Pregnancy Due Date Calculator

1. 01Enter the first day of your last periodNaegele's rule counts from the first day of your last menstrual period, not the date of conception.
2. 02Adjust your average cycle length (optional)The default is 28 days. If your cycles are typically longer or shorter, adjusting this improves the estimate.
3. 03Read the resultYour estimated due date, current week of pregnancy, trimester, and days remaining appear instantly.

• Early pregnancy planningGet a quick estimate of your due date and current week before your first prenatal appointment, where a doctor will typically confirm or adjust it with an ultrasound.
• Tracking trimester milestonesSee at a glance which trimester you are in as your pregnancy progresses.
• Estimating a conception dateGet an estimated conception date based on typical ovulation timing for your cycle length.

• Standard 28-day cycleLast period started January 1 → estimated due date is October 8, 280 days later (40 weeks).
• Longer cycle adjustmentWith a 35-day average cycle (7 days longer than the 28-day assumption), the estimated due date shifts 7 days later than the standard calculation, since ovulation likely happened later too.

Naegele's rule estimates a due date as 280 days (40 weeks) after the first day of the last menstrual period, assuming a typical 28-day cycle with ovulation around day 14. This calculator adjusts that estimate for a different average cycle length you enter — a longer cycle shifts the estimated due date later, a shorter cycle shifts it earlier, reflecting a later or earlier estimated ovulation day.

This is a population-average estimate, not a precise prediction. Only a small fraction of babies are born exactly on their estimated due date — most arrive within about two weeks before or after it. A first-trimester ultrasound can establish or confirm gestational age more accurately than LMP-based dating alone, which is why doctors commonly use one to set or adjust the due date, especially for people with irregular cycles.

This calculator assumes a roughly regular cycle and a known, accurate last-period start date. It is less accurate for irregular cycles, for pregnancies conceived shortly after stopping hormonal birth control, or if you are unsure of your last period date — in any of these cases, a first-trimester ultrasound from a healthcare provider is more reliable. This tool provides an educational estimate only, not medical advice, a diagnosis, or a guarantee of when labor will begin, and it does not replace the due date established or confirmed by your obstetric care provider.

What is Naegele's rule?

Naegele's rule is a standard method for estimating a due date: add 280 days (40 weeks) to the first day of the last menstrual period, assuming a 28-day cycle.

Why does it ask for the first day of my last period, not the conception date?

Most people know their last period date more precisely than their exact conception date, since ovulation and conception typically happen roughly two weeks into the cycle and can be hard to pinpoint exactly.

How accurate is the estimated due date?

It is a statistical estimate, not a guarantee — only a small percentage of babies arrive exactly on their estimated due date. Most births happen within about two weeks before or after it. A first-trimester ultrasound from a healthcare provider can establish or confirm gestational age more accurately, and this calculator does not replace the due date set by your obstetric care provider.

What if my cycle is irregular?

This calculator is less accurate for irregular cycles, since it assumes a roughly consistent average length. If your cycles vary significantly, a first-trimester ultrasound from a doctor will be more reliable than an LMP-based calculation.

What trimester am I in?

The first trimester runs through week 13, the second trimester from week 14 through week 27, and the third trimester from week 28 until birth.

Is my data saved?

No. Everything is calculated in your browser; nothing is sent to a server.