Your due date at the beginning of your pregnancy is your holy grail –  the far-away date you’ll finally get to meet your baby after labour and birth. But a very small percentage of babies are actually born on their due date. Here’s what you need to know about your baby’s due date:

An ultrasound makes your due date more accurate

Your due date is set at around 40 weeks and is calculated from the first day of your last menstrual period. While you can use your menstrual cycle to work out your due date, if your periods are irregular or you’re unsure of the end date of your last period, book in for an ultrasound – the earlier the ultrasound is, the more accurate your due date will be, helping prevent an unnecessary induction.

Most babies aren’t born on their due date

You might find it hard to believe, but only three out of every hundred babies actually arrive on their due date. “Only 3% of babies arrive on their due date,” confirms Australian College of Midwives spokeswoman Hannah Dahlen. Around that, there’s a fairly even spread, with 90% of babies being born between 38 and 42 weeks.

A lot of babies are in a hurry

One in around every 14 to 16 births is premature, or occurs before 37 weeks, and experts aren’t exactly sure why. “Around 6-7% of the general, healthy population have a pre-term birth,”confirms Professor Roger Smith, leader of the Mothers and Babies Research Centre at University of Newcastle, adding that mums who’ve already had one premature baby are more likely to have a second.

First time mums are more likely to go past their due date

“Around 70% of first-time mums tend to go over their due date,” confirms Dr Ted Weaver, chairman of the Royal Australian and New Zealand College of Obstetricians and Gynaecologists women’s health committee, adding that there’s also a tendency for mums who were overdue the first time to go past their due date with their second baby.

Doctors can get your due date wrong

With only 5% of women giving birth after 42 weeks, Dr Weaver says most very late births are because of an incorrect due date, with almost 50% of pregnant women unsure when they conceived. If that’s you, make sure you book in for a dating ultrasound. “I’ve seen women go to 43 weeks, even 44 weeks, but we often discover their due date was worked out incorrectly”, confirms Hannah.

This article was written by Joanna Bounds for Kidspot NZ, New Zealand’s best pregnancy and parenting resource.