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

Not very — only about 4% of births happen on the estimated due date. Most babies arrive somewhere in the 37 to 42 week window, which is why the date works best as the center of a range rather than an appointment.

Which method gives the most accurate due date?

For IVF pregnancies, the known transfer date is the most precise anchor. Otherwise, an early ultrasound — which measures the fetus directly — is generally more accurate than last-period dating, especially with irregular cycles.

Can my due date change later in pregnancy?

Yes. Providers sometimes revise the due date when an ultrasound shows growth that doesn't match the period-based estimate, particularly when the first scan happens early. Once set by an early ultrasound, it usually stays fixed.

Due Date Calculator

Estimate your delivery date from your last menstrual period, an ultrasound, your conception date, or an IVF transfer date — and see where you are today: current week, trimester, your baby's average size, and a full week-by-week schedule to week 42.

Estimate based on

First day of your last period

Average length of your cycles

Result

Note: the estimated due date is calculated based on a single pregnancy rather than a twin or multiple pregnancy.

Advertisement — In-Article

(Responsive Fluid)

Estimating the due date

The due date — formally the estimated date of confinement — is the best single-day guess for when delivery will happen. It helps to hold it as the middle of a range, though: only about 4% of births actually occur on the estimated date. Four anchors are commonly used to set it. Last menstrual period is the default: birth occurs on average at a gestational age of 280 days (40 weeks) counted from the first day of the last period, so the due date is that day plus 40 weeks, adjusted when cycles run longer or shorter than 28 days. Gestational age is defined this way regardless of debates about when pregnancy "technically" begins — fertilization or implantation — because it is anchored to the period, not to either event. Pregnancies typically run 37 to 42 weeks on this clock.

Ultrasound dating compares the fetus's measured size against typical growth rates; it is quick, has no known risk to the baby, and is most accurate early in pregnancy. Conception date dating works like period dating shifted by about two weeks — the typical gap between the period's start and conception. IVF dating is the most precise of all, because the transfer date is known exactly; this calculator uses it with the embryo's age (day 3, 5, or 6), still mapping onto the same 40-week gestational clock.

The due date as a reference point

Where a birth lands inside the 37–42 week window is usually not a concern: 37–39 weeks is early term, 39–41 full term, and 41–42 late term, and babies in any of these ranges are generally healthy — with full-term babies tending to have the best outcomes. Outside the window matters more. Before 37 weeks a baby is preterm; labor starting too early (say at 33 weeks) may be stopped medically to avoid the complications of underdevelopment. After 42 weeks the pregnancy is postterm and labor is often induced: the placenta that supplies nutrition and oxygen can gradually stop functioning properly while the baby keeps growing and needing more, until the pregnancy can no longer support it adequately. That risk curve — not the single date — is what providers actually manage.

References

Moore, Keith. "How accurate are 'due dates'?" BBC, February 3, 2015.
Wikipedia: "Pregnancy" — gestational age and term classifications.
PubMed Health. "Pregnancy and birth: When your baby's due date has passed." 2014.

Related Calculators

Pregnancy CalculatorSchedule from any anchor date Pregnancy Weight Gain CalculatorIOM week-by-week targets Pregnancy Conception CalculatorWhen conception likely happened All Fitness & Health CalculatorsThe complete section