What an estimated conception date means
Pregnancy is conventionally dated from the first day of the LMP, not from conception.2 Conception is assumed to happen about two weeks after that day.3 An estimated conception date is that assumed day: a reference point worked out on the calendar, not a record of when conception happened.
This is also why a pregnancy is already counted as about two weeks along at conception.
How the calculation works
The calculator adds 14 calendar days to the date you enter. The 14 days come from the standard convention, which assumes a 28-day cycle with ovulation on day 14.1
Worked example
First day of the LMP: August 1, 2026
Add 14 days: estimated conception date August 15, 2026
The result uses only the date you enter. It does not adjust for cycle length or use any other information.
Why the result is only an estimate
The 14-day convention does not account for cycles of irregular length or for ovulation that happens earlier or later than usual.1 Timing varies between people and from cycle to cycle. In a study of 221 women, only about 30% had their fertile days entirely within days 10 to 17 of the cycle, and the authors note that timing can be highly unpredictable, even when cycles are usually regular.4
What this estimate cannot tell you
An estimated conception date cannot establish the exact day conception happened, and it cannot establish paternity. It is a calendar estimate based on an average cycle, not information about your own pregnancy.
If you have questions about the timing of your pregnancy, talk with your doctor or midwife. This site cannot interpret your scans or records.
Sources
- American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion No. 700). 2017; reaffirmed 2025.
- NHS (England), Best Start in Life. Week 4 (week-by-week guide to pregnancy). accessed October 2026.
- American College of Obstetricians and Gynecologists (ACOG). How Your Fetus Grows During Pregnancy. patient FAQ, reviewed 2026.
- BMJ. Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. 321(7271):1259–62. 2000.