Find your likely conception date from your last period or due date
Calculation Method
The first day of your last period
Your Results
Estimated Conception Date
May 28, 2026
Thursday
Current Gestation (from LMP)
6 weeks
Fertile Window Breakdown
Window Opens
May 23, 2026
Peak Fertility
May 28, 2026
Window Closes
May 28, 2026
How it works: Based on Naegele's Rule — pregnancy lasts 280 days (40 weeks) from the first day of the last menstrual period. Ovulation and most likely conception occur around day 14 of a standard 28-day cycle. The fertile window spans 5 days before ovulation through ovulation day. Results are estimates; actual conception can vary. Always consult a healthcare provider for medical advice.
The conception date calculator helps you pinpoint the most likely day you conceived — based on either your last menstrual period (LMP) or your estimated due date. Using Naegele's Rule and standard ovulation timing, it calculates your fertile window, peak ovulation date, and estimated due date in seconds, giving you a clearer picture of your pregnancy timeline.
The estimate is based on Naegele's Rule, which assumes a 28-day cycle with ovulation on day 14. Most conceptions occur within the fertile window (5 days before ovulation through ovulation day), so the estimated conception date is typically accurate within 3–7 days. An ultrasound provides the most precise dating.
The LMP (Last Menstrual Period) date is the first day of your last period — before conception occurs. Conception typically happens about 14 days after the LMP, when ovulation takes place. Pregnancy is conventionally measured from the LMP, which is why a full-term pregnancy is 40 weeks (280 days) from LMP but only 38 weeks (266 days) from conception.
Yes. Subtract 266 days (38 weeks) from your due date to get the estimated conception date. Your LMP would be approximately 280 days before the due date. This calculator does that math instantly when you enter your due date.
The fertile window is the 6-day period during which pregnancy is possible — the 5 days before ovulation plus ovulation day itself. Sperm can survive in the reproductive tract for up to 5 days, so intercourse earlier in this window can still result in conception when the egg is released.