Free due date calculator using Naegele's Rule: estimate your baby's arrival date from your last period or conception date, week by week.
Pregnancy is measured in **40 weeks** from the first day of your last menstrual period (LMP) — not from the date of conception, which typically occurs about 14 days later. This dating convention, based on Naegele's Rule, is the global standard used by obstetricians and midwives for prenatal care. The reason LMP is used is simple: most women know the date of their last period, while the exact day of ovulation or conception is often difficult to pinpoint without specialized tracking.
The 40-week timeline is divided into three **trimesters**. The first trimester (weeks 1–13) covers the critical period of organ development. The second trimester (weeks 14–27) is when fetal movement becomes noticeable and many early pregnancy symptoms ease. The third trimester (weeks 28–40) is the final growth phase when the baby gains most of its weight and the body prepares for delivery. Knowing which trimester you're in helps you anticipate upcoming tests, physical changes, and milestones.
This calculator supports two calculation methods. Choose **Last Menstrual Period (LMP)** if you know the date your last period started — this is the most common approach. Choose **Conception Date** if you have been tracking ovulation or have a known date from fertility treatment. The **Cycle Length** slider lets you adjust for irregular cycles: a longer cycle shifts the due date later (later ovulation), while a shorter cycle shifts it earlier. The calculator then shows your estimated due date, current week of pregnancy, remaining days, and trimester.
The calculator uses Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the estimate adjusts by the difference — a 30-day cycle adds 282 days instead of 280, while a 25-day cycle subtracts. This is the same formula used by most OB-GYNs for initial pregnancy dating.
The due date is an estimate, not a guarantee. Only about 5% of babies are born on their exact due date. About 80% arrive between 37 and 42 weeks. Your first-trimester ultrasound (8–12 weeks) provides the most accurate dating and may adjust your due date. The due date is best used as a planning window, not a precise deadline.
If your cycle is irregular or not 28 days, use the cycle length slider to adjust. For example, if your cycle is typically 32 days (4 days longer than average), the calculator adds 284 days instead of 280. If you're unsure of your LMP or have very irregular cycles, use the conception date method if you know your ovulation date. Your doctor will confirm the due date with an ultrasound, which is the gold standard for irregular cycles.
Naegele's Rule is named after Franz Karl Naegele (1778–1851), a German obstetrician who taught at the University of Heidelberg. In 1812 he published a textbook that included this method for estimating the due date. The rule became the global standard because it requires only one piece of information most women reliably know — the date of their last period — making it practical in any clinical setting. While modern ultrasound dating has improved accuracy, Naegele's Rule remains the first-line method used worldwide for initial pregnancy dating.
Yes — switch to Conception Date mode. If you know your ovulation or conception date (e.g., from tracking or fertility treatment), the calculator adds 266 days (38 weeks) to that date. This can be more accurate than using LMP if your ovulation date is well established. Note that the calculator estimates your LMP as conception date minus 14 days for pregnancy week tracking purposes.
The 40-week convention counts from the first day of your LMP, which includes the roughly two weeks before ovulation and conception. That is why a full-term pregnancy is about 38 weeks from conception but 40 weeks from LMP.