DATING OF PREGNANCY FROM THE LAST MENSTRUAL PERIOD (LMP)

CONTENT

-INTRODUCTION

-HOW TO DATE PREGNANCY USING THE LMP?

-DRAWBACKS

 

INTRODUCTION

Dating of pregnancy from the last menstrual period (LMP) is the commonest and longest serving method of dating pregnancy. With over 200 years of use, it remains the first line and standard method of calculating the EDD as well as the gestational age of a pregnancy. Known as the “Naegele’s rule”, it states that the EDD is 280 days from your last menstrual period (LMP).

 

-HOW TO DATE PREGNANCY USING THE LMP?

Using the “Naegele’s rule; EDD= LMP + 280 days”, since there are 7 days in a week, another way of looking at EDD= LMP + 40 weeks (40x 7=280days). 

Similarly, since an average month contains about 30 to 31 days and every 3 month contains an average of 90 to 91 days, i.e. January, February and March contains 31 + 28+ 31 respectively = 90 days, while April, May and June contains 30 + 31+ 30 respectively= 91 days).

Therefore every 3 months ≈ 13 weeks ( Since13 x 7= 91days ≈ 3 months)

From the above, if 3 months is = 13 weeks, than 9 months is ≈ 3 x 13 weeks = 39 weeks.

Thus, EDD which is date of LMP + 40 weeks (i.e. 40x 7=280days), is also the same as LMP + 39 weeks + 1 week or LMP + 39 weeks + 7 days.

Since 9 months = 39 weeks, therefore EDD = LMP + 7 days + 9 months.  If 9 months is 1 year – 3 months, then another way to define EDD is LMP + 1 year – 3 months + 7days.

In summary, the EDD = LMP + 7days + 9months or LMP + 7days + 1year – 3months.

Hence in other to calculate your EDD from your LMP, add 7 days to the first day of your LMP and add either 9 months to your LMP if it is falls any time before April or add one year but subtract 3 months from your LMP if it falls any time after the 1st of April. 

For example: If your LMP were 15/02/18, to calculate your EDD, add 7 days to 15 and add 9 months to 02 (Februaury) to get your EDD which will be 22/11/18 (15 + 7days, 02 + 9months). 

But if your LMP were 01/06/18 then by adding 7days to 01 and subtracting 3 from 06 (June) and adding a year to 2018, then  your EDD will be 08 of 03 (March) of 2019, i.e 08/03/19.

 

DRAWBACKS

1.  REQUIRES AN ACCURATE RECOLLECTION OF THE LMP:

Unfortunately a lot of pregnancies are unplanned and many women do not keep records of their menstrual periods. This often time leads to guessing of the LMP along with the provision of wrong dates as LMP. 

Women who book late often find it difficult to recollect their LMP and end up guessing, usually ending up with a false date. It is not unusual to see many women provide different dates for their LMP in the course of their pregnancy.

2.  MOST REGULAR MENSTRUAL CYCLES ARE NOT 28 DAYS:

The Naegele’s rule was based on a calculation that the menstrual cycle has a length of 28 days and that ovulation occurred on the 14th day of the last menstrual period. Unfortunately, most regular cycles are not exactly 28 days and ovulation doesn’t occur on the 14th day of all menstrual cycles. To calculate the actual EDD, calculate the difference between your cycle length and 28 days and add the difference to the EDD gotten from the Neagele’s rule, if your cycle length is longer than 28 days or subtract the difference from the EDD, if your cycle length is shorter than 28days. For example, if your LMP were 15/02/2018 and you have a 33 day cycle, your actual EDD will be equal to the difference between your cycle length and 28days, in this case 33 – 28 days = 5days, added to the EDD calcultaed from the Naegele’s rule (15 + 7/02 + 9/18 which is = 22/11/18), i.e. 5days + 22/11/2018 = 27/11/18. But if your cycle length were 21 days and your LMP were to be the same as above, then you subtract the difference between your cycle length and 28days from the EDD calculated by the Naegele’s rule, which in this case will then be 22/11/18 -7 days to give 15/11/18.

3.  MANY WOMEN HAVE IRREGULAR MENSTRUAL CYCLES:

Although women with irregular menstrual cycles may have problems with ovulation, occasionally many of such women may ovulate and eventually conceive. Becasue calculation of the EDD using the Naegele’s rule is based on a 28 day regular cycle, with ovulation occuring regularly on the 14th day of each cycle, women with irregular cycles only ovulate in an unpredictable manner many weeks and occasionally some months after their LMP resulting in wrong dates if the Naegele’s rule were used.

4.  SOME WOMEN DID NOT HAVE ANY LMP BEFORE TAKING IN: 

Many women for example, those breast feeding and those who recently remove or discontinue the use of a long acting hormonal contraception may occasionally get pregnant prior to the resumption of their menses. With no existing LMP, calculation of their EDD by the Naegele’s rule is not applicable.

Depite its inadequacies, the dating of pregnancy using the Naegele’s rule calculated from the LMP remains the foremost and the first line method of dating pregnancy. Whatever methods are thereafter used to confirm the age and date of pregnancy, their results are expressed in pregnancy age comparable to that calculated by the Naegele’s rule.

 

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