Conception Date Calculator
Estimate the likely date of conception from a due date or last menstrual period.
Average length of your menstrual cycle (default 28)
Estimated Conception Date
Likely Conception Window
Corresponding LMP
Corresponding Due Date (EDD)
How Conception Date Is Estimated
Conception happens when a sperm fertilizes an egg, which can only occur around the time of ovulation. For a typical 28-day menstrual cycle, ovulation falls roughly 14 days before the next expected period, or about 14 days after the first day of the last menstrual period (LMP). Because most people do not know the exact moment of conception, it is estimated from a known reference date such as your LMP, your due date, or, after delivery, the baby's actual birth date.
From Last Menstrual Period (LMP)
Conception is estimated as the LMP plus (cycle length − 14) days. For a standard 28-day cycle this places conception about 14 days after the LMP. Adjusting the cycle length shifts the estimated ovulation day: longer cycles push conception later, shorter cycles earlier. The likely conception window spans the day before to the day after this estimate, reflecting the fertile window around ovulation.
From Due Date
A full-term pregnancy is counted as about 266 days from conception (or 280 days from the LMP). When working backward from an estimated due date, conception is approximated as the due date minus 266 days, with a window of plus or minus one day.
From Actual Birth Date
After delivery, conception can be estimated as the birth date minus 266 days. Because actual gestational length varies considerably from one pregnancy to another, this estimate carries a wider caveat range of about plus or minus one week.
The Fertile Window and Conception Timing
Conception does not occur on a single fixed day. Sperm can survive in the female reproductive tract for up to about five days, while an egg remains viable for roughly 12 to 24 hours after ovulation. This creates a fertile window of several days during which intercourse can lead to pregnancy. Because of this, the exact day of conception cannot be pinpointed with certainty, and any conception date is best understood as the center of a likely range rather than an exact event.
- Ovulation typically occurs about 14 days before the next period.
- Sperm can survive up to 5 days, widening the fertile window.
- The egg is viable for only 12 to 24 hours after ovulation.
- Cycle length and ovulation timing vary between individuals.
Important Notes
These figures are estimates based on average cycle and pregnancy lengths. Individual conception dates depend on the precise timing of ovulation, which can shift due to cycle irregularity, stress, illness, and other factors. Ultrasound dating performed early in pregnancy is generally more accurate than calculations based on the LMP. This tool is for general informational purposes and should not be used to establish paternity or for any medical or legal decision. Consult a healthcare provider for an accurate assessment of your pregnancy.
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