The obstetric ruler or pregnancy wheel: what is it and how does it work?

What is the obstetric ruler?

The obstetric ruler  is a cardboard or plastic disc that allows you to date the pregnancy and  establish the expected day of delivery .

The calculation is made starting from the first day of the last menstrual cycle and considering a pregnancy duration of 40 weeks (ie 280 days).

How is the obstetric ruler made?

This instrument consists of two superimposed disks , one larger and one smaller, capable of sliding over each other. For this reason, it also takes the name of gestogram , gestational disc , pregnancy ruler (abbreviated P-ruler ) or, more commonly, tumble  or  pregnancy wheel .

The months and days of the solar year are indicated on the larger disc, which is located on the outside . The 40 weeks of pregnancy (divided into three trimesters ) and two arrows are shown on the smaller disc, placed inside : the first to indicate the first day of the last menstrual period and the second for the presumed date of delivery .

Calculation of weeks of pregnancy with the obstetric ruler: how does it work?

The ruler for pregnancy is used by rotating the internal disc, so as to position the first arrow on the day of the month corresponding to the beginning of the last menstrual cycle . This way, the second arrow will automatically position itself on the due date .

In addition, the internal disc of the obstetric ruler will indicate:

• probable date of conception;
• the weeks of pregnancy (calculated as the gestational age of the fetus, i.e. weeks + complete days from the start of the last period);
• the main tests to be performed for each specific gestational period.

Some rulers also show the reference fetal measurements for each week on the back, such as:

• length and weight of the fetus;
• biparietal diameter or BPD (the distance between the two parietal bones of the skull);
• femur length.