Pregnancy Due Date Calculator
Estimate your due date and current pregnancy week from how many days ago your last menstrual period started, your average cycle length, and the dating method. A quick projection your OB-GYN will refine with early ultrasound.
Last updated: May 2026
Compare with similar
About this calculator
The formula uses Naegele's rule: due date = first day of last menstrual period (LMP) + 280 days, with adjustment for non-standard cycle lengths. Standard 28-day cycle: due date is exactly 40 weeks (280 days) from LMP. Non-standard cycles: adjust by (cycle length − 28) days. So a 32-day cycle adds 4 days to the standard calculation; a 26-day cycle subtracts 2 days. The formula assumes ovulation occurs on day 14 of a 28-day cycle (mid-cycle); longer cycles typically have later ovulation and therefore later due dates. Edge cases: very irregular cycles (varying more than 5 days month to month) make LMP-based dating unreliable; in those cases, early ultrasound (transvaginal ultrasound at 6-10 weeks measures crown-rump length, accurate to ±5-7 days) is the gold standard for dating. The formula returns approximate due date; only about 4% of babies are born on the exact estimated due date, with most arriving within 2 weeks before or after. Term pregnancy is defined as 37-42 weeks (preterm before 37; post-term after 42); deliveries before 37 weeks have increased neonatal complications, and post-term pregnancies may be induced if labor doesn't occur naturally by 41-42 weeks. The 280-day standard is approximate — actual gestational length varies by ethnic background, prior pregnancy history (women with prior pregnancies often deliver slightly earlier), maternal age, and individual variation. For multiples (twins, triplets), expected delivery is typically earlier: twins at 36-37 weeks, triplets at 32-34 weeks on average.
How to use
Suppose your last period started 42 days ago, your cycle is 28 days, using Naegele's rule. Total gestation = 280 + (28 − 28) = 280 days, so 280 − 42 = 238 days remain, and you are in week floor(42 / 7) = 6 (first trimester). Result: "238 days to due date — currently week 6 (1st trimester)". A longer cycle adds (cycleLength − 28) days; the Mittendorf method uses 288 days for a first pregnancy (283 for subsequent).
Frequently asked questions
How accurate is the due date estimate?
Roughly ±2 weeks. Only about 4% of babies are born on the exact estimated due date. About 60% deliver within 1 week of due date; 90% within 2 weeks; 5% before 37 weeks (preterm); 5% after 41 weeks (post-term). The estimate is based on a 28-day cycle with ovulation on day 14, which not all women follow. First-trimester ultrasound (8-12 weeks) is more accurate than LMP-based dating for women with irregular cycles, those who can't remember their LMP precisely, or those who conceived shortly after stopping hormonal contraception. Second-trimester ultrasound is also useful for confirming dating. By third trimester, ultrasound becomes less accurate for dating because fetal growth varies between individual babies. The due date isn't a deadline — it's a target range, and going past it doesn't mean something is wrong unless complications arise.
How does Naegele's rule work mathematically?
Naegele's rule (developed by Franz Naegele in 1812): take the first day of LMP, subtract 3 months, and add 7 days plus 1 year. Mathematically equivalent: LMP + 280 days (40 weeks). The rule assumes a 28-day menstrual cycle with ovulation on day 14 (mid-cycle) and conception occurring on that day. This means gestational age is counted from LMP (about 2 weeks before actual conception), not from conception itself. "How many weeks pregnant" in clinical use refers to gestational age, so at LMP + 14 days you're "2 weeks pregnant" (with conception just happening). This convention persists because LMP is usually a known date while conception date is often uncertain. For cycles longer or shorter than 28 days, the rule needs cycle-length adjustment to reflect actual ovulation timing — the calculator handles this automatically by adding (cycle length − 28) days to the LMP-based date.
When is ultrasound dating more accurate than LMP?
Almost always more accurate, especially when LMP date is uncertain or cycles are irregular. First-trimester ultrasound (6-12 weeks) measures crown-rump length (CRL) of the embryo/fetus, which closely correlates with gestational age; accuracy is ±5-7 days. After 12 weeks, biparietal diameter (BPD), head circumference, and femur length are used, with accuracy declining as pregnancy progresses (±1-2 weeks at 20 weeks; ±2-3 weeks at 30 weeks). LMP-based dating is reliable when: cycles are regular (varying less than 5 days month-to-month), LMP date is clearly remembered, no hormonal contraception used in 3 months prior. When LMP and ultrasound disagree by more than 7 days, most obstetric practices use ultrasound dating because it's a direct measurement rather than a calculated estimate. The "estimated due date" in your medical records is typically the ultrasound-confirmed value, which may differ from your LMP-calculated date.
What are the most common mistakes people make calculating due date?
The biggest is using conception date instead of LMP without adjusting — conception date plus 280 days overestimates due date by about 14 days (since LMP is 2 weeks before conception). If you know your conception date precisely (e.g., from IVF or single intercourse during ovulation tracking), add 266 days, not 280. The second is ignoring cycle length for non-28-day cycles; women with long or short cycles get inaccurate due dates without adjustment. The third is using LMP from an unusual menstrual cycle (e.g., the first cycle after stopping birth control) which may be atypical. The fourth is failing to update the due date after early ultrasound finds significant discrepancy with LMP-based estimate. The fifth is anxiety about reaching the due date as a deadline; most births occur within 2 weeks before or after the estimate, and going slightly past isn't cause for concern unless complications arise. Finally, some couples calculate using "estimated conception date" rather than LMP, producing dates 2 weeks off from clinical estimates.
When should I not use this calculator?
Skip it for clinical pregnancy management — work with an OB-GYN who can confirm dating with early ultrasound and monitor pregnancy progress with appropriate tests. It is the wrong tool for women with very irregular cycles (cycles varying more than 5 days month to month), those who conceived shortly after stopping hormonal contraception, or anyone whose LMP date is uncertain — for those situations, early ultrasound is the gold standard for dating. Do not use it for IVF or other assisted-reproduction pregnancies; conception date is typically known precisely, and embryo transfer date plus 266 days is the correct formula. For high-risk pregnancies (advanced maternal age, multiple pregnancies, prior preterm births), monitoring goes beyond simple due date estimation. The calculator gives a target range, not a fixed delivery date; for medical decision-making (when to consider induction, when to be concerned about post-term, etc.), follow your OB's clinical judgment which factors in many variables beyond simple cycle-based math.