How to Calculate How Many Months Pregnant

Weeks, months and trimesters are units of measurement used to determine the amount of time that you’ve been pregnant. A determination in weeks is probably the most often-used measurement because it is also the easiest to calculate. Unfortunately, due to the inexact allocation of weeks among the months on the standard calendar, no universal method exists for converting “weeks’ pregnant” to “months’ pregnant.”

Ensure that you understand the method used to calculate months of pregnancy when you discuss it with friends and family, as well as your obstetrician, to avoid confusion and determine the time you will deliver your baby.

Last Menstrual Period

The time that you’ve been pregnant is typically calculated using either your last ovulation date or the first day of your last menstrual period. Since most women don’t know the date of their ovulation, using the first day of your last menstrual period is the preferred method.

If you are unsure as to the date of your last period, try thinking back to what you were doing then; you may recall some event or activity that will help you pinpoint the exact date.

Calculating Weeks’ Pregnant

If you are using either your last ovulation date or the first day of your last menstrual period to calculate the time that you’ve been pregnant, use the day of the week that date fell on as your starting point.

Each time that day of the week comes again, you, of course, have been pregnant for an additional week. You can use a similar logic to calculate the months that you’ve been pregnant.

Calculating Months’ Pregnant

The most popular method for calculating month of pregnancy is to use the number of weeks since you conceived, but experts don’t agree on the conversion rate.

While your best friend might say that one month equals four weeks, others might insist on a more accurate interpretation: One month equals 4.3 weeks. For example, if you are 20 weeks’ pregnant, you have been pregnant for five weeks using the first method, but only four months’ pregnant using the second.

Alternate Methods

You can use one of a number of alternate methods to calculate the months of pregnancy other than simple division. One method is to count six weeks since the first day of the last menstrual period as the first month, and then begin counting months as usual.

This method is based on the fact that the ovulation date for most women starts about two weeks after the start of their menstrual period. Another method is to count five weeks for the first, third and sixth months and four weeks for the other months in an attempt to duplicate the calendar.

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