Conception Calculator
Calculate your conception date with precision. Use your due date, last period, or ultrasound results to find out the exact window when you likely conceived.
When did I conceive?
Determining the exact date of conception is often a retroactive calculation. This conception calculator uses standard medical formulas to work backward from your pregnancy milestones.
Key Fact: Conception (fertilization) typically happens within 24 hours of ovulation. In a standard 28-day cycle, ovulation occurs around day 14. However, the fertile window—the time when sex can lead to pregnancy—opens about 5 days before ovulation because sperm can survive that long.
How We Calculate Conception
- From Due Date: We subtract 266 days (38 weeks) from your estimated due date. This is the standard gestational period from fertilization to birth.
- From Last Period (LMP): We add 14 days to the start of your last period (adjusted for cycle length). This estimates the day of ovulation.
- From Ultrasound: We use the gestational age provided by the scan to pinpoint your due date, then calculate conception from there. Ultrasound dating in the first trimester is highly accurate.
Can I know the exact day?
Unless you had intercourse only once during your fertile window or used assisted reproductive technology (like IVF), it is difficult to know the precise day with 100% certainty. This calculator provides the most medically probable date based on your cycle data.
If you are looking forward to your baby's arrival, verify your timeline with our Pregnancy Due Date Calculator.
? Frequently Asked Questions
Yes, conception (fertilization) happens within 12-24 hours of ovulation. Once the egg is released, it must be fertilized quickly or it disintegrates.
Sperm can live in the female reproductive tract for up to 5 days. You may have had sex on a Friday, but if you didn't ovulate until Monday, conception likely occurred on Monday using the surviving sperm.
It is very accurate for women with regular cycles. For irregular cycles, using an ultrasound date will provide a much better estimate than using your last period.
For IVF, the conception date is technically the date of fertilization (retrieval day), or calculated based on the embryo transfer date. This calculator uses standard natural conception formulas.