Calculate when conception likely occurred
Estimated Conception Date
Fri, Dec 12, 2025
Window: Sun, Dec 7, 2025 - Sat, Dec 13, 2025
Due Date
Fri, Sep 4, 2026
Fertile Window
Dec 7 -Dec 13
Conception window: Sperm can survive up to 5 days in the reproductive tract, so conception could have occurred anywhere in the fertile window.
Accuracy: These calculations are estimates. Actual conception date can vary based on when ovulation occurred and individual cycle variations.
Medical dating: Your doctor may adjust dates based on ultrasound measurements, which are more accurate than LMP-based calculations.
Inputs
Result
Conception = October 8 - 266 days = January 15, 2025. Earliest = January 15 - 5 = January 10. Latest = January 15 + 1 = January 16. Due date is confirmed as October 8, 2025.
Inputs
Result
Ovulation day = 30 - 14 = day 16. Conception = March 1 + 16 = March 17. Earliest = March 17 - 5 = March 12. Latest = March 17 + 1 = March 18. Due date = March 1 + 280 = December 6, 2025.
Conception typically occurs around ovulation, about 14 days before your next expected period. From a due date, subtract 266 days (38 weeks). The fertile window spans about 5 days before ovulation.
It's difficult to pinpoint exactly. Sperm can survive 3-5 days, so conception could occur days after intercourse. Most estimates provide a window of 5-6 days when conception likely occurred.
Pregnancy is dated from your last menstrual period (LMP), but conception occurs about 2 weeks later at ovulation. A "6-week pregnancy" actually has a 4-week-old embryo.
This provides estimates based on average cycle lengths. Actual ovulation and conception can vary by several days. Ultrasound dating in early pregnancy is more accurate for determining gestational age.
Factors include: cycle length variability, stress, illness, travel, and individual biology. Women with irregular cycles have less predictable ovulation and conception dates.
Conception date estimates can help narrow the window, but are not definitive for paternity. DNA testing is the only reliable way to determine paternity.
Track pregnancy
Last Updated: Feb 12, 2026
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.