The impact of fetal, maternal and external factors on prediction of the day of delivery by the use of ultrasound.

In a non-selected population comprising 15,241 women, an evaluation was performed of the impact of fetal, maternal and external factors on the ultrasonic measurement of the biparietal diameter (BPD) and the day of delivery. The 7824 women who constituted the study population had singleton pregnancies and reliable menstrual histories, and they delivered spontaneously after 37 weeks. Multiple linear regression analysis was used. There was a difference in the size of the BPD at the ultrasound scan related to the gender, parity, maternal age, gestational age according to the last menstrual period and the experience of the operators. There was a total difference of +/- 1 day in the day of delivery as determined by ultrasound and the factors above. The effect on the day of delivery is explained by the differences in the BPD. An effect of gender on gestational length was present as well, which partly compensated for the difference in the BPD. In conclusion the accuracy of prediction of the day of delivery by ultrasound is influenced by the gender, parity, maternal age and the experience of the operator, but these differences are small and of no clinical importance.