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

Abstract and Introduction


Objectives: Infant birth weight is influenced by modifiable maternal pre-pregnancy behaviors and characteristics. We evaluated the relationship among pre-pregnancy body mass index (BMI), gestational weight gain, and infant birth weight, in a prospective cohort study.
Methods: Women were enrolled at ≤20 weeks gestation, completed in-person interviews and had their medical records reviewed after delivery. Infant birth weight was first analyzed as a continuous variable, and then grouped into Low birth weight (LBW) (<2,500 g), normal birth weight (2,500–3,999 g), and macrosomia (≥4,000 g) in categorical analysis. Pre-pregnancy BMI and gestational weight gain were categorized based on Institute of Medicine BMI groups and gestational weight gain guidelines. Associations among infant birth weight and pre-pregnancy BMI, gestational weight gain, and other factors were evaluated using multivariate regression. Risk ratios were estimated using generalized linear modeling procedures.
Results: Pre-pregnancy BMI was independently and positively associated with infant birth weight (β = 44.7, P = 0.001) after adjusting for confounders, in a quadratic model. Gestational weight gain was positively associated with infant birth weight (β = 19.5, P < 0.001). Lower infant birth weight was associated with preterm birth (β = −965.4, P < 0.001), nulliparity (β = −48.6, P = 0.015), and female babies (β = −168.7, P < 0.001). Less than median gestational weight gain was associated with twice the risk of LBW (RR = 2.04, 95% CI 1.34–3.11). Risk of macrosomia increased with increasing pre-pregnancy BMI and gestational weight gain (P for linear trend <0.001).
Conclusions: These findings support the need to balance pre-pregnancy weight and gestational weight gain against the risk of LBW and macrosomia among lean and obese women, respectively.


Birth weight plays an important role in infant mortality and morbidity, childhood development, and adult health.[1] Low birth weight (LBW) is a significant risk factor for adverse health outcomes, including many childhood diseases.[2] Thame et al.[3] documented relationships between birth weight, childhood growth, and blood pressure, suggesting early programming of blood pressure in pregnancy.[3] Reduced birth weight is related to the risk of type-2 diabetes and ischemic heart disease in later life.[4] In addition, LBW results in increased healthcare expenditures due to extended hospital stays for preterm delivery. At the other end of the birth weight spectrum, macrosomia increases the risk of c-section delivery, delivery complications (i.e. shoulder dystocia), and subsequent childhood obesity.[5,6,7] Birth weight, a leading determinant of infant survival, has implications for public health policy-making and practice.

Past study results support direct and indirect influences of maternal genetic, socio-cultural, demographic, and behavioral factors on birth weight.[2,3,8] For example, pre-pregnancy body mass index (BMI) and gestational weight gain influence infant birth weight and play significant roles in adverse pregnancy outcomes including LBW[9,10,11,12] and macrosomia.[5,7,13] In 1990, the Institute of Medicine (IOM) established guidelines for gestational weight gain based on pre-pregnancy BMI, aimed at achieving optimal pregnancy outcomes.[14] In a study of Icelandic women of normal pre-pregnancy BMI (BMI 19.5-25.5 kg/m2) investigators noted that those who gained weight within the IOM guideline of 11.5-16.0 kg experienced less delivery complications than those who gained >20 kg.[6] 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 among Pakistani women.[15] Previous study results are inconsistent, partly due to differences among study populations,[6,15] in time and measurement of weight gain,[6,15,16] in methods for controlling confounding, and relatively small sample sizes.[6,15,16,17,18] Literature is sparse on the independent effect of pre-pregnancy BMI on infant birth weight because many studies focused exclusively on gestational weight gain.[6,16,17,18] In addition, there is need to validate IOM gestational weight gain guidelines in different populations.

The objective of this study was to investigate the independent association between pre-pregnancy BMI and infant birth weight, after controlling for other maternal factors. We hypothesized that pre-pregnancy BMI is independently associated with infant birth weight and with the risks of LBW and macrosomia. In addition, we evaluated the extent to which women who were below, within, and above IOM gestational weight gain guidelines delivered normal birth weight infants in this cohort, as previously done among other populations.[19,20] Previous studies suggest combined effects of pre-pregnancy BMI and gestational weight gain on infant birth weight.[15,18,21] We assessed independent and combined effects of pre-pregnancy BMI and gestational weight gain on infant birth weight. Results from this study would extend existing literature by providing information that underscores the need to balance pre-pregnancy weight and gestational weight gain against the risk of LBW and macrosomia among target populations.


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