Conception Date Calculator — When Did I Conceive?
Find your estimated conception date from your due date or last period. Based on ACOG guidelines. Includes fertile window and trimester info. Free.
CalcVita. (2026). Conception Date Calculator — When Did I Conceive?. CalcVita. Retrieved July 16, 2026, from https://calcvita.com/en/calculators/conception-date
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Conception Date Calculator: 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 Practice Bulletin No. 700 (2017).
Conception window (fertile window)
The ±3-day window around the estimated conception date reflects that conception is possible within the fertile window of approximately 5 days before ovulation through the day of ovulation (Wilcox et al., 1995, NEJM).
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. Practice Bulletin 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 Practice Bulletin 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 Practice Bulletin No. 700, 2017 — Methods for Estimating the Due Date)
- What is the conception window?
- The conception window shown (±3 days around the estimated date) reflects the fertile window. Research shows that conception is possible from about 5 days before ovulation through the day of ovulation, as sperm can survive up to 5 days in the female reproductive tract. (Wilcox AJ et al., 1995 — Timing of sexual intercourse in relation to ovulation, 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 individuals. Only about 5% of babies are born on their exact due date. For a precise assessment, ultrasound dating — especially in the first trimester — is the most accurate clinical method. (ACOG Practice Bulletin No. 700, 2017)
- Why does cycle length affect the conception date?
- Ovulation typically occurs 14 days before the next expected period, regardless of cycle length. So in a 35-day cycle, ovulation occurs around day 21 (not day 14). Adjusting for cycle length improves the accuracy of the estimated conception date and due date. (ACOG Practice Bulletin No. 700, 2017 — Methods for Estimating the Due Date)
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