Estimated date of delivery: how to calculate the term of pregnancy
The estimated date of delivery is the date on which the pregnancy is assumed to end. The calculation of this date depends on the day of the last menstruation and on the date of conception. Although the estimated date of delivery for a woman who has a regular menstrual cycle may be more precise than that calculated for a woman with a irregular menstrual cycle, however, it is a "presumed" date, never 100% reliable.Its calculation can be done with different methodologies. Let's find out together!
What is the estimated date of delivery?
The presumed date of delivery is always in relation to the last menstrual cycle and, in particular, to the date of the first day of the last menstruation. Knowing this date, although it cannot be precise in establishing the exact day of the term of pregnancy, is important for every expectant mother, so that she cannot be caught unprepared by the happy event, as well as for the doctors who follow her, so that they can plan - based on this date - all planned visits and checks.
By convention, pregnancy is considered to last 280 days from the last menstruation: if you have a regular cycle (which therefore occurs between 28 and 30 days), to calculate the expected date of birth, just add 40 weeks to the day of the birth. "last menstruation. The birth will therefore be foreseeable between the two weeks preceding and those following the date thus obtained.
If, on the other hand, your menstrual cycle is irregular, other factors must be taken into consideration and the best way to calculate the date is to wait, as we will see, for the response of the gynecologist at the first ultrasound.
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What is the calculation of the estimated date of birth for?
Knowing the expected date of birth is important, first of all to be able to control and monitor the growth of the fetus. Once the indicative term of pregnancy has been calculated, it can be used as a reference to observe its development and follow all the necessary checks, without forgetting any deadline.
At the first visit to the gynecologist, your doctor will write the estimated date of birth in your maternity booklet, so as to plan when it will be necessary to have the routine exams and subsequent ultrasound scans, starting from the morphological ones. Furthermore, knowing the presumed date of birth of the child will be useful in understanding if the child will be born premature or late, if specific attention and care will be needed after birth, or if it will be better to proceed with a caesarean section or with induction. You will also need to calculate this date to be able to apply for maternity leave, starting two months before the birth.
Obviously, it should be remembered that the presumed date of delivery is only indicative and that labor is to be expected in the fifteen days (2 weeks) preceding that date or following it!
How to calculate the expected date of delivery?
A physiological pregnancy, without specific problems, can end between 37 and 42 weeks. In calculations to determine the estimated due date, the standard duration of 40 weeks (280 days) is usually considered.
The methods for calculating this date can be different: from Naegele's Rule to the use of the obstetric slide, up to the first trimester ultrasound (the most accurate system). There are also several sites and apps that can do this calculation for you! The important thing is that you can remember the first day of your last menstrual cycle: if you are looking for a baby, always remember to mark it!
Calculation of the expected date of delivery with the Rule of Naegele
The calculation of the day of birth according to the so-called "Naegele's Rule" is carried out as follows: you must first remember the day of the beginning of the last menstruation, then add 7 days to that date, subtract 3 months and finally add a year. To make the calculation easier, just add 9 months + 7 days to the first day of your last menstrual cycle.
The Rule of Naegele is obviously based on regular menstrual cycles of 28 days: it is enough not to have a precise ovulation on the 14th day of the cycle because the calculation is decidedly less reliable.
How to calculate the expected date of delivery with the obstetric slide rule
Gynecologists and obstetricians use another method to establish the expected date of delivery: it is the "obstetric ruler". The obstetric ruler is a cardboard instrument made up of two numbered discs, superimposed on each other. The lower wheel is divided into 12 sections corresponding to the 12 months of the year, with the days of the month inside. The upper one, on the other hand, is divided into 3 parts (corresponding to the 3 trimesters of pregnancy) which are in turn divided into the 40 weeks of "standard" pregnancy.
The gynecologist uses the obstetric slide by turning the lower wheel in order to position the arrow of the upper disc in correspondence with the first day of the last menstruation, thus calculating the date of birth by reading what is the term of the 40 weeks scheduled starting from that day .
The ultrasound of the first trimester of pregnancy to establish the expected date of delivery
However, the best method to obtain the indicative date of delivery with more precision is the ultrasound of the first trimester of pregnancy, the so-called "dating ultrasound". This ultrasound allows the doctor to correctly date the age of the fetus by measuring the length of the skull. --caudal and biparietal diameter of the fetus.
This ultrasound should be done within the tenth week of pregnancy, but the sooner it is done, the more accurate it is! Obviously, however, not before the sixth week of pregnancy, or the future baby would not be visible!
For more information on the estimated date of delivery, you can consult the website of the Santagostino Bimbi Pediatric Center.
