Anomalies in Fetal Dating

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Gestational age (GA), which is important in the management of pregnancy, is determined from the date of onset of the last menstrual period (LMP) and may be estimated from ultrasound measurements. The LMP is not always reliable. A number of ultrasound measurements are used in the estimation of GA, and there is evidence that a biparietal diameter (BPD) measurement between 12 and 18 weeks is a more accurate predictor of delivery date than LMP. We perform a dating scan in the late first or early second trimester and an anomaly scan at 20-22 weeks GA, and have anecdotal evidence of inconsistencies in GA estimates from the two scans. The objectives of this study were to assess the fit of our measurements to the recommended BPD and femur length charts and to assess the relative uncertainty associated with GA predicted by the two scans. A total of 283 BPDs and 246 FLs were plotted on recommended charts and a visual assessment of fit made. GA estimates from dating and anomaly scans were compared (77 patients) using the available measurements (BPD, FL, crown-rump-length). BPD measurements fitted the recommended chart. FL measurements fitted the recommended chart at the dating scan, but were longer than expected at the anomaly scan. GA estimates at the two scans agreed where both were based on BPD (mean difference: -0.3 days; 95% CI: 8.8 days), but for other variables there were larger mean differences or a broader spread of differences. The anecdotal finding that our measurements are not consistent with recommended FL charts was confirmed. A commonly applied rule—if GA by LMP and GA by ultrasound agree to within 7 days, use the former, otherwise the latter—used at the dating scan, added to the uncertainty when compared with GA by BPD at the anomaly scan (mean difference: -0.2 days; 95% CI: 12 days). Ultrasound GA should always be used. A difference in dating estimates of up to 9-12 days between two scans must be accepted.