Real-World Use Cases

A woman in London whose last menstrual period started on March 10, 2026 enters the date and sees her estimated due date is December 15, 2026. She uses this to plan maternity leave, schedule prenatal appointments, and prepare the nursery.

A couple who conceived on April 1, 2026 after tracking ovulation enters the conception date and sees a due date of December 23, 2026. The calculator shows they are currently in their second trimester at 18 weeks and 2 days gestational age.

Someone who had a Day 5 embryo transfer on May 15, 2026 uses the IVF mode. The calculator accounts for the 5-day embryo age and shows a due date of February 8, 2027, with key trimester milestones mapped out.

A mother with an irregular cycle of 35 days adjusts the cycle length slider. Her LMP of February 1, 2026 gives a due date of November 15, 2026 instead of November 8, because the longer cycle means ovulation occurred later than the standard day 14.

How It Works

The calculator uses Naegele's Rule, the standard obstetric formula for estimating due dates. For the last menstrual period (LMP) method, it adds 280 days (40 weeks) to the first day of your last period. This assumes a 28-day cycle with ovulation on day 14.

If your cycle is longer or shorter than 28 days, the calculator adjusts the due date by the difference. A 35-day cycle adds 7 days, shifting ovulation and the due date later. A 24-day cycle subtracts 4 days, moving the due date earlier.

For the conception date method, the calculator adds 266 days (38 weeks), since conception typically occurs about 14 days after the LMP in a standard cycle.

For IVF, the calculator subtracts the embryo age from the 266-day total. A Day 5 transfer means the embryo was already 5 days old, so the calculator adds 261 days to the transfer date to reach the estimated due date.

Gestational age is counted from the first day of the last menstrual period, not from conception. This is the standard used by healthcare providers worldwide. At 4 weeks gestational age, the pregnancy is actually only 2 weeks old from conception.

Step-by-Step Usage Guide

1. Choose your calculation method: LMP, conception date, or IVF transfer.
2. Enter the relevant date in the date picker.
3. If using LMP, adjust the cycle length slider if your cycle is not 28 days.
4. If using IVF, select the embryo age (Day 3, 5, or 6).
5. The calculator shows your estimated due date, days remaining, gestational age, and current trimester.
6. Review the key pregnancy dates section for trimester boundaries and full-term date.
7. Use the copy button to share your due date with family or your healthcare provider.

Examples

Common Mistakes and Edge Cases

Things to keep in mind

• Naegele's Rule assumes a 28-day cycle: Only about 15% of women have a 28-day cycle. If your cycle is different, use the cycle length slider for a more accurate estimate. Cycles vary from 21 to 45 days.
• Only 5% of babies arrive on their due date: The due date is an estimate, not a deadline. Most babies are born within two weeks before or after the estimated date. First babies tend to arrive a few days later than the estimate.
• Ultrasound dating is more accurate: First-trimester ultrasound measurements can date a pregnancy more accurately than LMP-based calculation. If your ultrasound due date differs from this calculator, trust your healthcare provider's estimate.
• Irregular cycles: If your cycles are highly irregular (varying by more than 7 days between cycles), LMP-based dating is less reliable. Conception date or IVF transfer methods give better estimates in these cases.
• Gestational age vs fetal age: Gestational age counts from the LMP (about 2 weeks before conception). Fetal age counts from conception. At 8 weeks gestational, the fetus is 6 weeks old. Healthcare providers use gestational age exclusively.

FAQ

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Data Sources and Accuracy