Pregnancy Conception Calculator
Conception (medically) involves fertilization and implantation; for timing estimates this calculator treats the primary estimate as fertilization around ovulation. Sexual intercourse in the several days before ovulation can lead to pregnancy because sperm may remain viable about 3–5 days; the tool shows a 5-day backward window plus the estimated conception day.
Calculate based on
Estimated conception date:
Possible intercourse window (sperm viable ~3–5 days):
Estimated due date (280-day rule from inferred LMP):
Irregular cycles and late ovulation increase uncertainty. Dating by early ultrasound is usually more reliable than LMP alone. This page is informational—not medical advice.
Why three different starting points
Conception timing can be inferred from whichever piece of information you have: a due date (working backward by convention), the first day of your last period plus your typical cycle and luteal phase length (the classic clinical approach), or an ultrasound-measured gestational age (often the most reliable, since it's based on measured fetal size rather than reported cycle history).
Frequently asked questions
Why is there a 5-day window instead of a single conception date? Sperm can survive in the reproductive tract for roughly 3–5 days, so intercourse any day within that window before ovulation can potentially result in conception — not just intercourse on the ovulation day itself.
Why does clinical "gestational age" not match actual conception date? Medical convention counts gestational age from the first day of the last menstrual period, roughly two weeks before ovulation/conception actually occurs — so a pregnancy dated "6 weeks" clinically is really about 4 weeks from conception.
Which method is most accurate? Early first-trimester ultrasound dating is generally considered the most reliable, since it doesn't depend on accurately remembering cycle details or assuming a "typical" cycle length and ovulation timing.