Normal ultrasonic fetal growth ratios evaluated in cases of fetal disproportion

During a 2-year period, 5476 normal routine obstetrical ultrasound investigations were performed in the 2nd trimester (16th to 20th week). Data on biparietal diameter (BPD), abdominal diameter (AD) and femur length (FL) have been obtained from this material. Ratios between BPD/AD and BPD/FL have been calculated, and from these ratios, graphs were constructed. Only nine normal fetuses (0.2%) were found to be outside mean +/- 3 SD, and none of the normal cases were +/- 4 SD, so this is perhaps a better guideline for those warranting further investigation. To evaluate if these ratios could better reflect disproportional fetal growth, three cases of triploidy and four cases of dwarfism were tested against these ratios. Triploidy was obvious on the BPD/AD graph and dwarfism on the BPD/FL graph. The ratios were not found to be conclusive in the intrauterine diagnosis of trisomy 21 or of trisomy 18, as only 4 of 17 cases were obvious on the graphs.