Pre-Pregnancy Body Mass Index, Gestational Weight Gain, and Other Maternal Characteristics in Relation to Infant Birth Weight

Ihunnaya O. Frederick; Michelle A. Williams; Anne E. Sales; Diane P. Martin; Marcia Killien


Matern Child Health J. 2008;12(5):557-567. 

In This Article


The results of our study suggest that maternal pre-pregnancy BMI is positively associated with infant birth weight, after adjustment for other maternal characteristics. The effect of pre-pregnancy BMI on infant birth weight was greatest among lean to average women of relatively low weight gain compared to overweight or obese women with high gestational weight gain. Our study found evidence of independent as well as combined effects of pre-pregnancy BMI and gestational weight gain on infant birth weight. In addition, our study results suggest that gestational weight gain within the IOM guidelines was associated with decreased risk of delivering LBW infants and with decreased risk of macrosomia. We compared results from using the three-category (level of compliance with IOM guidelines) approach against those from the two-category (above or below median gestational weight gain) approach in assessing risk of LBW or macrosomia. While results from both approaches suggest significant associations between gestational weight gain and the risk of macrosomia, they differ on the association between gestational weight gain and the risk of LBW. However, the three-category method allowed for refinement of the association between gestational weight gain and infant birth weight beyond the two categories of median weight gain approach.

Our study results are in agreement with previous findings. For example, Merchant et al.[15] found association at low pre-pregnancy BMI, but no significant association between infant birth weight and pre-pregnancy BMI among women of >26 kg/m2 BMI who gained weight within and above IOM guidelines among Pakistani women.[15,28] The interaction of pre-pregnancy BMI and gestational weight gain in relation to pregnancy outcome has been assessed in previous studies.[15,18,21,22,28,29,30,31] Merchant et al.[15] reported lower mean birth weight of newborns for women with pre-pregnancy BMI < 19 kg/m2 who gained <12.5 kg compared to those who gained >12.5 kg in a retrospective study.[15] Similarly, Zhou and Olsen[28] reported a positive association between infant birth weight and gestational weight gain that decreased with increasing pre-pregnancy BMI.[28] Consistent with previous findings,[15,21,22,28,29] our results lend support to the need to balance pre-pregnancy BMI as well as gestational weight gain with respect to the risk of LBW and macrosomia, among lean and obese women, respectively.

Interpretation of our results is limited by several factors despite the relatively large sample size and the high response rate of the study cohort. The use of total gestational weight gain measure assumes a constant weight gain although differences in weight gain across trimesters have been reported. However, our analysis using rate of gestational weight gain instead of gestational weight gain yielded comparable results to those using total gestational weight gain (data not shown). Pre-pregnancy BMI measure was based on self-reported weight and height at approximately three months before conception. Self-reported weight and height tend to be slightly lower than directly measured weight and height, especially among women. However, the validity of self-reported weight and height has been noted to be generally high among large groups of US adults.[32,33] Although the results of our study are generalizable to large sections of the US population, this may be limited with respect to subpopulations of ethnic minorities of low socioeconomic status. Finally, due to the hypothesized complex nature of the relationship between infant birth weight and maternal factors, we cannot rule out the possibility of residual confounding due to variables not included in this analysis.

The relationship between pre-pregnancy BMI and fetal growth is biologically plausible, although the direct pathway by which pre-pregnancy BMI influences infant birth weight is not known. In a review of the relationship between maternal BMI, energy intake and pregnancy outcomes, Neggers and Goldenberg[29] found that pre-pregnancy weight consistently predicted most neonatal measurements compared to other maternal factors.[29] The authors hypothesized a complex interaction of genetics, maternal nutrition, gestational weight gain and other factors for this relationship, likely mediated through maternal nutritional pathway.[29] This notion of complex interaction among multiple maternal factors is supported by work performed by other investigators. Kramer et al.[5] attributed temporal trends of increasing infant birth weight and preterm birth to a combination of factors, including increased maternal adiposity, reduced cigarette smoking and changes in socio-demographic factors.[5] Our reported combined effect of pre-pregnancy BMI and gestational weight gain on infant birth weight lends support for this notion of a complex pathway involving multiple maternal factors.


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