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Estimate a due date from the last menstrual period using Naegele's rule.
2026-10-08
This calculator estimates a pregnancy due date using Naegele's rule -- adding 280 days (40 weeks) to the first day of the last menstrual period (LMP) -- the standard estimation method used in obstetric care, alongside the current gestational age in weeks and days.
Enter the first day of your last menstrual period. The result shows the estimated due date and, as of today, your current gestational age and days remaining until the due date.
Due date = LMP date + 280 days (40 weeks), based on a standard 28-day cycle. Gestational age = days elapsed since the LMP, expressed in completed weeks plus remaining days.
An LMP of January 1, 2026 gives an estimated due date of October 8, 2026.
For general elapsed-time tracking of any milestone date, see the Days Since Date Calculator.
This page is an educational estimate, not medical advice. Anyone who is or may be pregnant should confirm dating, prenatal care and any symptoms with a qualified healthcare provider. If something feels wrong, contact your provider or local emergency services rather than relying on a calculator.
Consider a hypothetical last menstrual period that began on March 10, 2026. Adding 280 days gives the estimated due date. March 10 plus 21 days reaches March 31, and the remaining 259 days carry through the following months to arrive at December 15, 2026. If you check the tool on June 2, 2026, 84 days will have passed since the LMP, which is 12 weeks and 0 days of gestational age. Clinicians express gestational age as completed weeks plus days, such as 12w0d, and that is the format to expect in the result.
Due date = first day of last period + 280 days. Gestational age = days since that date, shown as whole weeks and leftover days.
A source of confusion is that pregnancy is counted from the first day of the last period, not from conception. Conception typically happens roughly two weeks later in a standard cycle, so a baby is about 38 weeks old at term while gestational age reads 40 weeks. This convention exists because the LMP date is usually known while the conception date rarely is. Do not be alarmed if a fetal age quoted elsewhere is about two weeks less than the number shown here.
The days-remaining figure counts down to the estimated date, but many babies arrive before or after it. Full term is generally considered to be a range of weeks rather than a single day, and your provider will explain what applies to your situation. Use the tool to prepare questions for appointments, to plan leave from work, or to line up childcare and travel, always keeping a buffer around the date. If a later ultrasound gives a different estimated date, your provider's date takes precedence over the one calculated here.
It's a widely used estimate, but actual due dates vary -- only about 5% of babies are born on their exact estimated due date. It also assumes a standard 28-day cycle with ovulation on day 14, which doesn't apply to everyone. A clinical ultrasound dating is generally more precise.
No -- use this as a quick reference only; your healthcare provider's clinical dating (often via ultrasound) is the authoritative estimate for prenatal care decisions.
Naegele's rule assumes a 28-day cycle -- if yours is significantly longer or shorter, the estimate may be less accurate; your healthcare provider can adjust for your specific cycle length.
No. Use the first day of your last menstrual period, because the 280-day rule is measured from that day.
Gestational age is counted from the last period, before conception occurred, so it runs about two weeks ahead of the time since conception.
Follow your healthcare provider's date. Ultrasound dating, especially early in pregnancy, is often considered more reliable than an LMP-based estimate, particularly when cycles vary.
Yes, but leave room for babies arriving earlier or later, and check your employer's policy and local rules for leave eligibility with the relevant official source.
Reference: U.S. Department of Labor