Background:The effect of matal weights on the risk of iron deficiency anemia (IDA) during pregnancy remains unclear.The study aimed to investigate the association between matal weight indicators and IDA during pregnancy.Methods:We conducted a cohort study to examine the association between matal weight indicators,including prepregnancy body mass index and the rate of gestational weight gain (GWG),and the risk of IDA among Chinese pregnant women.Data about new-onset IDA at different trimesters from a national cross-sectional survey were collected;information regarding baseline variables and rate of GWG from women participating in the survey were retrospectively collected.Tested IDA and reported IDA were documented.Multilevel logistic regression to examine the association between matal weight indicators and the risk of IDA after adjusting for potential confounders was conducted.Results:This study enrolled 11,782 pregnant women from 24 hospitals from September 19,2016,to November 20,2016.Among those,1515 (12.9％) IDA events were diagnosed through test (test IDA);3915 (33.3％) were identified through test and patient reporting (composite IDA).After adjusting for confounders and cluster effect of hospitals,underweight pregnant women,compared with normal women,were associated with higher risk of test IDA (adjusted odds ratio [aOR]:1.35,95％ confidence interval [CI]:1.17-1.57 and composite IDA (aOR:1.35,95％ CI:1.2 1-1.51);on the contrary,overweight and obese women had lower risk of test IDA (aOR:0.68,95％ CI:0.54-0.86 overweight;aOR:0.30,95％ CI:0.13-0.69 obese) and composite IDA (aOR:0.77,95％ CI:0.67-0.90 overweight;aOR:0.34,95％ CI:0.21-0.55 obese).The higher rate of GWG was associated with higher risk of IDA (test aOR:1.86 95％ CI:1.26-2.76;composite aOR:1.54,95％ CI:1.16-2.03).Conclusions:Pregnant women who are underweight before pregnancy and who have faster GWG are more likely to develop IDA.Enforced weight control during pregnancy and use of iron supplements,particularly among underweight women,may be warranted.