Frequently Asked Questions about Calculating Your Due Date

Yes, your due date can be “wrong”; it isn’t a guarantee, just an estimate. In fact, only about 5% of babies are actually born on their estimated due dates (EDD). 4

The method you used to get pregnant will affect your estimated due date (EDD). Here are the most common ways it may be calculated:

From the date of your last menstrual period

In most cases, your doctor will calculate your due date from the date of your last menstrual period (LMP). However, if you have irregular periods, this method is less reliable, and they may opt for calculation based on an ultrasound (described below).

Note that even if your periods are regular, menstrual cycles can have variable lengths. The month you got pregnant, you might have ovulated a little earlier or later than average, which could have rendered your EDD inaccurate. 5

From the date of conception

If you know exactly when you conceived, it’s possible to use that as the starting point to figure out your EDD. Doing this involves counting 38 weeks from the date of conception.

This method is quite tricky to pull off. To do it, you’ll need to know exactly when you were ovulating, which isn’t easy. That’s because sperm can survive inside your reproductive tract for up to five days, and the process of conception can take 12–24 hours (from the moment a mature egg gets released from your ovaries to the moment it’s fertilized by sperm).

All this means that it’s hard to come up with an accurate EDD. You can’t just count forward from when you had unprotected sex—you have to figure out when the egg was fertilized within the ovulation window. That just isn’t always practical, especially if you weren’t regularly making use of tools like ovulation test strips to pinpoint exactly when you ovulated.

From your IVF transfer date

If you’re using in vitro fertilization (IVF), your EDD is calculated by adding 266 days to the date of conception. This may be earlier than the transfer date; if your doctor transferred the embryo three days after retrieving your egg, add 263 days to the transfer date. If your doctor transferred the embryo five days after egg retrieval, add 261 days to the transfer date. 6

Estimating your due date for an IVF pregnancy is much more accurate than counting forward from your LMP if you conceived naturally. Still, there’s no guarantee that your baby will be born on that exact day.

Via ultrasound

When the date of conception is unknown or imprecise, an ultrasound can be the most accurate way to determine your EDD, since babies’ growth rates do not diverge much in the first and second trimesters. 5 Your doctor will measure your baby and try to determine her gestational age based on how big she is. 7

If you find that your baby’s gestational age doesn’t match up with the date of your LMP, the age your doctor obtains from your ultrasound will have a margin of error of about a week in the first trimester and two weeks in the second trimester.

Your doctor may adjust your due date if they find that your baby is significantly larger or smaller than expected and her EDD does not match up with your last menstrual period or estimated date of conception in the first or second trimester.

Fundal height and your EDD

You may have read that your due date may change based on your doctor’s measurement of your fundal height (the distance between your pubic bone and the top of your uterus in centimeters), which your doctor will measure during each of your prenatal visits after 20 weeks.

It’s true that (conveniently) your fundal height may match your gestational age, meaning that if you’re 24 weeks pregnant, your fundal height should be roughly 24 cm (9.45 inches). 5 However, it isn’t true that a discrepancy between your fundal height and EDD is likely to lead to your due date being changed.

Usually, by the time your baby is large enough for your fundal height to be measured, there won’t be any changes to your EDD. If the fundal height is off, your doctor will be more concerned about potential pathologies or complications, such as gestational diabetes.

Does carrying multiples affect your due date?

If you’re carrying twins or multiples, your doctor will probably use the same method of calculating your due date as in a singleton pregnancy (i.e., your LMP and an ultrasound).

However, it’s statistically more likely that you will naturally go into labor on the earlier side, with most twin pregnancies being delivered at around 36 weeks. 8

Natural vs. planned childbirth in multiple pregnancies

It’s possible to give birth to twins naturally. However, if your body doesn’t go into labor around 36 weeks, many experts recommend giving birth through induced labor or a C-section, even though 36 weeks is typically considered “premature” or “preterm.” Giving birth a little early reduces your babies’ risk of complications like stillbirth. 9

When can you expect to give birth to multiples?

If you’re carrying multiples, you’re statistically likely to give birth earlier than if you’re carrying just one baby (either because you go into labor earlier in your pregnancy or because you have a pregnancy complication that makes it safer for your physician to induce labor earlier).

On average, here’s when you’re likely to give birth depending on how many babies you’re carrying: 6

• Singleton (one baby): 39 weeks (gestational age)
• Twins: 35 weeks
• Triplets: 32 weeks
• Quadruplets: 30 weeks
• Quintuplets or more: 27 weeks

These figures are just rough averages. Your doctor will assess your pregnancy and make adjustments to your planned birth date depending on what they discover.