Pregnancy Due Date Calculator

The most common method uses Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle is longer or shorter than 28 days, the calculator adjusts automatically. Other methods include calculating from a known conception date (add 266 days), IVF transfer date (add 261 or 263 days depending on embryo type), or an ultrasound measurement.

A full-term pregnancy is defined as 39 weeks 0 days through 40 weeks 6 days. Early term is 37-38 weeks, full term is 39-40 weeks, late term is 41 weeks, and post-term is 42 weeks and beyond. The total duration from LMP to delivery is approximately 280 days or 40 weeks, though most healthy babies are born between 37 and 42 weeks.

Only about 4-5% of babies are born on their exact due date. The due date is an estimate — most babies arrive between 37 and 42 weeks of gestation. LMP-based calculations assume a 28-day cycle with ovulation on day 14, which varies between women. An early ultrasound (before 12 weeks) is the most accurate dating method, accurate to within 5-7 days.

Yes. Your doctor may adjust your due date if an ultrasound-based estimate differs significantly from the LMP-based date. ACOG guidelines define the threshold by gestational age: more than 5 days before 9 weeks, more than 7 days at 9-13 weeks, and more than 10-14 days in the second trimester. This is common when cycle lengths are irregular or when the exact LMP date is uncertain. Once established by early ultrasound, the due date generally should not be changed based on later scans.

Gestational age counts from the first day of the last menstrual period (LMP) — which is about 2 weeks before conception actually occurs. Fetal age (also called conceptional age) counts from the actual date of fertilization. So fetal age is always about 2 weeks less than gestational age. Medical professionals and due date calculators use gestational age as the standard.

If you're unsure about your last period, you can use other methods: a known conception date, an IVF transfer date, or — most commonly — an early ultrasound. Your doctor can estimate gestational age from crown-rump length measurements during a first-trimester ultrasound. This is often the most reliable dating method, especially for women with irregular cycles.

If your cycle is longer than 28 days, ovulation occurs later, and your due date shifts later by the same number of extra days. For example, a 35-day cycle means ovulation around day 21 instead of day 14, so the due date is 7 days later than the standard LMP calculation. The reverse applies for shorter cycles. Our calculator adjusts automatically when you enter your cycle length.

Naegele's Rule is the standard formula for estimating a pregnancy due date. It was proposed by German obstetrician Franz Naegele in 1812. The formula adds 280 days (or equivalently, subtracts 3 months and adds 7 days) to the first day of the last menstrual period. While widely used, it assumes a 28-day cycle and ovulation on day 14, which is why adjustments may be needed.