Pregnancy Due Date Calculator
Calculate your estimated due date and full trimester schedule from the first day of your last menstrual period.
Naegele's rule has been the standard for estimating due dates since the early 19th century. The calculation is simple enough to do in your head, add 280 days to the first day of your last period, but accounting for cycle length variation and getting the trimester schedule right takes a moment. This lays it all out clearly.
The estimated due date is an approximation, only about 5% of babies are born on their exact due date. A normal delivery window is 37–42 weeks. Due dates are most accurate when confirmed by a first-trimester ultrasound.
How due dates are calculated
Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. This calculation assumes a 28-day cycle with ovulation on day 14. For cycles longer or shorter than 28 days, the estimated due date adjusts, this calculator makes that adjustment automatically. Conception typically occurs around day 14 of a 28-day cycle, so gestational age (counted from LMP) is about 2 weeks more than fetal age.
The three trimesters
First trimester (weeks 1–13): Organ development begins. Morning sickness and fatigue are common. The highest risk period for miscarriage. Most prenatal genetic screening occurs in this window. Second trimester (weeks 14–27): Often called the "honeymoon trimester", energy typically improves, morning sickness resolves. Fetal movement (quickening) begins around weeks 16–22. Anatomy scan ultrasound typically at 18–20 weeks. Third trimester (weeks 28–40+): Rapid fetal weight gain. Preparation for birth. Third trimester discomforts (back pain, difficulty sleeping) are common. Labor typically begins between weeks 37–42.
Accuracy and confirmation
LMP-based dating is most accurate for women with regular 28-day cycles. Irregular cycles reduce accuracy significantly. First-trimester ultrasound (8–12 weeks) measures fetal size directly and is considered more accurate than LMP dating, particularly for women with irregular cycles or uncertain LMP dates. If there's a significant discrepancy between LMP dating and ultrasound dating, most providers use the ultrasound date.
Important medical note
This calculator provides an estimate for planning and information purposes only. All prenatal care decisions, due date confirmation, and medical guidance should come from your healthcare provider. If you have questions about your pregnancy, contact your OB, midwife, or healthcare team.
Frequently asked questions
What if I don't know my last period date?
Your healthcare provider can estimate gestational age through early ultrasound measurement of the embryo or fetus. Crown-rump length in the first trimester is the most accurate non-LMP dating method. If you have an irregular cycle or uncertain dates, bring that information to your first prenatal appointment.
Does a longer cycle mean a later due date?
Yes. If your cycle is 35 days rather than 28, ovulation occurs approximately 7 days later, which shifts the estimated due date about 7 days later as well. This calculator adjusts for cycle length automatically.
What does full term mean?
The American College of Obstetricians and Gynecologists defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37–38 weeks, late term is 41 weeks, and post-term is 42 weeks or beyond. The 40-week due date is the midpoint of the full-term window, not the absolute target.