How to Estimate Your Due Date From Your Last Period or Conception Date

A due date gives you a timeline for check-ups, scans and planning. It is an estimate, because only about 4 percent of babies arrive on the exact day.

Quick answer

Enter the first day of your last period (and your cycle length if it is not 28 days), or the date of conception. The tool gives your estimated due date, current gestational age and the start of the second and third trimesters.

The rule

• Naegele's rule: due date = first day of last period + 280 days (40 weeks).
• From conception: due date = conception date + 266 days (38 weeks).
• For a cycle longer or shorter than 28 days, the date is adjusted by the difference.

Example

Last period started on 1 January: due date is about 8 October.

Trimesters

• First trimester: week 1 to 13 or 14.
• Second trimester: week 14 to 27.
• Third trimester: week 28 until birth.

Steps

1. Enter the first day of your last period.
2. Enter cycle length if not 28 days.
3. Or enter the conception date.
4. Read the due date and weeks.

Important

• Early ultrasound measurements can adjust the date and are more accurate when periods are irregular.
• Full term is 37 to 42 weeks; babies often arrive within two weeks either side of the due date.
• Contact your midwife or doctor if you have concerns.

This tool provides an estimate and is not medical advice.