Pregnancy Dating – Getting it Right the First Time!

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)