Ezconfinement - Naegele's Rule

How to Calculate Your Baby’s Due Date With Naegele’s Rule?

Have you just found out you’re pregnant? Congratulations! Now, we’re sure that you and your partner can’t wait to know when you’re due! To be honest, calculating your due date isn’t complicated science that requires some intensive formula. Fun fact: actually, very few women deliver on their actual due date however, it is good to take note and have a clear idea of when your baby will be born but don’t have your heart set and attached to the exact date.

When seeing a doctor for the first time and finding out the gestational age is considered to be one of the most critical aspects in providing quality care. This is because finding out the gestational age allows the obstetrician to provide care to the mother without compromising or bringing harm to the fetus or mother. Gestational age is common term mothers and their partners will frequently hear during pregnancy as it is used to describe how far along the pregnancy is and is measured by weeks.

Naegele’s rule Calculations

A woman’s pregnancy will last an average of approximately 280 days or 40 weeks from the first day of her last menstrual period (LMP). Even though it is considered your first day of pregnancy, the baby probably hasn’t been conceived until about two weeks later. This is because fetal development lags around 2 weeks behind your pregnancy dates.

It is important for obstetricians to get the woman’s detailed menstrual history which includes the duration, flow, and previous periods, and also any hormonal contraceptives if any were taken. All these factors contribute to determining the woman’s cycle and also ovulation period.

Pregnancy due dates are calculated with a formula called the Naegele’s rule. It was formulated by Frederich Naegele, an obstetrician in the 19th century where ovulation was not as easily observed back in the day. Since then, the reliability of this method only depended on a few factors. Firstly, the woman’s memory of her last LMP, the regularity of her period cycles, and the presence of early or light bleeding.

This simple rule can be calculated by simply adding 7 days to your last menstrual period date, then subtracting 3 months from that date. From there, a provisional date of delivery can be identified.

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For example, if your last LMP was on the 1st of November 2020, adding 7 days would give you the 8th of November 2020. Subtracting 3 months from the 8th of November would provide you with a date of August the 8th. Obviously, you would be changing the year if necessary so, your estimated due date for your newborn would be the 8th of August 2021.

The rule has been based on a woman’s ovulation and fertilization that typically happens on day 14 of her cycle with an average cycle length of 28 days. It is good to take note that this rule will not be as accurate if the newly pregnant woman has a history of irregular menstrual periods or irregular ovulation that happens either earlier or later in her cycle.

With the irregularities in her cycle, it could throw off the formula with the estimated due date not being as precise as it should be. Sometimes, although not very often, women may have a slight bleed of a threatened miscarriage in her early pregnancy stage, which adds more confusion often mistaking it as her menstrual period.

Naegele’s rule assumes an average length of the months in a Gregorian calendar year of just over 30 days by dividing 365 in a year by 12 months. This will, therefore, provide an average of 280.75 days. Given the unequal length of months in the Gregorian calendar, it results with Naegele’s rule overestimating the estimated due date by 3 days for all LMP’s in May and depending on the effect of leap years that will give precise dates on only either 2,5 or 7 months of the entire year.

An average pregnancy is assumed to be 40 weeks or 280 days from your last menstrual period. Some recent studies have suggested that instead of using 280 days to calculate your estimated due date, it would be better to use 282 days (40 weeks and 2 days). This, however, is not currently accepted into clinical practices around.

Why doctor change my due date?

If the doctor has changed your estimated due date, it could mean that either your fetus is smaller or larger than the average fetus during this particular stage of pregnancy. A doctor will typically ask for an ultrasound to be done to determine the gestational age of the fetus when the date of your last menstrual period is uncertain or if there is a history of irregular periods. 

The best way of calculating the woman’s estimated due date is still by using Naegele’s rule when assuming a gestational age of 280 days at childbirth. That’s basically all that there is when using Naegele’s rule to find out your estimated due date!

Pro tip: Even if you don’t remember your last menstrual period, fret not! There are other ways to figure out your due date as it’s common some women tend to not remember such as ultrasounds. 

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