Due Date Calculator
Estimated due date: --
How This Due Date Calculator Works
The default method is Naegele's rule, the standard formula used by clinicians for a quick estimate: it takes the first day of your last menstrual period (LMP) and adds 280 days (40 weeks), assuming a typical 28-day cycle with ovulation around day 14. If you enter a different average cycle length, the calculator adjusts the estimate by adding or subtracting the difference from 28 days. If you know your actual conception date instead (for example, from ovulation tracking or IVF), the calculator switches to adding 266 days (38 weeks) directly to that date, since conception itself marks the starting point rather than the LMP. Only about 5% of babies arrive on their exact estimated due date — it's a statistical midpoint, not a deadline, with full term spanning 37 to 42 weeks.
Why Cycle Length Matters
Naegele's rule assumes ovulation happens on day 14 of a 28-day cycle. If your cycles typically run longer or shorter, ovulation (and therefore conception) shifts along with it, which is why this calculator lets you adjust for a cycle length between 20 and 45 days. Someone with a consistently 35-day cycle likely ovulates later, pushing the real due date later than a plain LMP-plus-280-days calculation would suggest.
Tracking Your Pregnancy Milestones
The trimester and full-term markers above are calculated from the same starting point as your due date, so use them as a general roadmap rather than fixed appointments. If you're also tracking your cycle to plan or confirm a pregnancy, the ovulation calculator and period calculator can help estimate fertile windows and cycle timing. This tool provides an educational estimate only, not medical advice — your doctor or midwife will refine your due date using ultrasound measurements and your individual history, so always follow their guidance for actual prenatal care decisions.
Frequently Asked Questions
How accurate is a due date calculated this way?
It's a statistical estimate, not a guarantee — only about 5% of babies are born on their exact estimated due date. Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14, so actual delivery commonly falls anywhere within the 37-to-42-week full-term window. Your doctor may adjust the estimate using an early ultrasound, which is generally considered more precise than LMP-based dating alone.
What's the difference between calculating from LMP versus conception date?
Naegele's rule (the LMP method) adds 280 days (40 weeks) to the first day of your last menstrual period, because pregnancy is conventionally dated from LMP even though conception happens about two weeks later. If you know your actual conception date — from ovulation tracking, fertility treatment, or IVF transfer date — the calculator instead adds 266 days (38 weeks) directly to that date, which skips the assumption about when ovulation occurred and can be more accurate for irregular cycles.