How Accurate Is Your Due Date?

Your due date is the date you reach 40 weeks of pregnancy. It is a useful guide for planning your care, but it is an estimate, not a deadline.

How many babies arrive on their due date?

Only around 4 to 5% of babies are born on the exact day. Most arrive in the two weeks before or after it. First babies are a little more likely to come late.

Which dating method is most accurate?

• First-trimester ultrasound: ACOG considers this the most accurate way to date a pregnancy.
• IVF transfer date: very precise, because the age of the embryo is known.
• Last menstrual period: accurate if your cycle is regular and you are sure of the date, less so with irregular cycles.
• Conception date: useful, but the exact day of conception is often uncertain.

Can my due date change?

Yes. If your dating scan measures your baby as older or younger than your last-period date by more than a set amount, your doctor will usually change the due date to the scan date. Once set by an early scan, it does not normally change again.

What do term and overdue mean?

• Early term: 37 weeks to 38 weeks and 6 days
• Full term: 39 weeks to 40 weeks and 6 days
• Late term: 41 weeks to 41 weeks and 6 days
• Post-term: 42 weeks or more

If you go past your due date, your doctor or midwife will discuss monitoring and induction of labour, usually around 41 weeks.