Introduction: In the UK an accurate gestational age is confirmed by ultrasound measured foetal crown rump length (CRL) at 11 + 2-14 + 1 weeks of gestation. The currently recommended Robinson and Fleming crown rump length reference chart was develop in 1975. Advances in ultrasound technology and standardized crown rump length measurement training could mean this is now out of date. Our study aimed to assess its accuracy in current routine antenatal care.
Methods: Retrospective data from 178 IVF pregnancies seen for routine antenatal care at a UK Regional Maternity Unit between 1 January 2006 and 1 January 2016 was retrieved. We compared ultrasound calculated crown rump length gestational age taken at the routine First Trimester Screening Clinic (FTSC) with the 'true' gestational age calculated from the known IVF fertilization date.
Results: We identified a systematic overestimation of gestational age by ultrasound using the currently recommended crown rump length reference chart when compared to IVF gestational age. The mean overestimation was 3.0 days (95% CI: 2.7 to 3.4), p < 0.001. A range of alternative ultrasound reference charts also generated a systematic overestimation, ranging from 1.6 to 2.9 days (p < 0.001, for each).
Conclusions: The current crown rump length reference chart systematically overestimates gestational age by an average of three days when assessed in IVF pregnancies. A systematic overestimation was also identified in alternative crown rump length reference charts. These differences, although slight, were systematic with implications for the accuracy of gestational age estimation particularly in pregnancies at risk of pre-term delivery or growth restriction. Our findings need confirming in larger, non IVF cohorts and could lead to the need for an updated crown rump length reference chart.
Keywords: Scanning; pregnancy; research.