Estimate your baby's due date based on your last menstrual period.
Fill in the fields on the left, then press Calculate to see your result instantly. This tool provides general estimates for informational purposes and does not replace advice from your doctor or midwife.
The most common method for estimating a due date is Naegele's rule, developed by German obstetrician Franz Naegele in the 19th century: adding 280 days (40 weeks) to the first day of your last menstrual period (LMP), based on the assumption of a standard 28-day cycle with ovulation on day 14. This calculator also adjusts slightly for cycle lengths longer or shorter than 28 days, since ovulation timing shifts accordingly and a more personalized LMP-based estimate accounts for this rather than assuming everyone's cycle matches the textbook average. It's important to keep expectations realistic: only about 5% of babies are actually born exactly on their calculated due date, with the large majority born within a window of roughly two weeks before or after it — a due date is a statistically reasonable estimate of when labor is likely, not a guarantee or a deadline, and healthcare providers generally consider a birth anywhere from 37 to 42 weeks to be within the normal full-term range.
Few numbers in pregnancy generate as much anticipation as the due date, yet it's worth understanding upfront that it functions statistically more like the center of a bell curve than a precise appointment. Naegele's rule, the calculation behind most due date estimates, was developed in the 19th century by German obstetrician Franz Naegele and remains the standard method used by healthcare providers worldwide, over 150 years later.
The rule itself is simple: add 280 days, or 40 weeks, to the first day of your last menstrual period. That number assumes a standard 28-day cycle with ovulation occurring on day 14 — a reasonable population average, but one that doesn't perfectly match everyone's actual cycle, which is why more refined versions of the calculation adjust slightly for cycles that run longer or shorter than 28 days.
The statistic that surprises most expectant parents is just how rarely babies arrive precisely on their calculated due date — commonly cited figures put it around 5%, meaning the overwhelming majority of births happen either before or after the estimated date. Medical guidelines generally define a full-term, low-risk delivery window spanning from 37 to 42 weeks, a range spanning five full weeks, which underscores just how much natural variation exists even in healthy, uncomplicated pregnancies.
Ultrasound dating, particularly when performed in the first trimester, is generally considered the most accurate dating method available, since it measures the fetus directly (commonly using crown-rump length in early pregnancy) rather than relying on menstrual history, which can be affected by irregular cycles, uncertain LMP recall, or ovulation timing that doesn't match the 14-day assumption. This is why a due date is sometimes revised after an early ultrasound if the measured fetal size suggests a meaningfully different gestational age than LMP-based dating implied.
Practically, the most useful way to think about a due date is as the midpoint of a normal, wide range rather than a deadline to plan precisely around. Most healthcare providers actively discourage treating it as anything more definite than that, since anxiety around an approaching or passed due date is common but rarely medically warranted on its own — spontaneous labor onset within the broader full-term window is the biological norm, not the exception.
Most babies are born within a window of about 2 weeks before or after the estimated due date.
Yes, an early first-trimester ultrasound is generally considered the most accurate way to date a pregnancy.
Early ultrasound measurements (particularly in the first trimester) are considered more accurate than LMP-based dating alone, since they directly measure fetal size rather than relying on menstrual history and assumed ovulation timing, which can vary from the textbook 14-day assumption. If ultrasound dating differs meaningfully from LMP dating, providers often adjust the due date to the ultrasound estimate.