Should fetal weight growth curves be population specific?

The use of ultrasound to estimate fetal weight has become very common. Several studies have examined which fetal parameters predict best fetal weight, and based on those parameters fetal growth curves have been established. It is possible however, that fetal growth patterns differ across population groups, and therefore should be population specific. To establish fetal weight growth curve for the Israeli population and to compare it to the one currently used, suggested by Hadlock et al.

A cross-sectional study of singleton pregnancies was conducted. For each fetus the estimated weight was calculated using sonographic measurements of bi-parietal diameter, femur length, abdominal circumference, and head circumference. A regression analysis was used to evaluate the relationship between weight and fetal age.

A growth curve was established based on measurements of 857 fetuses. There was a gradual increase in fetal weight from 99 grams at 14 weeks to 3505 grams at 40 weeks. There was a slight difference between the new curve and the one currently used. Comparing 3rd percentile curve of the one in use with ours, we found that the first curve fell outside our 95% confidence limits for weeks 14 to 23 (when it was too low) and for weeks 27 to 35 (when it was too high). Comparing the 97th percentile curves, we found that the current curve fell outside our 95% confidence limits for weeks 14 to 22 (when it was too low), and for weeks 25 to 37 (when it was too high).

We recommend that the physicians in Israel use the new Israeli fetal weight curve. This curve may be useful, especially in situations where fetal weight is close to or below the 3rd percentile of Hadlock's curve in weeks 27-35, and when fetal weight is close to or above the 97th percentile of Hadlock's curve in weeks 14-22.