Comprehensive Calculator Guide
This calculator estimates the likely conception date and the start of the fertile window before it, from the first day of your last period or from the due date. With a last period on 1 March 2026 and a 28-day cycle, conception was around 15 March and the baby is due on 6 December 2026.
How it is estimated
Pregnancy is dated from the first day of the last period. Conception usually happens near ovulation, about two weeks later in a 28-day cycle and later in longer cycles. Naegele's rule puts the due date 280 days after the last period, plus any extra cycle length.
Sperm can survive about five days, so the fertile window is roughly the five days before ovulation and the day itself. A first-trimester ultrasound dates a pregnancy more accurately than these sums.
How to Use
- 01Enter the first day of your last period, or the due date.
- 02Enter your cycle length if it is not 28 days.
- 03Read the estimated conception date and fertile window.
Formula Used
Conception ≈ last period + cycle − 14 | Due = last period + 280 + (cycle − 28)Practical Examples
Example 1
Last period 1 March 2026: conception around 15 March, due 6 December 2026.
Example 2: from the due date
Due 10 January 2027: last period around 5 April 2026, conception around 19 April.
Important Tips
- Use the ultrasound due date if you have one.
Common Mistakes to Avoid
- Dating the pregnancy from conception instead of the last period.
Frequently Asked Questions
- How do I work out when I conceived?
- Add your cycle length minus 14 days to the first day of your last period.
- Can I find conception from the due date?
- Yes, enter the due date.
- Is an ultrasound more accurate?
- Yes, especially in the first trimester.