How to estimate your conception date
- Choose Due date if you have one from your midwife or doctor, or Last period to work from the first day of your last period.
- Pick the date. For the last period, change the Cycle length if your cycle is usually shorter or longer than 28 days.
- Read the estimated conception date, the fertile window it falls in and how far along you are today.
How conception dates are estimated
Doctors date pregnancy from the first day of the last period, and a typical due date is 280 days after it. Conception is assumed to happen at ovulation, around 14 days into a 28-day cycle, which leaves 266 days from conception to the due date. Working backwards from a due date simply reverses that count. If your due date was moved after a dating scan, use the new date: it already allows for when you actually ovulated.
Why the answer is a range
An egg can be fertilised for about a day after ovulation, but sperm can wait in the body for up to 5 days. So the fertile window runs from about 5 days before ovulation to the day itself, and the calculator shows that window as well as the single most likely date. To go forwards from a date instead, the due date calculator gives the due date and milestones, and the pregnancy week calculator shows how far along you are.