximation of the calculation obtainable with the obstetric slide, however, it is possible to take into account some individual factors (such as the date of the presumed ovulation or the fertilizing sexual intercourse).

The information provided by the obstetric ruler is part of the data collected in the anamnesis by the gynecologist and therefore contributes to monitoring the fetal growth parameters, which will then be supported by laboratory analyzes, ultrasound investigations and other medical assessments.

The obstetric ruler is an instrument made up of two numbered and superimposed cardboard discs. The wheels – one lower and one upper – are joined in the center by an eyelet, which allows them to be able to turn independently clockwise and counterclockwise.

The presumed date of delivery is calculated by adding nine months and one week (280 days, in fact) to the first day of the last menstruation. Based on this information, the doctor turns the lower wheel of the obstetric slide, confirming or not the calculation of the weeks of pregnancy made by the pregnant woman or by the couple.

In the obstetric slide rule, the expected date of delivery is indicated by an arrow. Based on this result, the gynecologist plans together with the pregnant woman the calendar with the various ultrasound scans and checkups to monitor the pregnancy.

The obstetric slide rule is also known as the pregnancy wheel, gestogram, gestational disc and P-Ruler (short for the English term “Pregnancy Ruler”). In common jargon, at times, you can hear about “tumbles” or “wheel” obstetrics.

The use of the obstetric slide is an important reference for establishing the gestational age at a given time.

In other words, this tool allows you to calculate the times during pregnancy and, considering that each gestation period corresponds to a specific phase of growth of the fetus, the obstetric slide is useful for:

The obstetric ruler is also useful to indicate the ideal care plan for the expectant mother: a premature baby may need more care, just as a gestation that lasts beyond 42 weeks makes it necessary to induce labor.

By convention, the calculation of the weeks of pregnancy takes as reference the first day of the last menstruation: considering that a “standard” gestation lasts 280 days, it is necessary to add seven days and subtract three months from this date (Naegele’s rule).

In practice, the obstetric slide is used by the gynecologist by entering the date corresponding to the day on which the last menstrual flow appeared. The presumed time of full-term birth is thus obtained.

Naegele’s rule is based on menstrual cycles of 28 days, with an assumed ovulation on the 14th day.

The duration of pregnancy is calculated in complete weeks, starting from the first day of the last menstrual flow: for this reason, it is important to know this date with certainty.