Conception Date Calculator — When Did I Conceive?
Find your estimated conception date from your due date or last period, with a ±3-day window and the matching due date. Based on ACOG Committee Opinion 700. Free.
CalcVita. (2026). Conception Date Calculator — When Did I Conceive?. CalcVita. Retrieved October 16, 2026, from https://calcvita.com/en/calculators/conception-date
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Conception Date: How to Estimate When You Conceived
Wondering exactly when conception happened? Learn how ovulation, the fertile window, and your LMP date combine to give a reliable conception estimate.
Read the full article →How does the conception date calculator work?
Enter either your estimated due date or the first day of your last menstrual period (LMP) along with your average cycle length. The calculator applies ACOG-based formulas to estimate when conception most likely occurred.
The formulas
From due date: conception date = due date − 266 days (38 weeks). From LMP: conception date = LMP + (cycle length − 14) days. Both are derived from ACOG Committee Opinion No. 700 (2017).
Conception window
The ±3 days around the estimated conception date are a margin for the usual variation in the day of ovulation. They are not the fertile window: according to Wilcox et al. (1995, NEJM), conception follows intercourse in the 6 days ending on the day of ovulation, so intercourse may have taken place up to 5 days before the date shown.
Accuracy and limitations
This is an estimate. Actual conception depends on the exact day of ovulation, which varies per cycle. For clinical dating, first-trimester ultrasound remains the gold standard (ACOG, 2017).
Scientific sources
- American College of Obstetricians and Gynecologists (2017). Methods for Estimating the Due Date. Committee Opinion No. 700. Obstetrics & Gynecology, 129(5):e150–e154. acog.org
- Wilcox AJ, Weinberg CR, Baird DD (1995). Timing of sexual intercourse in relation to ovulation — effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med 333(23):1517–1521. PMID: 7477165. doi:10.1056/NEJM199512073332301
- How is the conception date calculated?
- From a due date: conception date = due date − 266 days (38 weeks). From LMP: conception date = LMP + (cycle length − 14) days, since ovulation typically occurs 14 days before the next period. These formulas are based on ACOG Committee Opinion No. 700 (2017) — Methods for Estimating the Due Date.
- Why 266 days from conception to due date?
- A full-term pregnancy lasts approximately 266 days (38 weeks) from the moment of fertilisation, or 280 days (40 weeks) from the first day of the last menstrual period. ACOG (2017) uses 280 days from LMP as the standard for due date estimation using Naegele's rule. (ACOG Committee Opinion No. 700, 2017 — Methods for Estimating the Due Date)
- What is the conception window?
- The window shown (±3 days around the estimated date) is a margin for the usual variation in the day of ovulation, which is never known exactly; it is not the fertile window. The fertile window comes before ovulation: in the Wilcox study, conception occurred only after intercourse within the 6 days ending on the day of ovulation, so the intercourse that led to the pregnancy may have taken place up to 5 days before the conception date. (Wilcox AJ et al., 1995, NEJM, PMID: 7477165)
- How accurate is the estimated conception date?
- The estimate is an approximation. Actual conception depends on the exact day of ovulation, which varies between cycles and between women. For a precise assessment, ultrasound dating — especially in the first trimester — is the most accurate clinical method. (ACOG Committee Opinion No. 700, 2017)
- Why does cycle length affect the conception date?
- Ovulation usually occurs about 14 days before the next period, so a longer cycle means later ovulation: in a 35-day cycle, around day 21 rather than day 14. The calculator applies that assumption to both the conception date and the due date. The phase after ovulation also varies somewhat between women, so the result remains an approximation.
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