Pregnancy Due Date Calculator
Enter the first day of your last period, and this calculator estimates your due date using Naegele's rule.
About
About Pregnancy Due Date Calculator
The standard medical method for estimating a due date, Naegele's rule, adds 280 days, exactly 40 weeks, to the first day of the last menstrual period, a formula that's been the obstetric standard for generations even though it's genuinely just a statistical estimate. Only a small fraction of babies actually arrive on their calculated due date; most full-term births happen somewhere in the 38-to-42-week window. This calculator applies that same formula so you get the standard clinical estimate your provider will reference too.
Gestational milestones, from the first ultrasound to key prenatal screenings, are all scheduled relative to estimated due date rather than conception date, which is why having an accurate calculation matters for more than just curiosity, it actually drives your entire prenatal care calendar.
A due date is a statistical midpoint, not a prediction of the actual delivery day. Understanding that upfront, that 38 to 42 weeks is the normal full-term range rather than a narrow target, helps set realistic expectations rather than treating the calculated date as a hard deadline.
Enter the first day of your last menstrual period into the date picker. The calculator returns your estimated due date, current gestational age in weeks, and key trimester transition dates.
Pregnancy details are deeply personal health information. Calculations run entirely in your browser, with nothing transmitted to a server.
FAQ
Frequently asked questions
How is a due date actually calculated?
Using Naegele's rule, which adds 280 days (40 weeks) to the first day of the last menstrual period, the standard formula obstetric providers reference.
How accurate are due date calculators really?
They're statistical estimates, not predictions, only a small percentage of babies arrive exactly on the calculated date. Most full-term births happen anywhere between 38 and 42 weeks.
Does this work for IVF pregnancies?
Not directly with this formula, IVF due dates are typically calculated from the specific embryo transfer date and embryo age (commonly a 3-day or 5-day transfer) rather than last menstrual period.
Why did my doctor give me a different due date than this calculator?
An early ultrasound dating, especially in the first trimester, is generally considered more accurate than a calculation based on last menstrual period alone, since it directly measures fetal size rather than assuming a textbook-average cycle length and ovulation timing.
How many babies are actually born on their exact due date?
Only about 5 percent. Full-term delivery spans a window of roughly 37 to 42 weeks, so a due date is genuinely a statistical estimate of the most likely range rather than a precise prediction of the actual birth day.
Does this work if I have an irregular cycle?
Less reliably, since the standard calculation assumes a 28-day cycle with ovulation around day 14. With an irregular cycle, a due date based on a dated ultrasound from your provider is considerably more accurate than a last-menstrual-period calculation.
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