Calendar Method to Determine How Many Weeks Pregnant

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Knowing how far along your pregnancy is is crucial for receiving appropriate prenatal care. And yet when you think about how to calculate the length of pregnancy, it can be easy to become confused. Should you go by months? Should you go by weeks? How many weeks pregnant are you? How many months? The method of dating a pregnancy by months is of no value to your doctor or midwife. The method that they use is based on the number of weeks that have passed since you have gotten pregnant, as measured by your period.

 

How Long is Pregnancy?

Pregnancy is, on average, forty weeks or two hundred eighty days 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 forty. If you use the date of ovulation, pregnancy is two hundred sixty-six days long on average.

How to Calculate the Weeks of Pregnancy

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.

Example:

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 twenty-six percent 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 nineteen weeks and two days. Simply count back two days to figure out what day of the week nineteen day zero was and count up from there weekly.

Alternative Ways to Date Your Pregnancy

There are reasons why using the first day of your last normal period is not the best method to use when determining your estimated due date. This might be because you have cycles that vary greatly or do not know the date of your last period. You might have even had a very odd cycle that you are not sure how to count. Another way that can be incredibly accurate is an ultrasound examination in the first trimester.

Used in conjunction with your period dates, this is considered the best way to date a pregnancy.

If you got pregnant after using in vitro fertilization (IVF) or another assisted reproductive technology (ART), your reproductive endocrinologist will help you determine your due date based on the age of the embryo and the date of the transfer. 

Any pregnancy that does not have this information prior to twenty-two weeks gestation is to be considered suboptimal. The American College of Obstetricians and Gynecologists (ACOG) also recommends against changing due dates except for rare occasions. 

Weeks of Pregnancy

Sources:

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

Methods for estimating the due date. Committee Opinion No. 700. American College of Obstetricians and Gynecologists. Obstet Gynecol 2017;129:e150–4.

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