Reverse Due Date Calculator

Already have a due date? Work backwards to when conception likely happened, when the pregnancy count started, and how far along you are today.

Reverse calculations are exact arithmetic on estimates: conception is placed 266 days before the due date, and the implied last period 280 days before. Real biology varies by days, so read the results as centres of ranges.

Your due date

Estimated conception

Implied last period (day 1)

You are (today)

Trimester

First trimester ended / ends

Second trimester ended / ends

Please read: This calculator gives estimates only — it is not medical advice and cannot diagnose or date a pregnancy with certainty. Only around 5% of babies arrive on their estimated due date, and most arrive within a week or so either side. Ultrasound dating performed by a clinician takes precedence over any calculator result. Always confirm your dates, screening windows and care plan with your healthcare provider. All dates are processed locally in your browser; nothing you enter is uploaded or stored.

What a reverse calculation can and cannot tell you

It can pin down the calendar: if a due date came from a reliable early ultrasound, counting back 266 days gives a conception estimate that is usually within a few days. It cannot identify a specific encounter with certainty — sperm survival means fertilisation may have occurred days after intercourse, so several dates can be consistent with the same due date.

If your due date itself came from a last-period estimate with an irregular cycle, the backwards figures inherit that uncertainty. The methods page explains each approach and when providers prefer one over another.