How is a due date calculated? From last period, conception or IVF

The moment a pregnancy is confirmed, one question tends to follow almost immediately: when is the baby due? The answer comes from a simple calculation, but it's worth understanding what it really means. A due date feels like a deadline. In practice, it's the middle of a window, and only a small share of babies, roughly 4–5%, arrive on the exact day.

How is a due date calculated from your last period?

Most due dates are worked out using Naegele's rule, which adds 280 days, or 40 weeks, to the first day of your last menstrual period (LMP):

Due date = first day of last period + 280 days

It assumes a 28-day cycle with ovulation around day 14. If your last period began on January 1, your estimated due date is October 8. The pregnancy calculator does this instantly and shows how far along you are.

A quick way to do it in your head

Take the first day of your last period, add one year, subtract three months and add seven days. January 1 becomes January 1 of the next year, then October 1, then October 8. It's the same rule, rearranged to be easy to remember.

What if your cycle is longer or shorter than 28 days?

A longer cycle usually means ovulation happened later, so the due date moves later by the same number of days. A shorter cycle moves it earlier:

How to calculate a due date from conception

If you know when conception happened, add 266 days, or 38 weeks. That's the same as Naegele's rule minus the two weeks between the period and ovulation. In practice, few people know the exact date, since sperm can survive for several days before an egg is released, so the last period is used more often.

How is an IVF due date calculated?

IVF gives the most precise dates of all, because the embryo's age is known exactly:

The due date calculator supports all of these methods.

How far along am I? Gestational age explained

Pregnancy is counted from the first day of the last period, not from conception. This is called gestational age, and it means the first two weeks of "pregnancy" technically happen before you're pregnant. When someone is described as 10 weeks along, the embryo is about 8 weeks old. It's a convention, but a useful one, because the start of the last period is a date most people know.

Why can an early ultrasound change your due date?

In the first trimester, embryos grow at a very predictable rate, so measuring one on an ultrasound is the most accurate way to date a natural pregnancy. If the scan suggests a date that differs meaningfully from the one based on your last period, your provider will usually switch to the ultrasound date. Later scans are less useful for dating, because babies start to grow at different rates.

What if you don't know the date of your last period?

That's common, especially with irregular cycles, after stopping hormonal contraception, or while breastfeeding. There's no need to guess. Tell your provider, and they'll usually date the pregnancy with an early ultrasound.

Trimesters and term: the timeline in weeks

Babies born before 37 weeks are considered preterm. If a pregnancy continues past 41 weeks, providers typically increase monitoring and may discuss induction.

How is the due date used during pregnancy care?

Beyond telling you when to pack a hospital bag, the due date sets the schedule for much of your prenatal care. Many routine checks are timed by gestational week: early screening tests happen in the first trimester, a detailed anatomy scan is commonly done around 18 to 22 weeks, and screening for gestational diabetes is often done between about 24 and 28 weeks. Some of these tests are only accurate within specific windows, which is one reason providers care about getting the date right early on.

The due date also helps your care team judge whether the baby is growing as expected. If measurements later in pregnancy seem ahead or behind, knowing the pregnancy was dated accurately in the first trimester makes those measurements much easier to interpret.

How likely is the baby to arrive on the due date?

Not very. Most babies are born somewhere between 37 and 42 weeks, and first pregnancies sometimes run a little longer. It helps to think of the due date as the center of a roughly five-week window. Planning around the whole window, for things like work leave or family visits, avoids a lot of stress if the baby comes early or keeps everyone waiting.

Whatever the calculator says, your healthcare provider's dating and guidance always come first. Bring any questions about timing to your prenatal appointments.