Due Date Calculator

How Due Dates Are Calculated

The standard due date calculation uses Naegele's Rule, published in 1812 and still the foundation of obstetric dating: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14, which is accurate for roughly 20% of women. The rest have cycles ranging from 21 to 35 days, which can shift the true due date by a week or more in either direction.

Only about 5% of babies arrive on their exact due date. Approximately 80% arrive within two weeks of the due date — between 38 and 42 weeks. First-time mothers average 41 weeks and 1 day. Second and subsequent pregnancies average 40 weeks and 3 days. The due date is not a deadline — it is the center of a probability distribution where the peak of the bell curve falls at 40 weeks.

Trimester Milestones

First trimester (weeks 1-12): The embryo develops all major organ systems. Morning sickness peaks around weeks 8-10. The heartbeat is detectable by week 6-7 on ultrasound. The first prenatal visit typically occurs between weeks 8-12, where an ultrasound may adjust the due date by measuring crown-to-rump length — this ultrasound dating is more accurate than LMP dating for women with irregular cycles.

Second trimester (weeks 13-27): Often called the "golden trimester" because morning sickness fades, energy returns, and the pregnancy is not yet physically limiting. The anatomy scan at weeks 18-22 checks for structural development and can reveal the sex. Fetal movement (quickening) is typically felt between weeks 16-25, earlier in subsequent pregnancies.

Third trimester (weeks 28-40+): The baby gains roughly half a pound per week. Braxton Hicks contractions (practice contractions) may begin. The baby typically moves into a head-down position by weeks 32-36. Full term begins at week 37, though 39-40 weeks is considered optimal for delivery timing when there is no medical reason for earlier birth.

How accurate is this calculator?

This calculator uses the standard 280-day rule from your last menstrual period or 266 days from conception. Accuracy depends on knowing the correct start date and having a regular cycle. An early ultrasound (6-12 weeks) is the most accurate dating method and may adjust the LMP-based date by several days.

What if my cycles are irregular?

If your cycles are longer than 28 days, your due date may be later than this calculator shows. If shorter, it may be earlier. For irregular cycles, the dating ultrasound at 8-12 weeks is the best way to establish an accurate due date because it measures the embryo directly rather than estimating from cycle dates.