About due dates

Due dates can be calculated using the last normal menstrual period or through ultrasound.  If the last normal menstrual period is being used to calculate the due date, there are a few conditions that must be met for this to be considered an accurate input to the due date:

  1. The last period needs to have been a normal period
  2. The women must have had three normal cycles since ceasing breastfeeding or the oral contraceptive Pill
  3. The last three cycles must have been regular

If these conditions are not met, the date cannot be calculated using the last normal menstrual period, and so the due date would then be based on an early pregnancy scan.

When should an early pregnancy scan be attended?

The earlier the scan, the more accurate the estimate of the due date.  I usually refer women for dating scans at 7 weeks of pregnancy, however they can be performed any time between 7 and 12 weeks for the purposes of dating.  The earlier in pregnancy,  the more accurate the date.  There is very little to be gained from doing a scan prior to seven weeks as there is less chance of the baby having a heart beat, and so a good chance that the scan will need to be repeated.

An early pregnancy scan can be reassuring and exciting for parents.  Studies suggest that when the due date is taken from a good-quality, early pregnancy scan, there are fewer inductions for women going overdue.  In other words, due dates that are derived from dating scans tend to push back the due date when compared with pregnancies that are dating from the last menstrual period.

Melissa Maimann is an endorsed eligible midwife in private practice in Sydney.  One of the first eligible midwives in Australia, Melissa now offers a range of care options for women including private midwifery care, antenatal shared care and birth support.  Visit Melissa's website to learn more about her services.