How it works
This tool traces your reproductive timeline backward from a known delivery expectation point. Rather than projecting forward from menstruation records, it takes an estimated arrival day for your baby and counts in reverse to pinpoint when fertilization most likely happened.
The clinical anchor is a 280-day gestational span measured from the onset of your final menstrual bleeding. Ovulation typically falls 14 days into that span, leaving 266 days between the release of an egg and birth. By subtracting that interval from your delivery target, you land on the probable fertilization moment.
The genuine value emerges when you already carry a clinical due date—perhaps from an ultrasound scan or a midwife assessment—and want to understand which intimate encounters line up with becoming pregnant. Sperm can persist for up to five days within the cervical environment, while the released egg remains viable for roughly 24 hours. That biology creates a six-day fecundability corridor, giving you a range rather than a pinpoint.
| Cycle Characteristic | Typical Value | Impact on Fertile Window |
|---|---|---|
| Standard length | 28 days | Ovulation near day 14 |
| Longer pattern | 32–35 days | Ovulation shifts later |
| Shorter pattern | 21–24 days | Ovulation shifts earlier |
| Luteal phase | 12–16 days | Stays relatively fixed |
This information is an estimate, not professional medical advice. Consult your healthcare provider for clinical dating and reproductive guidance.
The formula
Likely conception date = Due date − 266 days
Fertile window start = Conception date − 5 days
Worked example
Suppose your midwife sets March 20, 2025 as your expected delivery target. You wonder which dates mattered for becoming pregnant.
Conception date: 2025-03-20 − 266 days = 2024-06-27
Fertile window start: 2024-06-27 − 5 days = 2024-06-22
Last period onset: 2024-06-27 − 14 days = 2024-06-13
Your fecundability corridor runs from June 22 through June 27, 2024. Intercourse during that stretch carries the highest probability of leading to fertilization.
Things to watch
Irregular cycles throw off the math significantly. If your pattern swings between 24 and 38 days, ovulation drifts unpredictably, and the reverse calculation from a due date assumes a textbook 28-day rhythm that may not reflect your body.
Ultrasound dating often overrides calendar estimates. A first-trimester scan measures fetal crown-rump length directly, producing a clinical gestational age that can differ from backward counting by a week or more. When the two disagree, the scan usually wins.
Implantation timing also matters. The fertilized egg needs 6 to 10 days to travel and embed in the uterine lining, so the moment of fertilization precedes the start of pregnancy by over a week.
Paternal sperm longevity extends the corridor backward. Since sperm can wait five days inside the female tract, an encounter on June 22 could still result in fertilization on June 27 when the egg actually releases.