Due Date Calculator

How a due date is calculated

The most common method, called Naegele rule, adds 280 days (40 weeks) to the first day of your last menstrual period. The rule was published by German obstetrician Franz Naegele in the 1800s and remains the standard worldwide. If conception is known, an alternative is to add 266 days, since pregnancy is conventionally dated from about two weeks before conception. The calculator handles both methods.

Why pregnancy counts from the last period

Counting from the last menstrual period is a historical convention from a time before ovulation testing. The LMP is a date most women can date reliably, while conception is rarely known to the day. By convention, a baby is therefore 4 weeks old at conception and 40 weeks at delivery, even though the actual fertilisation happens midway through the second week of the count. Every doctor, hospital, and pregnancy resource worldwide uses the same convention.

How accurate is a due date?

A due date is an estimate, not a deadline. Only about 1 in 20 babies (5 percent) arrives on the exact date. Roughly 80 percent are born within two weeks either side, fairly evenly split before and after. The full-term window is week 37 to week 42, and care guidelines treat any birth in that span as on time. Doctors use the due date primarily to schedule check-ups and screen for late-term pregnancies, not as a sharp prediction of the actual birth day.

What is gestational age?

Gestational age is how far along a pregnancy is, counted in weeks and days from the first day of the last menstrual period. It is the figure used at every prenatal visit and in every clinical guideline. Conception age, sometimes called fetal age, is two weeks less than gestational age. The calculator reports gestational age in weeks and days based on today and the LMP you provided, so you always know how many weeks pregnant you are.

The three trimesters

The first trimester runs from week 1 through week 13, the second from week 14 through week 27, and the third from week 28 through delivery. Each trimester has its own typical events: organ formation and the first major scans in the first trimester, rapid growth and quickening in the second, and final preparation for birth in the third. Many guidelines and pregnancy resources are organised by these three blocks.

Cycle length and dating

Naegele rule assumes a regular 28-day cycle with ovulation on day 14. If your cycle is consistently longer or shorter, the LMP-based due date can be off by a few days. For example, with a 32-day cycle, ovulation typically happens around day 18, so the calculated due date may be about 4 days early. A first-trimester ultrasound is much more accurate than any calendar method and is what doctors use to confirm or revise the due date if needed.

Ultrasound scans

A dating ultrasound is usually offered between 8 and 12 weeks of gestation. The fetal size at this stage gives a very accurate due date because all babies develop at a similar rate in the first trimester. The anatomy ultrasound, which checks the baby's development in detail, follows between 18 and 22 weeks. Many places also offer growth scans in the third trimester if there are concerns.

IVF and assisted conception

For an IVF pregnancy the embryo transfer date is known precisely, so the conception-date option works well. Enter the transfer date adjusted for the embryo age at transfer (3 days for a day-3 transfer, 5 days for a day-5 blastocyst) and the calculator gives 266 days later as the due date. This is more reliable than LMP-based dating for IVF because the timeline of fertilisation is known exactly.

Going past your due date

It is common. Around 50 percent of pregnancies extend to week 41, and care guidelines usually allow up to a week or two past the due date with extra monitoring before considering induction. Policies vary between countries and hospitals. Talk to your obstetrician or midwife about local practice and what monitoring will be offered if you go past your due date.