Trimester-specific rate of gestational weight loss or gain and birth size: differences by prepregnancy BMI.

ABSTRACT

OBJECTIVE: The objective of this study was to estimate the effects of trimester-specific gestational weight gain (GWG) on small and large (compared with appropriate) for gestational age (i.e., SGA, LGA, and AGA) by prepregnancy BMI classifications. METHODS: We conducted a cohort study of pregnancies in a national network of community health care organizations, stratifying by prepregnancy BMI (n = 20,676 with normal weight; 19,156 with overweight; 11,647 with obesity class I; 5124 with obesity class II; and 3197 with obesity class III). SGA and LGA (vs. AGA) were modeled as a function of trimester 1, 2, or 3 GWG rate, previous trimester(s) GWG rate, and maternal characteristics using modified Poisson regression. RESULTS: GWG rates ranged from weight loss to substantial gains. GWG-LGA associations were strongest in trimester 1 (risk ratio [RR] range for 10th vs. 50th percentile GWG, across BMI categories 0.60-0.73). GWG-SGA associations were strongest in lower BMI categories and in trimester 2; RRs were 1.62, 1.40, and 1.17 for prepregnancy normal weight, obesity class I, and obesity class III, respectively, with curvilinear associations for class II and III. CONCLUSIONS: Among people with prepregnancy obesity class II or III, GWG rate is associated with higher LGA risk in a dose-dependent manner, including understudied ranges of weight loss, but with weak associations with SGA.