Common Pregnancy Due Date Mistakes (and a Better Way)

Updated 8 September 2026 · Health and fitness

A Pregnancy Due Date Calculator gives a fast answer, but the answer is only as good as what goes in. These are the mistakes we see most often with pregnancy due date calculations in Australia, and how to avoid them.

1. Relying on one measure

BMI, waist size and body fat each tell part of the story. Look at them together.

2. Underestimating drink sizes

Many pours and cans are more than one standard drink.

3. Treating estimates as diagnoses

Formulas describe populations, not individuals. They can’t replace advice from a health professional.

4. Mixing kilojoules and calories

1 calorie equals about 4.2 kilojoules. Australian labels use kJ.

5. Overlooking the detail that matters most here

Only about 1 in 20 babies arrive on the due date. An early ultrasound gives a more accurate date.

A quick sense check

Due date = last period + 280 days, adjusted for cycle length (Naegele’s rule). With typical inputs the calculator returns estimated due date of 8 March 2027. If your own result looks wildly different, check that each figure is in the right unit and period (weekly, monthly or yearly) before drawing conclusions.