Accuracy of second trimester fetal head circumference and biparietal diameter for predicting the time of spontaneous birth.

AIMS: Previous studies suggested that head circumference (HC) predicts the date of confinement better than the biparietal diameter (BPD). The present study aimed to prospectively test this assumption.

METHODS

A prospective study including 4179 consecutive women attending the second trimester routine ultrasound examination at 17-20 weeks of gestation. Outer-outer BPD and HC were determined as the average of three repeated measurements. Twins were excluded. Time of delivery was noted, the differences between this and the predicted date of delivery calculated with HC and BPD (based on a pregnancy duration of 282 days), and the two methods were compared using Wilcoxon signed rank test.

RESULTS

For the entire group, the median differences between actual and predicted delivery with HC and BPD were 0.9 and 1.2 days, respectively. For deliveries with a spontaneous onset of labor (n=3336) the corresponding differences were 0.9 and 1.4 days. The difference between the HC and BPD methods was significant (P<0.0001). In the group of spontaneous onset of labor, there were 05.6% post-term pregnancies (>or=296 days) according to HC and 5.7% according to BPD.

CONCLUSION

Provided that the expected pregnancy duration was 282 days, both HC and BPD predicted spontaneous birth with a mean accuracy of one day, HC being significantly better than BPD.