How is my due date calculated?

Most due dates come from one simple sum: 280 days from the first day of your last period. Here is the exact method, a worked example, and how ultrasound, conception and IVF dating change the answer.

By the Calculator Den team · Updated 2026-10-17

If you have just seen a positive test, the first question is almost always the same: when is the baby due? The answer comes from a surprisingly simple sum that has been used for nearly two hundred years. It is worth understanding how it works, because the method makes two assumptions that are not always true for you, and knowing that tells you when your real due date is likely to shift. This is general information, not medical advice, so always go by what your own midwife or doctor tells you.

In short: the standard due date is 280 days (40 weeks) from the first day of your last menstrual period (LMP). That is Naegele's rule. A quick way to do it by hand: take the first day of your last period, add one year, subtract three months, and add seven days. A free due date calculator does it instantly, and a first-trimester ultrasound later becomes the most accurate check.

The standard method: Naegele's rule

Pregnancy is counted as 40 weeks, but here is the twist that confuses almost everyone: it is counted from the first day of your last period, not from the day you conceived. Because you usually ovulate and conceive about two weeks after your period starts, you are already considered "two weeks pregnant" on the day you actually conceive. It is a dating convention, not a biology error, and it exists because almost everyone knows the date their last period started, while very few know the exact day they conceived.

So the sum is simply: first day of last period + 280 days. Naegele's rule, named after the German obstetrician Franz Naegele who described it in 1812, is the hand shortcut for that: add a year, go back three months, add seven days.

A worked example

Say the first day of your last period was 1 March 2026.

• Add one year: 1 March 2027.
• Subtract three months: 1 December 2026.
• Add seven days: 8 December 2026.

That gives roughly 8 December 2026. One honest caveat: Naegele's rule is a hand shortcut, and because months have different lengths it can differ from an exact day count by a day or two. Counting exactly 280 days from 1 March 2026 actually lands on 6 December 2026, while the rule gives 8 December. Calculators and the paper "wheels" clinics use normally count the exact 280 days, so treat the rule as a close mental estimate and the day count as the precise version. The due date calculator does the exact counting for you and also shows how many weeks along you are today, which is the number your appointments are scheduled around.

The two assumptions baked into the sum

Naegele's rule quietly assumes two things, and this is the part most pages skip:

• A 28-day cycle. The rule assumes you ovulate on day 14. If your cycles run longer, say 32 days, you ovulate later, so your true due date is a few days later than the sum suggests. Shorter cycles pull it earlier. A rough adjustment: add the number of days your cycle is over 28 (or subtract if it is under). For a 32-day cycle, that is about four days later.
• A known, regular period date. If your last period was irregular, you were on hormonal birth control recently, or you simply are not sure of the date, the LMP sum can be well off.

This is exactly why your provider does not rely on the period date alone.

Ultrasound dating: the more accurate check

A dating scan in the first trimester is generally considered the most accurate way to estimate your due date. The sonographer measures the length of the baby from head to bottom (the crown-rump length) and compares it to known growth charts, which in early pregnancy are very consistent from one pregnancy to the next. An early scan is typically accurate to within about five to seven days.

Providers usually combine both methods: they start with your LMP date, then check it against the scan. The American College of Obstetricians and Gynecologists (ACOG) gives specific thresholds in its guidance on estimating the due date: at 8 weeks 6 days or earlier, the scan date is used if it differs from the period date by more than 5 days; from 9 to 13 weeks 6 days, the cutoff is more than 7 days. Otherwise the period date is kept. The first-trimester scan is the most reliable dating tool, and dating scans later in pregnancy are less accurate, which is why early dating matters.

Dating from conception or ovulation

If you happen to know your conception or ovulation date, for example from ovulation tests or tracking, you can date from there instead: add about 266 days (38 weeks) to the conception date. Notice it is 266, not 280, because conception dating skips the roughly 14 days the LMP method counts before you conceived. A conception calculator works the same idea in reverse, estimating when conception likely happened, and an ovulation calculator helps pin down your fertile window in the first place.

IVF due dates are based on a known embryo age

If you conceived through IVF, your due date is actually the easiest to pin down, because the exact age of the embryo is known. The clinic counts from the egg retrieval or transfer date and the embryo's age:

• For a day-5 (blastocyst) transfer, the due date is about 261 days from the transfer date.
• For a day-3 transfer, it is about 263 days from the transfer date.

Because there is no guesswork about when fertilisation happened, IVF dating does not carry the cycle-length uncertainty that the LMP method does. With IVF the transfer date is normally used for dating, and a later scan does not usually change it. The same applies to a frozen embryo transfer, where the transfer date and the embryo's age at freezing set the dating.

Why a due date is a window, not a deadline

Here is the most useful thing to internalise: your due date is a single calculated day, but birth is a range. Babies born from 37 to 42 weeks are generally considered term, and relatively few arrive on the exact estimated date itself. ACOG splits that range into early term (37 to 38 weeks 6 days), full term (39 to 40 weeks 6 days), late term (41 weeks to 41 weeks 6 days) and post-term (42 weeks and over), so "full term" has a narrower meaning than the whole window. Most births land in the couple of weeks on either side of the due date, so treat it as the middle of a window rather than a deadline. That is why providers watch how you progress as you approach and pass 40 weeks instead of treating the calculated day as a hard line.

Thinking of it as "early December" rather than "the 8th" also helps keep the final weeks in perspective. That said, a due date is not a safety guarantee. Contact your midwife or doctor straight away if you have vaginal bleeding, strong or constant pain, your waters break, you notice your baby moving less than usual, or you have regular contractions before 37 weeks. If you reach 41 weeks or beyond, your provider will normally discuss closer monitoring or induction with you, though exactly when varies by country and by your own situation.

How accurate is a due date?

Not very, as a single day, and that is normal. Only around 4 to 5 percent of babies are born on their exact estimated due date; the rest arrive across the weeks around it, which is why it is an estimate rather than an appointment. An early dating scan is the most precise single check, typically within about five to seven days, but even that is a best estimate of a natural range. The practical takeaway is to prepare for a window of a few weeks, not to count down to one date.

Comparing the methods at a glance

Whatever the calculator says, your midwife or doctor will confirm and may adjust your date, usually after the dating scan. That confirmed date is the one your care is planned around. This article is general information and does not replace medical advice.

Common questions

What if my cycles are irregular or I do not know my last period? The LMP sum is unreliable here, so an early dating scan matters more and will usually set your date. Tell your provider if your cycles are long, short or irregular, or if you recently stopped hormonal birth control or were breastfeeding, as all of these can shift ovulation.

Why does the clinic's date differ from an online calculator? A calculator uses your period date and a fixed 280-day count. The clinic adds the dating scan and your cycle details, so their date is usually the more accurate one once you have been seen. If you are tracking your fertile days, the ovulation and conception calculators can help you understand the timing behind the number.

Does the due date change through pregnancy? Your estimated date is normally set once, early on, from the best available information, and then kept. Later scans check the baby's growth rather than re-dating the pregnancy.