Generality
The obstetric slide is an instrument used by midwives and gynecologists to date pregnancy and estimate the most likely day of birth .
Although they are approximate calculations, depending on these indications it is possible:
- Schedule the controls scheduled during gestation;
- Monitor that the development of the fetus inside the uterus proceeds according to normal physiological rhythms;
- Know the alleged date in which the child will be born, then prepare for the event.
When using the obstetric slide, the gynecologist takes the beginning of the last menstruation as a reference and takes into account an average gestational duration of 280 days, ie 40 weeks .
The result obtained gives a dating of the week of pregnancy in which the woman finds herself and, consequently, establishes the day in the vicinity of which the birth of the child could occur.
What's this
Premise: how is the beginning of the pregnancy dated?
To date the pregnancy, doctors generally refer to the day of the beginning of the last menstruation . If the gestation proceeds normally, without complications, it has an average duration of 280 days, equal to 40 weeks starting from the day taken as a reference. However, a more physiological pregnancy ends with tranquility even between 37 and 42 weeks .
What is the Obstetric Rule?
The obstetric slide is a tool that allows:
- Know at what time of pregnancy the future mother and her child are;
- Calculate the presumed date of the term delivery .
If the menstrual cycles are 28-30 days and the pregnancy proceeds without problems, the birth can occur at any time, between two weeks before and two weeks after the scheduled date. For a better approximation of the calculation obtainable with the obstetric rule, however, it is possible to take into consideration some individual factors (such as the date of the presumed ovulation or fertilizing sexual relationship).
The information provided by the obstetric ruler is part of the data collected in the anamnesis by the gynecologist and therefore contributes to monitoring fetal growth parameters, which will then be supported by laboratory analyzes, ultrasound investigations and other medical evaluations.
Obstetric Regulus: how is it done?
The obstetric slide is an instrument composed of two numbered and overlapping 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 anticlockwise.
- The lower wheel (larger) of the obstetric rule is divided into 12 sections, one for each month of the year, in turn graded with the days of the month ;
- The upper disc (smaller) shows, instead, the 40 weeks of pregnancy divided into the 3 trimesters . This part of the obstetric rule can also report useful information, such as diagnostic tests during pregnancy or the stages of fetal development .
The expected date of birth 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 spins the lower wheel of the obstetric slide, confirming or not the calculation of the weeks of pregnancy carried out by the mother or couple.
In the obstetric slide rule, the expected date of birth is indicated by an arrow . Depending on this result, the gynecologist plans the calendar with the various ultrasounds and check-ups to monitor pregnancy together with the pregnant woman.
Obstetric rule: synonyms and terminology
The obstetric slide is also known as pregnancy wheel, gestogram, gestational disc and P-Ruler (abbreviation of the English term " Pregnancy Ruler "). In common jargon, sometimes, one can hear of obstetric "ruzzola" or "wheel".
To better understand each other, then, it should be remembered that:
- The duration of pregnancy is expressed in 40 complete weeks, which is equivalent to: 280 days, 10 lunar months or 9 solar months plus a week.
- From the moment of conception to the time of delivery, about 266 days should pass, if we consider that the duration of the luteinic phase (post-ovulatory) is on average 14 days (it can vary from 9 to 16 days).
What is it for?
Obstetric rule : why is it used ?
The use of the obstetric slide is an important reference to establish gestational age at a given time.
In other words, this tool allows us to calculate the times in pregnancy and, considering that each gestation period corresponds to a specific phase of fetal growth, the obstetric ruler is useful for:
- Monitor the development of the child inside the uterus;
- Establish the probable date of conception and the expected day of birth ;
- Indicate to the future mother the deadlines not to be forgotten, such as the medical checks scheduled during gestation (blood tests, ultrasound scans, etc.).
The obstetric ruler is also useful to indicate the ideal care plan for the future mother: a premature baby could need more care, just as a gestation that lasts over 42 weeks requires an induction of labor .
How to use it
Obstetric Rule: how to calculate the presumed date of birth
By convention, the calculation of the weeks of pregnancy takes as a reference the first day of the last menstruation: whereas a "standard" gestation lasts 280 days, seven days must be added and three months removed from this date ( Naegele rule ).
In practice, the obstetric rule is used by the gynecologist inserting the date corresponding to the day on which the last menstrual flow appeared. The presumed time of the term birth is thus obtained.
To know
Naegele's rule is based on 28-day menstrual cycles, with an ovulation assumed on the 14th day.
Obstetric Rule: what data are needed
The date of birth resulting from the obstetric rule is based on two main information:
- Start day of the last menstruation
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.
- "Standard" duration of pregnancy
The duration of pregnancy is conventionally established in 40 weeks (ie 9 months plus 7 days) . This time frame is however approximate, so much so that the parts that occurred slightly in advance or postponement, between the thirty-seventh and the forty-first week of gestation, are still normal .
To date the pregnancy, during the anamnesis, the gynecologist applies the rule of Naegele:
- Taking as a reference the date corresponding to the 1st day of the beginning of the last menstruation (1) 7 days are added, 3 months are subtracted and a year is added. Simplifying this formula, we add 9 months plus 7 days (2) to the first day of the last menstruation.
In the case, for example, the starting date of the last menstruation is June 4th, the presumed time of birth will be March 11th of the following year.
In reality, with the obstetric rule this calculation is "automatic" and very simple:
- The doctor places the arrow on the internal disk at the day and month reported by the woman as the starting date of the last menstruation, then reads the duration of gestation in complete weeks by default (eg 40 weeks + 6 days = 40 weeks). The obstetric ruler will also indicate the probable date of conception, the expected day of birth and the deadlines to be remembered, such as the various check-ups.
As already mentioned, the calculation of the obstetric rule is indicative .
For a better approximation of the result, it is possible to take into consideration:
- Date of alleged ovulation
Assuming a certain regularity of menstrual cycles, the term of pregnancy can be calculated with the obstetric rule starting from the presumed day of ovulation, bearing in mind that this always occurs 14 days before the last flow ( luteinic phase ) and, on average, 14 days after the start of the last menstruation ( follicular phase ).
In practical terms, instead of adding 7 days, in the obstetric ruler, on the 1st day of the beginning of the last menstruation , 14 are calculated from the date of the presumed ovulation. It should be pointed out, however, that establishing this day with precision is more complex for a woman, if we exclude the use of ovulatory sticks or other systems to know exactly when the ovulatory phase occurs.
Furthermore, the variability occurring in the ovulatory cycle is greater: although it has an average duration of about 14 days, the follicular phase can undergo strong oscillations, ranging from 1 to 3 weeks. For most women, however, the luteinic phase (from ovulation to the beginning of menstruation) is more constant and lasts an average of 14 days.
- Date of presumed conception (if known)
The exact day of conception is a good starting point for calculating with the obstetric rule the presumed date of the birth which, approximately, will take place at the end of the 266th day after that of the fertilizing coitus .
However, for the woman or the couple, remembering this information may not be such an immediate process, since a single and isolated unprotected sexual relationship should have been consummated after the last period.
Regardless of the fact that this date can be used to calculate the date of the presumed birth, it is possible to use the obstetric rule to go backwards, ie to establish the conceptional age : with a certain margin of error, calculated in excess or defect, this tool allows to establish the day of conception.
In this regard, it should be remembered that we speak of conceptional age when the duration of pregnancy is calculated from the moment of the presumed conception. Gestational age is established by counting, instead, from the first day of the last menstruation.
Obstetric Regulus: how accurate is the calculation?
In women with a regular menstrual cycle, the calculation of the obstetric rule can be considered reliable in providing a rough dating of the week of pregnancy and, consequently, a date from which the child could be born.
It must always be remembered that this is an indication: gestation lasts 40 weeks on average, but it could be born on any day starting from the 38th week up to the 42nd week .
To learn more about how the menstrual cycle works »In any case, the estimation of the obstetric rule is a starting point: this information can be established with a better approximation by means of an ultrasound check and the other medical tests that are performed during gestation.
One of the tests that allows to "adjust" a result of the obstetric rule is the ultrasound of the first trimester (also called dating ultrasound ). This exam allows, in fact, the observation of the gestational chamber and the size of the embryo, allowing the gynecologist to confirm or correct the initial estimate (presumed too early or late delivery), dating the beginning of the pregnancy with a margin of error very low, close to 3 days.
Some considerations
At this point, we need to underline some concepts related to the use of the obstetric ruler:
- The date of birth envisaged by the obstetric ruler is unlikely to coincide with the actual date. However, if the menstrual cycles are regular (that is, they occur on average every 28-30 days) and the pregnancy proceeds normally (without problems), the birth can take place at any time within the 15 days before or after the scheduled date. During the echography check, the gynecologist will be able to provide a more precise dating based on parameters related to the growth and development of the fetus, confirming or not the dating made with the obstetric rule and / or the calculation of weeks of pregnancy.
- The calculation of the weeks of pregnancy starts from the date of the last period . This reference is the only certain in the woman's possession, excluding the use of ovulation tests or monitoring of the basal temperature. If the future mother does not remember the day of the beginning of the last period, it is possible that the dating of the pregnancy with the obstetric ruler is not correct or precise.
- The calculation made on the basis of the obstetric rule is all the more truthful, the more regular are the menstrual cycles : in dating the weeks of pregnancy, the average length of the menstrual cycles is considered in the interval between 22 and 45 days, but the value of reference (physiological) is a cycle that is repeated regularly every 28 days. It should be remembered that a certain individual variability is possible, ie the duration of the cycle can change from month to month, due to the intervention of various factors. In other words, if a woman has a cycle of 28 days, but in the preconception phase something has interfered with this periodicity, the result of the obstetric rule could be less precise.
- The result of the obstetric rule can be uncertain or unreliable in the case of highly irregular cycles, resort to assisted reproduction techniques, use of hormonal contraceptives, pathological or iatrogenic amenorrhea, maternal diseases, particular accidents or problems that can influence the normal course of gestation .
