Calendar Method to Determine How Many Weeks Pregnant

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Pregnancy is, on average, 40 weeks or 280 from the first day of your last menstrual period (LMP). The number of weeks you are pregnant depends on the date of your last period. You technically ovulate near what would be called week two of pregnancy, but this is how the vast majority of people, your doctor or midwife included, calculate pregnancy length. Your due date is week 40. If you use the date of ovulation, pregnancy is 266 days long on average.

To calculate how many weeks you are you can use a calendar. List your due date or the first day of your last period. Count forward for LMP or backwards from your due date to determine which week is when. (Hint: This is the same day of the week every week!) You can write this on your calendar or use a pregnancy weeks calculator.


LMP: January 1
Week 1: January 8
Week 2: January 15
Week 20: May 21
40 Weeks (Due Date): October 8

Did you know that as many as 26% of women have their due dates changed in pregnancy? Do you know what to ask before letting them change your due date? (You may also want to figure out from your due date when you got pregnant.) This can be very important in having a healthy baby because we know that a baby who is born even slightly early can have serious health effects from an early term birth.

Calculating a pregnancy by weeks is an easier way for doctors and midwives coordinate what is going on when in your pregnancy.

Since each day of pregnancy can be an important one it is imperative that they have a good idea of how far along you should be in pregnancy to match their expectations. Given all of the trouble with counting by months, you can see how a weekly pregnancy format is much easier for them to use in your medical and prenatal care.

If you are unsure of how to calculate the weeks of pregnancy, just ask for your next appointment how far along you are in weeks and days. An example might be 19 weeks and 2 days. Simply count back two days to figure out what day of the week 19 day 0 was and count up from there weekly.

Weeks of Pregnancy


Declercq ER, Sakala C, Corry MP, Applebaum S, Herrlich A. Listening to Mothers III: Pregnancy and Childbirth. New York: Childbirth Connection, May 2013.

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