Conception Calculator
Calculate your estimated conception date based on your due date. Find out when conception likely occurred and understand your fertile window.
Conception Estimate
Estimated Conception
Mar 24, 2025
Week 37 • Trimester 3
Conception Window
Mar 19, 2025 - Mar 25, 2025
6-day fertile window
Your Due Date
Cycle Information
Average cycle is 28 days. Cycles between 21-35 days are considered normal. This affects the estimated last period date and ovulation timing.
Conception Timeline
Last Menstrual Period
Monday, March 10, 2025
Fertile Window Begins
Wednesday, March 19, 2025
Ovulation / Estimated Conception
Monday, March 24, 2025
Fertile Window Ends
Tuesday, March 25, 2025
Due Date
Monday, December 15, 2025
Understanding the Results
Conception Window: The 6-day period when conception was most likely. This accounts for sperm survival (up to 5 days) and egg viability (24 hours).
Accuracy: These dates are estimates based on average cycles. Actual conception may vary by several days, especially with irregular cycles.
Medical Dating: Your healthcare provider may adjust dates based on early ultrasound measurements, which can be more accurate than calculations.
Key Dates Summary
| Event | Date | Notes |
|---|---|---|
| Last Period (LMP) | Mar 10, 2025 | Start of gestational age |
| Fertile Window Start | Mar 19, 2025 | 5 days before ovulation |
| Estimated Ovulation | Mar 24, 2025 | Peak fertility day |
| Likely Conception | Mar 24, 2025 | Within 24 hours of ovulation |
| Due Date | Dec 15, 2025 | 40 weeks from LMP |
Conception Estimate
Estimated Conception
Mar 24, 2025
Week 37 • Trimester 3
Conception Window
Mar 19, 2025 - Mar 25, 2025
6-day fertile window
Quick Answer
Conception typically occurs about 2 weeks after the first day of your last period (around ovulation). From due date, conception = due date minus 266 days. From LMP, conception ≈ LMP + 14 days.
Key Facts
- Conception usually 14 days after LMP
- Pregnancy dated from LMP, not conception
- Due date is 280 days (40 weeks) from LMP
- Conception = due date - 266 days
- Sperm can survive 5 days
- Exact conception day often uncertain
Frequently Asked Questions
Conception typically occurs during ovulation, about 14 days before the next period. From due date, we subtract 266 days (38 weeks). Since sperm can live 5 days and the egg 24 hours, the actual conception window spans several days.
Sperm can survive in the reproductive tract for up to 5 days, and the egg is viable for about 24 hours after ovulation. Intercourse anywhere in this 6-day window can result in conception, making the exact date uncertain.
Gestational age starts from the first day of your last menstrual period (LMP), about 2 weeks before conception. So at "2 weeks pregnant" conception hasn't occurred yet. Fetal age is actual time since conception, about 2 weeks less than gestational age.
Accuracy depends on cycle regularity. For regular 28-day cycles, accuracy is within a few days. Irregular cycles, stress, or late/early ovulation can shift actual conception. Ultrasound dating in the first trimester is most accurate.
The conception date range can help narrow down when conception occurred, but is not definitive proof of paternity. Given the 6-day fertile window and variation in cycle timing, DNA testing is the only certain way to determine paternity.
Conception Estimate
Estimated Conception
Mar 24, 2025
Week 37 • Trimester 3
Conception Window
Mar 19, 2025 - Mar 25, 2025
6-day fertile window
Medical Disclaimer
This calculator is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult with a qualified healthcare provider before making any health-related decisions.