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Key messages

  • Pregnancy can not be said to be prolonged without accurate dating
  • Establishing an accurate Estimate Due date (EDD) should be confirmed when pregnancy care commences.

In June 2023, we commenced a project to review and update the Maternity and Neonatal eHandbook guidelines, with a view to targeting completion in 2024. Please be aware that pending this review, some of the current guidelines may be out of date. In the meantime, we recommend that you also refer to more contemporaneous evidence. 

Establishing the estimated due date (EDD)

Accurate pregnancy dating is essential to determine if a pregnancy is prolonged.  

The agreed estimated due date should be established as early in the pregnancy as possible.

For women with Oligomenorrhea, lactation amenorrhea or oral contraceptive withdrawal bleeding, where a calculation cannot be based on the last normal menstrual period (LNMP), the first ultrasound prediction becomes the EDD.

If the LNMP was certain and menstruation regular, use the table below to establish an agreed EDD.

TABLE 1. LNMP vs Ultrasound as a predictor for EDD
Ultrasound between 6 and 13 weeks If the two dates differ by 5 days or less, use the LNMP EDD
If the two dates differ by > 5 days, use the ultrasound EDD
Ultrasound between 13 and 24 weeks If the two dates differ by 10 days or less, use the LNMP EDD
If the two dates differ by >10 days, use the ultrasound EDD.
No ultrasound between 6 and 24 weeks Use the LNMP EDD

If the LNMP was not certain, or menstruation irregular, use on EDD estimate from an ultrasound performed between six and 24 weeks.1


1. Department of Health (2018) Clinical Practice Guidelines: Pregnancy Care. Canberra: Australian Government Department of Health.

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Clinical Guidance Team
Safer Care Victoria

Version history

First published: Nov 2020
Due for review: Dec 2022

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