Pregnancy Estimate
Enter your last menstrual period (LMP) date and cycle length to estimate your due date using Naegele's rule.
Normal range: 20–45 days (typical is 28)
Estimated Due Date
September 7, 2025
Sunday
This due date is in the past.
This calculator uses Naegele's rule and assumes a regular cycle. The due date is an estimate—only about 5% of babies arrive on their exact due date. Always follow your healthcare provider's guidance.
A due date calculator estimates the date a baby is expected to be born, typically 280 days (40 weeks) after the last menstrual period. Naegele's rule, the most widely used method, adds 280 days to the LMP date and adjusts for cycle length variations. The calculator also tracks your gestational age (how far along you are in weeks and days), your current trimester, and time remaining until the due date. Due dates are estimates, not certainties—less than 10% of pregnancies result in labour on the calculated date.
Enter the first day of your last menstrual period and your average cycle length (default 28 days). The calculator applies Naegele's rule: due date = LMP + 280 days + (cycle length − 28 days). It then compares today's date to show your current gestational age in weeks and days, identifies your trimester (first 13 weeks, second 13–27 weeks, third 27+ weeks), and counts down weeks and days remaining until the due date.
Due Date = LMP + 280 days + (Cycle Length − 28 days)LMP is the first day of your last menstrual period. The base is 280 days (40 weeks), which represents the typical human gestation period. Cycle length (usually 20–45 days) is used to adjust for individual variation: longer cycles push the due date forward; shorter cycles pull it back. Gestational age is calculated as the number of days from LMP to today, divided by 7 for weeks.
| Input | Result | Notes |
|---|---|---|
| LMP: 1 December 2024, Cycle: 28 days | Due date: 8 September 2025 (Tuesday); Gestational age ~7 weeks 4 days (if calculated on 28 January 2025) | Standard 28-day cycle; due date falls 280 days after LMP, which is the baseline gestation period. |
| LMP: 15 October 2024, Cycle: 32 days | Due date: 29 July 2025 (Wednesday); Gestational age ~12 weeks 3 days (if calculated on 28 January 2025) | Longer 32-day cycle adds 4 days to the baseline; due date shifts later than a 28-day cycle would predict. |
| LMP: 1 November 2024, Cycle: 25 days | Due date: 2 August 2025 (Friday); Gestational age ~12 weeks (if calculated on 28 January 2025) | Shorter 25-day cycle subtracts 3 days from the baseline; due date is earlier than standard Naegele's estimate. |
Naegele's rule, developed by Hermann Ludwig Naegele in 1812, is the standard formula for estimating due dates: add 280 days (40 weeks) to the LMP, adjusted for cycle length. It remains the most widely used method in obstetrics globally.
Because conception date is often uncertain—ovulation and fertilisation are not directly observable. LMP is the most reliable reference point available to most people. Clinical convention defines gestational age from LMP, so all healthcare providers use the same standard.
Very accurate for establishing a baseline estimate—typically within 3–5 days for regular cycles. However, early ultrasound (8–14 weeks) is more accurate (±3–4 days). Due dates may be adjusted after ultrasound; always follow your healthcare provider's assessment.
If your cycle varies by more than a few days month to month, enter your average length. If cycles are highly irregular or you're unsure of your LMP, an early ultrasound is essential for accurate dating. Providers will often date from ultrasound findings rather than LMP in these cases.
Full term is 37–42 weeks of gestation. A baby born before 37 weeks is considered preterm; after 42 weeks is postterm. Labour can be induced at 39 weeks if there's a medical reason, or at 42 weeks if labour hasn't started.
If you don't know your LMP, an early pregnancy ultrasound (8–14 weeks) can date the pregnancy accurately. If you have irregular cycles, use your average cycle length or ask your healthcare provider for guidance. An ultrasound is the gold standard when history is unclear.