Working a pregnancy timeline backward
Usually pregnancy math runs forward — period, ovulation, due date. This calculator runs it the other way. You've been handed a due date at a scan, or you're holding a birth date years later, and you want to reconstruct where it all started. The anchor number is 266: standard dating places conception about 266 days (38 weeks) before the due date.
Give the calculator whichever date you have — due date, last period, or a specific conception estimate — and it rebuilds the whole timeline in both directions: the likely conception window, the fertile days around it, and the week-by-week schedule ahead.
Why the answer is a window, not a day
Three layers of uncertainty stack up, and it's worth seeing each one. The due date itself is an estimate — ultrasound dating is accurate to about ±5 days in the first trimester, wider later. Ovulation timing varies even in regular cycles. And the fertile window spans about six days, because sperm can wait up to five days for the egg. Stack those together and honest reverse dating gives you a window of roughly a week, centred 266 days before the due date.
That's plenty of precision for reconstructing your timeline or answering 'which trip was it?'. It is not enough precision to distinguish between possible fathers when dates fall close together — DNA testing answers that question; calendars don't.
The companion tools
This page starts from the due date and looks back. The conception calculator starts from your last period and looks forward, the pregnancy calculator tells you how many weeks along you are today, and the due date calculator handles the classic forward projection. Same 280-day arithmetic underneath; different question answered on top.
Frequently asked questions about reverse conception dating
How is conception date estimated from a due date?
Working backward, conception is often estimated about 266 days (38 weeks) before the due date, or roughly 14 days after the start of the last period in standard dating.
How reliable is reverse conception dating?
It is an estimate. Ultrasound dating and irregular cycles can shift the true conception window by several days.
Is this medical advice?
No. For pregnancy, fertility, or health concerns, speak to your doctor, midwife, or OB-GYN.
Sources and further reading
- ACOG — Methods for estimating the due date
- Wilcox AJ et al. — Timing of intercourse and probability of conception (NEJM, 1995)
This page is for general information only and is not medical advice. Always confirm pregnancy dating and care decisions with your doctor or midwife.