In this episode of the podcast, Penny has a rant about one of her pet peeves – the ever changing due date – and gives lots of tips on how to determine the estimated due date (EDD) with the greatest accuracy.
Take home messages:
- The only way to know the exact EDD for certain is an IVF pregnancy with a known implantation date.
- The next most accurate method is an ultrasound measurement of crown-rump length between ~7 and 13+6 weeks (or maybe even up to 23 weeks, if you believe the Canadians). Dates based on LMP are fraught with assumptions and risks of error.
- Transabdominal measurements are just as good as transvaginal scans at determining the EDD.
- The margin of error increases the later the scan is done – about 5 days in first trimester, 7-10 days up to 20 weeks, 2 weeks in late second trimester, and up to 3 weeks in third trimester.
- International organisations agree that ultrasound should be offered universally between 8 and 13+6 weeks and an EDD based on first trimester crown-rump length should be the definitive date. Note, however, that in practice, some radiologists will leave the EDD as per the LMP if it’s within 5-7 days.
- Offering universal early dating scan reduces the rate of interventions in the post-dates period.
- Check for inaccurate dates written on request forms, or typos on reports.
- Never use HCG to determine gestational age.
- Don’t be fooled into adjusting the due dates based on growth parameters later in pregnancy!
Australasian Society of Ultrasound Medicine recommendations:
- Earliest measurement of gestational age in pregnancy should be the definitive assessment
- If the first scan is after the first trimester –> if the ultrasound measurements are within one week of EDD as determined by LMP date, the scan confirms the LMP date.
- If the first scan is 14 – 20 weeks and differs from the LMP date by more than one week, a new EDD is assigned, if the different measurement parameters are in agreement.
References & Resources:
ASUM Statement on Normal Ultrasound Fetal Measurements (Australia)
ASUM Guidelines for the Performance of First Trimester Ultrasound (Australia)
ISUOG Practice Guideline: Performance of First Trimester Fetal Ultrasound (international)
SOGC Clinical Practice Guideline Determination of Gestational Age by Ultrasound (Canada)
ACOG Committee Opinion: Method for Estimating Due Date (USA)
NICE Guideline: Antenatal Care for Uncomplicated Pregnancies (UK)
Bits & Bumps 
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