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.
Page last updated: 12 Nov 2020