The Calendar Method to Find Out How Many Weeks Pregnant You Are

Weeks are more accurate than months

Woman looking at check up appointment.
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Knowing how far along you are in your pregnancy is crucial for receiving appropriate prenatal care. Not only does it give you the due date, it also helps your healthcare team gauge how things are going.

And yet, when you think about how to calculate the length of pregnancy, it's easy to become confused. Should you go by months or weeks? Some people will ask how many weeks pregnant you are while others want to know how many months. And, everyone will want to know your due date!

Though it's easy to think of a pregnancy as lasting 9 months, it's not quite accurate. The method of dating a pregnancy by months is of no value to your doctor or midwife. Instead, they use a method based on the number of weeks that have passed since you became pregnant. This is measured by your period. 

How Long Is Pregnancy?

On average, a pregnancy lasts 40 weeks or 280 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.

While you technically ovulate near what would be called week two of pregnancy, this is how the vast majority of people—your doctor or midwife included—calculate pregnancy length. Your due date is at week 40. If you use the date of ovulation, pregnancy is 266 days long on average.

How to Calculate the Weeks of Pregnancy

To calculate how many weeks pregnant you are, you can use a calendar. List your due date or the first day of your last period. Count forward from your LMP or backward 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:

If you know your due date but not when you got pregnant, you can use the reverse due date calculator.

If you are unsure where you are right now, ask how far along you are in weeks and days at your next appointment. An example might be nineteen weeks and two days. With that information, you can simply count back two days to figure out what day of the week 19, day zero was and count up from there weekly.

Why Weeks, Not Months?

A month is simply too long when it comes to pregnancy. Calculating a pregnancy by weeks is an easier and more accurate way for doctors and midwives to coordinate what is going on 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 so they can compare it to their expectations.

Babies grow very fast and one week can make a big difference in fetal development. The weekly format allows your practitioners to check that your test results are falling in line with what the average. It also helps detect any issues with the pregnancy and coordinates care between your entire healthcare team.

Alternative Ways to Date Your Pregnancy

There are times when using the first day of your last normal period is not the best method to use when determining your estimated due date. It's possible that your cycles vary greatly or you do not know the date of your last period. You might even have had a very odd cycle that you are not sure how to count.

One way that can be incredibly accurate is an ultrasound examination in the first trimester. It is typically performed around weeks 10 to 13, though not every woman has one in the first trimester. During the ultrasound, a measurement of your baby's length can be compared to fetal growth milestones to indicate how many weeks he's been growing. Used in conjunction with your period dates, this is considered the best way to date a pregnancy.

When you're at your next appointment, ask if they have a gestational wheel calculator. These are often found in doctor's offices and can give you a visual representation of your LMP and due date. It's also possible to find apps for your phone or tablet that provide a similar function.

If your pregnancy is the result of in vitro fertilization (IVF) or another assisted reproductive technology (ART), your reproductive endocrinologist will help you determine your due date. This is based on the age of the embryo and the date of the transfer. 

Changes in Your Due Date

The American College of Obstetricians and Gynecologists (ACOG) says that any pregnancy that does not have an accurate due date prior to 22 weeks gestation is to be considered suboptimal. It is possible that your due date may change, though, and it happens to as many as 26 percent of women.

There are some key questions you should ask before letting them change your due date. This can be very important in having a healthy baby because a baby who is born even slightly early can have serious health effects from an early term birth. ACOG also recommends against changing due dates except on rare occasions.

A Word From Verywell

Understanding how many weeks pregnant you are will help you have informed conversations with your prenatal team. Using a calendar and noting each week as you go can help you keep track of the weeks as well. If you have any questions, be sure to ask your practitioner. They'll be happy to clarify any lingering questions you may have.

Sources:

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

American College of Obstetricians and Gynecologists. Committee Opinion No 700: Methods for Estimating the Due Date. Obstetrics & Gynecology. 2017;129:e150–4. doi: http://dx.doi.org/10.1097/AOG.0000000000002046.