Objective  To explore whether there are different patterns in gestational weight gain (GWG) among pregnant women, and to investigate the effect of pre-pregnancy body mass index (BMI) on the changes in pregnant women′s weight.

  Methods  From the participants of the prospective birth cohort of pregnant women in Guangxi Zhuang Autonomous Region (Guangxi), we selected 2 409 women having maternal care services at county hospitals in four counties of Guangxi for the study from February 2015 through April 2018. Demographic information and the prenatal examination records were collected for all the women. Latent class growth mode (LCGM), Wald test, and χ² test were used in data analyses.

  Results  For all the women, four different GWG trajectories during pregnancy were identified, which covered 23.50%, 43.46%, 25.90%, and 7.14% of the women with different pre-pregnancy weight from low to high; each of the trajectories presented a turning point of weight gain rate at 10 gestational weeks. The women at elder age were more likely to have a greater pre-pregnancy weight (P < 0.05); higher proportion of high pre-pregnancy weight was observed among the women living in urban areas (P < 0.001), having higher pregnancy BMI (P < 0.001), and doing physical exercise regularly (P = 0.088). Various GWG trajectories were established for the three groups of the women with low, normal, and high pre-pregnancy BMI using LCGM analysis and the intercepts and the slopes of the trajectories were significantly different from each other (P < 0.001 for all); while, the slope was positively correlated with the intercept for all the trajectories established.

  Conclusion  Among Zhuang pregnant women, gestational weight gain would be accelerated at 10 gestational weeks and the women with higher pre-pregnancy weight could have a faster weight gain during early pregnancy. The study results suggest a closer monitoring on body weight among the pregnant women at elder age and those with pre-pregnancy weight.