How Can I Calculate My Pregnancy Due Date?

After helping many women conceive after months on end of trying we get a lot of questions asking how to calculate the due date. Besides, it's only natural to wonder how many weeks pregnant you are and/or how pregnancy is calculated. While it is important to have an idea of when your baby will be born, try not to get too attached to the exact date as very few women actually deliver on their due date. Since there’s no harm in knowing, we’d like to help you figure out how to calculate your due date.

Calculating Due Date

Your due date is considered to be 40 weeks (280 days) after the first day of your last menstrual period (LMP) or 38 weeks (266 days) after ovulation. This makes your first day of your LMP to be considered day one of pregnancy, even though you probably didn’t conceive until about two weeks later (fetal development lags two weeks behind your pregnancy dates).

How is the calculation done?

For those with an average of 28-day menstrual cycles, there are two ways to calculate your due date.

Naegele’s rule

It is a simple calculation done by adding seven days to the first day of your LMP and then subtracting three months.

For example, if your LMP was November 1, 2022:

1. Add seven days (November 8, 2022).

2. Subtract three months (August 8, 2022).

3. Change the year, if necessary (to the year 2023, in this case).

In this example, your due date would be August 8, 2023.

Pregnancy wheel

This is the other method used to calculate and most doctors prefer this method. Step one would be to locate the date of your LMP on the wheel. Once you line up that date with the indicator, the wheel displays your due date. Don’t forget that the due date is only an estimate of when you will deliver your baby. The chances of actually having your baby on that exact date are very slim.

What If I’m not sure about my LMP?

Relax! There are ways to figure out your due date when you can’t remember the first day of your last menstrual period.

• If you know you had your LMP during a particular week, It’s possible for your doctor to estimate your due date accordingly.

• If you have no idea when your last period was, your doctor may use an ultrasound to determine your due date.

What If I have a longer or irregular cycle?

If your cycle is longer than the average 28-day cycle, no problem. A pregnancy wheel can still be used, but some simple calculations are necessary. The second half of a woman’s menstrual cycle always lasts for 14 days. Usually the time from ovulation to the next menstrual period. If you have a general idea of when you ovulated, you can use an adjusted LMP to find your due date with a pregnancy wheel.

Let’s test it practically if your menstrual cycle is usually 35 days long and the first day of your LMP was November 1:

1. Add 21 days (November 22).

2. Subtract 14 days to find your adjusted LMP date (November 8)

After you have identified your LMP date, simply mark it on the pregnancy wheel and then look at the date where the line crosses. That is your estimated due date.

Why is the ultrasound date different from the due date?

A little background, after an ultrasound, the findings include two estimated due dates. The first date is calculated using the date of the LMP. The second date is based on the ultrasound measurements. These dates are rarely the same. The ultrasound determines whether or not these dates are in agreement. It’s rare for the doctor to change the due date unless they are both very different.

And get this the more ultrasounds, each ultrasound report will contain a new due date based on the most recent measurements but the expected due date shouldn’t be changed based on measurements from a second- or third-trimester ultrasound. They tend to be more accurate earlier on in the pregnancy.

What would it mean if my doctor changed my due date?

This happens if your fetus is significantly smaller or larger than the average fetus at your particular stage of pregnancy. The main reason why your doctor orders an ultrasound is to determine the gestational age of your baby when there’s a history of irregular periods, when the date of your LMP is uncertain, or when conception occurred despite oral contraceptive use. Since the measurements were done in the first trimester are the most accurate the only reason the due date would change is based on the ultrasound measurement.

In summary

Your due date helps you and your healthcare providers track your pregnancy and plan for labour and delivery. It’s an important date, but it’s just an approximation of when you may go into labour. You may have your baby before or after your due date and it is still okay.

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