Objective:
To examine the difference of the effect of maternal smoking during pregnancy on trajectories of gestational weight gain (GWG) and estimated fetal weight (EFW) by using multilevel models

Methods:
Prenatal check-up data including EFW measured by ultrasonography and GWG were collected from three hospitals in Yamanashi prefecture, Japan. Multilevel analyses were conducted to determine the estimates of slopes of EFW and GWG, separately, in each gestational period by maternal smoking status during pregnancy.

Results:
We obtained 10525 prenatal check-up data from 1021 women. Of these, 494 (48.4%) were primipara. Mean maternal age at delivery was 31.1 years. The number of smoking mother was 109 (10.7%). Mean weight gain during pregnancy was 10.1 kg. Means of birthweight in boys and girls were 3033.0g and 2922.5g, respectively. Although women who smoked during pregnancy were likely to have increase weight, the slope of EFW in non-smoking mothers was significantly increased during third trimester while this trend was not observed in smoking mothers.

Conclusions:
It was suggested that the effect of maternal smoking during pregnancy on GWG was inversely associated with fetal growth.