Estimate the date your baby was likely conceived.
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.
Conception typically occurs around ovulation, roughly 14 days after the start of your last menstrual period in an average 28-day cycle, though this timing shifts earlier or later for shorter or longer cycles respectively, since ovulation timing (not the follicular phase length) is what varies most between individuals. If you already know your due date instead, this calculator works backward — subtracting 266 days (38 weeks) from the due date, since pregnancy is conventionally dated as 40 weeks from the last menstrual period (LMP), but actual biological gestation counted from conception itself is closer to 38 weeks, a two-week difference that reflects the gap between LMP and ovulation rather than any inconsistency in how pregnancy is measured. Because sperm can survive in the reproductive tract for up to five days while an egg remains viable for roughly 24 hours after release, conception could technically have occurred anywhere within a several-day window around ovulation, which is why this calculator, like most conception estimates, provides a most-likely date rather than a certainty.
Estimating a conception date sounds like it should be straightforward arithmetic, but pregnancy dating conventions create a small but important wrinkle: pregnancy is measured in weeks from the first day of your last menstrual period (LMP), not from the date sperm and egg actually met, which happens roughly two weeks later at ovulation.
This means a pregnancy "dated" at 40 weeks from LMP represents an actual gestational age from conception closer to 38 weeks — the same underlying event, just measured from two different reference points that are consistently two weeks apart. This calculator accounts for that gap when working backward from a known or estimated due date to approximate the likely conception window.
The reason this can only ever be an estimate, even with perfect math, comes down to reproductive biology rather than calculation error. Ovulation timing varies somewhat cycle to cycle even for people with regular periods, and conception itself could have occurred anywhere within roughly a five-to-six-day fertile window — sperm can survive in the reproductive tract for up to five days awaiting ovulation, while a released egg remains viable for only about 24 hours.
For people using ovulation tracking methods — basal body temperature charting, ovulation predictor kits, or cervical mucus monitoring — pinpointing a likely conception date with more confidence becomes possible, since a documented ovulation date narrows the window considerably compared to a generic 14-day assumption based on cycle length alone.
In medical or legal contexts requiring more certainty than an LMP-based estimate can offer, early first-trimester ultrasound measurements (particularly before 12 weeks) are considered more accurate for pregnancy dating than menstrual history alone, since fetal measurements at that stage correlate very consistently with gestational age regardless of cycle irregularities or uncertain LMP recall. This calculator's estimate is a reasonable starting point, but it isn't a substitute for clinical dating when precision genuinely matters.
It's an estimate based on average ovulation timing; actual conception can vary by a few days depending on your cycle.
Because 280 days is counted from your last period, while true gestation from conception is about 2 weeks shorter — 266 days.
No — genetic testing can confirm biological parentage but cannot determine the precise date of conception. Medical dating relies on either known ovulation tracking, early ultrasound measurements, or (for IVF) the known embryo transfer date, none of which this calculator's inputs alone can replace with certainty.