Due date calculator

How to calculate your due date

1. Choose your calculation method: last menstrual period or conception date.
2. Select the date using the date picker.
3. Your estimated due date, gestational age, and trimester appear instantly.
4. Confirm the result with your healthcare provider.

Common uses

• Planning ahead for appointments, leave, and preparations around your estimated due date, then work out the baby's exact age from the birth date with an age calculator
• Sharing a quick gestational age and trimester estimate with a midwife or partner before a formal appointment
• Calculating the number of days or weeks until the due date alongside a date calculator

Technical specification

• Algorithm or formula: Naegele's rule, as described in ACOG Committee Opinion No. 700 (2017): due date = first day of last menstrual period + 280 days. From a conception date: conception + 266 days. Gestational age = days since the last period, with trimester boundaries at 13 and 27 completed weeks.
• Browser API or library: Native JavaScript Date. No external library.
• Input limits: A date on or before today, by your device's local date. A future date shows a hint instead of a result. Dates more than 42 weeks past LMP fall outside typical clinical use.
• Output: Estimated due date, gestational age (weeks and days), trimester, and weeks remaining, rounded up to whole weeks.
• Known limitations: Naegele's rule assumes a 28-day cycle with ovulation on day 14. Cycle-length adjustment is not exposed. Ultrasound dating is more accurate after the first trimester.

Frequently asked questions

What is Naegele's rule?

Naegele's rule is the standard method for estimating a pregnancy due date. It adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). The rule assumes a 28-day cycle with ovulation on day 14.

Should I use last menstrual period or conception date?

Use last menstrual period if you know that date, as it is the standard method used by healthcare providers. Use conception date only if you know it precisely, for example from IVF or closely tracked ovulation.

What does gestational age mean?

Gestational age is counted from the first day of the last menstrual period, not from conception. At 4 weeks gestational age, actual conception typically occurred around 2 weeks earlier. This is the standard measure used in clinical settings.

How accurate is this due date estimate?

The result is an estimate based on Naegele's rule and the dates you enter. Only around 5% of babies are born on their exact due date. Confirm your dates with a healthcare provider, who may use ultrasound measurements for a more precise estimate.