Ultrasonic measurement of the fetal head to abdominal circumference ratio in normal pregnancy.

The relation between fetal HC/AC ratio and menstrual age was determined from 14th-40th weeks gestations by analysis of 1,211 measurements examined with real-time ultrasound scanner Aloka 630, 650. The mean HC/AC ratio with +/- 2SD and percentile ranks were determined and presented in both graphic and tabular forms. The mean ratio was 1.207 at 14 week's but decreased slowly until 30 week's when the ratio was 1.110 thereafter there was a rather sharp fall in the mean ratio 1.010 at 36 weeks and 0.967 at 40 weeks and then the variability decreases. Interestingly, HC/AC ratio value for each gestational week in this study was not so different from those of western studies. We hope that the value from this large series is important baseline data for evaluation of fetal growth in our population and is more appropriate for application with Thai women than caucasian data.