Pregnancy Conception Calculator
Estimate your conception date from a due date or last menstrual period. Free conception calculator with the fertile window and gestational age.
About the Pregnancy Conception Calculator
This free pregnancy conception calculator estimates when conception likely occurred, working from either your due date or the first day of your last menstrual period (LMP). It also shows the estimated fertile window and your current gestational age, using the standard obstetric conventions.
Pregnancy is dated from the first day of the LMP, and conception typically happens around 2 weeks later at ovulation. So the estimated conception date is roughly LMP + 14 days (adjusted for your cycle length), and the due date is about 280 days (40 weeks) from the LMP. From a known due date, the calculator works backwards.
Expectant parents use this out of curiosity and to understand pregnancy timelines. These are estimates only and not a substitute for medical advice — an ultrasound and your doctor or midwife give the accurate dates. Everything runs privately in your browser.
How to Use the Pregnancy Conception Calculator
- 1Choose whether you know your due date or your last menstrual period date.
- 2Enter that date and your average cycle length (default 28 days).
- 3Read the estimated conception date and fertile window.
- 4Review the current gestational age if you entered an LMP.
Frequently Asked Questions
How is the conception date estimated?
Conception is assumed to occur at ovulation, about 14 days after the first day of the last menstrual period for a 28-day cycle. So conception date ≈ LMP + 14 days, adjusted if your cycle is longer or shorter. From a due date, subtract 266 days.
How accurate is a conception date calculator?
It gives an estimate, not an exact day. Ovulation timing varies between women and cycles, and sperm can survive several days, so the true conception day may differ by a few days. An early ultrasound is the most reliable dating method.
How does cycle length change the estimate?
Ovulation is roughly 14 days before your next period, so a longer cycle pushes ovulation — and therefore conception — later. For a 32-day cycle, conception is closer to LMP + 18 days. The calculator adjusts for the cycle length you enter.
Why is pregnancy dated from the last period, not conception?
Because the LMP date is far easier to know than the exact conception date. This convention makes a full-term pregnancy '40 weeks', even though actual gestation from conception is about 38 weeks — the extra 2 weeks bridge the LMP to ovulation.