Obstetric rule: what it is, what it is for and how it works
The obstetric rule is a useful and particular tool that, through approximate calculations, is able to tell what the presumed date of birth will be for a pregnant woman. The gynecologist starts from the date relative to the beginning of the last menstruation and considers a gestation of about 280 days, or the classic 40 weeks.
Before delving into the topic, we would like to point out this short video on the topic: how big is a child month by month?
What "is and how" is the obstetric slide made?
To date the pregnancy, we generally refer to the date of the beginning of the last menstruation and from here we consider about 40 weeks before arriving at the birth. If there are no complications and gestation goes well (some pregnancies end earlier, and others even reach 42 weeks), after about 280 days, you will be able to hold your baby in your arms.
What does the obstetric slide do specifically? It lets you know two fundamental things:
- What period of pregnancy are the expectant mother and her baby in
- Calculate the estimated date of delivery.
All the information collected through the obstetric slide is part of the data collected during the anamnesis to monitor the fetal growth parameters.
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To understand how an obstetric ruler is made, we must consider two cardboard discs overlapped and joined in the center by a pin that allows them to rotate independently, clockwise or counterclockwise.
- The lower disk is larger than the upper one and is divided into 12 sections, one for each month of the year; each month in turn is divided into days.
- The upper (smaller) disc, on the other hand, reports the 40 weeks of pregnancy divided into 3 trimesters. This part of the obstetric slide can also report useful information, such as the diagnostic tests expected during pregnancy or the stages of fetal development.
The doctor, by rotating the two discs, will be able to arrive at an presumed date of delivery which, as we have seen, is around the 40th week of gestation. Depending on the result, the gynecologist will plan together with the expectant mother the calendar with the various ultrasound scans and checkups to monitor the entire development of the fetus.
Obstetric rule: what is it for?
The obstetric slide rule is an important tool above all to establish the gestational age and therefore to calculate the times during pregnancy through all the stages of the fetus' growth. Specifically:
- Monitor the development of the baby inside the uterus.
- Establish the probable date of conception and the expected day of delivery.
- Schedule medical checkups, ultrasounds, blood tests, for the expectant mother, so that she avoids forgetting them.
The obstetric ruler is also useful to indicate the ideal care plan for the expectant mother: a premature baby may need more care, as well as a gestation that lasts beyond 42 weeks requires an "induction of labor.
How to use the obstetric slide rule in the correct way
The main information on which to base the calculation made with the obstetric slide rule are essentially two:
- Start day of the last menstruation
It is important to know exactly this date in order to have a precise calculation.
- "Standard" duration of pregnancy
The duration of pregnancy is conventionally established at 40 weeks (i.e. 9 months plus 7 days). However, this time frame is approximate, some deliveries occur in advance while others postpone by a few days.
To date the pregnancy, during the anamnesis, the gynecologist applies Naegele's rule:
- 9 months and 7 days are added to the first day of the last menstruation.
In reality, with the obstetric slide this calculation is "automatic" and very simple, the following will also be indicated:
- the probable date of conception
- the day scheduled for delivery
- deadlines not to be forgotten
- the various control visits
It must always be borne in mind that the calculation of the obstetric rule is indicative. What can be done to be more precise? These other data must also be considered:
- Date of presumed ovulation
It is not easy to establish the date of ovulation, it is generally assumed that it is 14 days after the start of the last menstruation, but in reality every woman is different and the timing of ovulation will be different.
- Date of presumed conception
The exact day of conception is a good starting point for calculating the presumed date of birth with the obstetric ruler. We know that it is not at all easy to remember this information, but if you are trying to have a baby, it is a fact that you may score from time to time.
How accurate is the calculation made with the obstetric slide?
For all women who have a regular menstrual cycle, the obstetric rule calculation can be considered very reliable. This method will provide an approximate dating of the week of pregnancy you are in, and, accordingly, will establish a date from which the baby could be born.
We use the conditional because we remember that gestation lasts on average 40 weeks, but delivery could take place on any day starting from the 38th week and up to the 42nd week.
Insights on the obstetric rule
The obstetric ruler is also called in other ways that we have decided to mention to you so that you are informed at 360 °. Among these we have:
- wheel of pregnancy
- gestogram
- gestational disc
- P-Ruler (abbreviation of the English term "Pregnancy Ruler")
- "tumbles" or obstetric "wheel" (in jargon)
To make a quick recap, remember that:
- The duration of pregnancy is expressed in full 40 weeks, which is equivalent to 280 days, 10 lunar months or 9 solar months plus one week.
- From the moment of conception to that of delivery, about 266 days should elapse, considering that ovulation occurs on the 14th day after the last menstruation.
- Ask your gynecologist about the obstetric ruler, or download a special app, but remember that everything is subjective and varies from woman to woman.
