Pregnancy Due Date Calculator

How to use Pregnancy Due Date Calculator

1. Choose last menstrual period (LMP) or conception date.
2. Pick the date. For LMP, also enter your average cycle length in days.
3. Read your estimated due date and your gestational age today.
4. Check the trimester date ranges and share the estimate with your doctor or midwife.

How due dates are estimated

Pregnancy is dated from the first day of the last menstrual period, not from conception, because that date is usually known while conception is not. This is why a full-term pregnancy is described as 40 weeks even though the baby has only been developing for about 38 weeks. Naegele’s rule, the traditional method, adds 280 days to the first day of the last period and assumes a regular 28-day cycle with ovulation around day 14.

If your cycles are longer or shorter, ovulation happens later or earlier, so this calculator shifts the due date by the difference between your cycle length and 28 days. If you know the conception date, for example after fertility treatment, it adds 266 days instead.

Formula, trimesters and worked example

Due date = LMP + 280 days + (cycle length − 28). From conception: due date = conception date + 266 days. Gestational age today = today − (due date − 280 days). The trimesters used here are first: weeks 0 to 13 and 6 days, second: weeks 14 to 27 and 6 days, third: week 28 to the due date.

Example: an LMP of 10 January 2026 with a 28-day cycle gives a due date of 17 October 2026. With a 35-day cycle the due date moves 7 days later, to 24 October 2026. A conception date of 24 January 2026 also gives 17 October 2026, since 24 January plus 266 days is that date.

Important limits and a clear disclaimer

This tool provides an estimate only and is not medical advice. Only a healthcare professional can confirm gestational age, usually with an early ultrasound, and they may revise the due date. Irregular cycles, recent hormonal contraception, uncertain dates and multiple pregnancies all reduce the accuracy of a calculated date.

• Contact your doctor or midwife as soon as you think you may be pregnant.
• Seek urgent care for bleeding, severe pain or any symptoms that worry you.
• Trimester boundaries differ slightly between sources; the ones here are a common convention.
• Dates beyond 11 months ago or in the future are rejected as likely typing errors.
• Your dates are calculated in your browser and are never sent or stored.

Why use this pregnancy due date calculator tool

• Two methods. Use the last period or a known conception date.
• Cycle adjustment. Corrects the date if your cycle is longer or shorter than 28 days.
• Weeks pregnant today. Shows gestational age in weeks and days.
• Trimester dates. Start and end dates for all three trimesters.

Frequently asked questions

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