Estimation of gestational age during the first trimester by real-time measurement of fetal crown-rump length and biparietal diameter

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Abstract
A series of 237 patients in the first trimester of pregnancy were examined to derive normal growth curves for fetal crown‐rump length (CRL) and biparietal diameter (BPD) using sector scanner real‐time ultrasound equipment. Conventional B‐scan CRL measurements were obtained from 50 of these patients: In 41 (82%), the values fell within a range ± 5 mm of those of real‐time. Another group of 97 patients between 7 and 13 wk of gestation was then examined in a blind trial to confirm the accuracy of real‐time measurements in estimating gestational age. An estimate was made in 95% of cases to within 4.6 days with CRL and to within 5 days with BPD. A similar comparison was made with both of these dimensions using a comparative nomogram, giving a prediction within 3.9 days in 957percnt; of cases. This method is now used routinely.