How Accurate Is Your Due Date? Understanding Due Dates in Australian Maternity Care

18 Apr

Your estimated due date (EDD) is one of the first things you’re given in pregnancy, but how reliable is it? In Australia, due dates are used as a guideline rather than a deadline, and most babies arrive within a few weeks of this estimate. Understanding how due dates are calculated, what happens if you go overdue, and your options for induction can help you feel more informed and prepared.

How Is Your Due Date Calculated?

In Australia, due dates are usually estimated using Naegele’s Rule:

EDD = First day of your last menstrual period (LMP) + 280 days (40 weeks).

This assumes a 28-day cycle with ovulation on day 14, but since many women have shorter or longer cycles, this isn’t always accurate.

Ultrasound Dating

A more reliable way to estimate gestational age is through an early pregnancy ultrasound, ideally between 7 and 12 weeks. This scan measures the crown-rump length (CRL) of the embryo.

First-trimester scans: Most accurate, with a margin of error of about five days.
Second-trimester scans (13–24 weeks): Slightly less accurate, with a margin of seven to ten days.
Third-trimester scans: Least accurate, as babies grow at different rates.

If an ultrasound estimate differs significantly from your due date based on LMP, the ultrasound date is usually used instead.

How Accurate Is Your Due Date?

Your due date is an estimate, and only 4–5% of babies are born exactly on this date. Most births occur between 37 and 42 weeks, with first-time mothers more likely to go closer to 41 weeks.

Gestational Age Categories:

Before 37 weeks: Preterm birth
37 to 38 weeks: Early term
39 to 40 weeks: Full term
41 weeks: Late term
42+ weeks: Post-term

Because due dates are not exact, Australian maternity guidelines focus on monitoring rather than rigid deadlines.

What Happens If You Go Overdue?

If you reach 41 weeks, your care provider may recommend:

Membrane sweep (stretch and sweep): A vaginal examination where the amniotic sac is gently separated from the cervix to help bring on labour.
Increased fetal monitoring: This may include cardiotocography (CTG) to check your baby’s heart rate and ultrasounds to assess amniotic fluid and placenta function.
Induction of labour (IOL): If you reach 41+3 - 42+ weeks, induction is often recommended using:

Prostaglandin gel or balloon catheter to soften the cervix.
Artificial rupture of membranes (ARM) to break the waters.
Oxytocin (Syntocinon) drip to stimulate contractions.

Policies may vary between public and private hospitals, with some offering induction closer to 41 weeks and others waiting until 42 weeks.

Can You Decline Induction?

Yes, you have the right to make an informed choice about any aspect of your care.

If there are no concerns, you may choose to wait for spontaneous labour with regular monitoring. However, after 42 weeks, risks such as placental insufficiency and stillbirth increase slightly, which is why induction is strongly recommended.

Your midwife or obstetrician can discuss the risks and benefits with you to help you make the best

My Thoughts:

Your due date is only a guide, and most babies arrive on their own timeline. Australian maternity care balances safety with flexibility, ensuring you have options whether labour starts naturally or requires medical assistance. Understanding your due date and what happens if you go overdue can help you feel more confident and prepared.

Further reading:

Middleton P, Shepherd E, Morris J. Induction of labour at or beyond 37 weeks’ gestation. Cochrane Database of Systematic Reviews 2020, Issue 7. Art. No.: CD004945. DOI: 10.1002/14651858.CD004945.pub5.