Why Babies Rarely Arrive on Their Due Date

Guides. Updated October 11, 2026. Calculated with the standard pregnancy dating method used by ACOG and the NHS.

A due date is the midpoint of a range. Only a small share of babies, often quoted at around 4 to 5 percent, are born on that exact day.

The normal range

Birth between 37 and 42 weeks is common. Full term is from 39 weeks to 40 weeks and 6 days.

Why it varies

Ovulation timing, the length of individual pregnancies and whether it's a first baby all play a part.

What it means for conception dates

If you're working out conception from a birth date, remember the birth may have been a week or two either side of the due date, so give your estimate a range.

This article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.