The Relationship between the Mother’s Pre-pregnancy Body Mass Index (BMI) and Infant’s Birth Weight

Document Type: Original Article


1 Medical Student of Mashhad University of Medical Sciences, Student Research Committee, Mashhad, Iran

2 PhD Student of reproductive health, Student Research Committee, Nursing and Midwifery School, Mashhad University of Medical Sciences, Mashhad, Iran

3 Department of Biostatistics, School of Public Health, Mashhad University of Medical Sciences, Mashhad, Iran


Introduction: Birth weight, as one of the main indices of growth, is among the determining factors of survival. Given the absence of documented studies and lack of information regarding the influence of mothers’ body mass index (BMI) on newborns’ birth weight, this study was carried out with the aim to determine the relation between the mother’s BMI at the beginning of pregnancy and the infant’s weight at birth.
Methods: In the current cross-sectional study, 800 pregnant women, who were referred to medical and healthcare centers of Mashhad during 2011 (with a gestational age less than 12 weeks), were selected using stratified random multi-level cluster sampling. In the first prenatal checkup, a questionnaire, containing questions about personal information, was completed and mothers’ BMIs were calculated. At birth, the infants’ weight and other variables were surveyed. Analysis of data was done using SPSS version 11.5 and descriptive and analytical tests.
Results:Among 800 mothers, 14% were underweight (BMI<19), 51.8% were in the normal range (19<BMI<25), 19.6% were overweight (25<BMI<30), and 14.6% were obese (BMI>30). Based on the variance analysis test, the average birth weight of an infant increased with the mother’s weight gain (P<0.001). Also, the average of infant’s birth weight was significantly higher in mothers aged ≥35 years, mothers who gave birth to male infants, and multiparous women. Low birth weight and preterm labor were significantly more prevalent among underweight mothers, and macrosomia was more common among the infants of obese mothers. Furthermore, cesarean section was more common among mothers with increased BMI. The results of the extrapolated linear model indicated that mother’s BMI at the beginning of pregnancy plays a role in predicting the infant’s birth weight.
Conclusion:The results of this study highlight the importance of mother’s BMI at the beginning of pregnancy in improving her (and the child’s) health indices. Abnormal BMI leads to undesirable prenatal complications. As a result, BMI can be used as a measure to identify pregnant women, who are at risk of maternal and neonatal complications, and prevent the associated problems.