Introduction: Elevated pre-gestational body mass index (obese/overweight) and improper gestational weight gain are important risk factors for predicting adverse pregnancy outcomes.

Objective: We aimed to identify the association between pre-pregnancy body mass index (BMI) and gestational weight gain on birth outcomes.

Materials and Methods: Data from this retrospective cohort study were extracted from 1457 (out of 1800) pair health records belonging to the pregnant mother and infants at Ahvaz Iran health care centers from 2010 to 2018. Ten public health care centers were randomly selected from the headquarters west and east of Ahvaz City. The samples were selected based on the inclusion criteria. They divided into different groups based on BMI (underweight, normal, overweight, and obese) and gestational weight gain (GWG) groups (inadequate and excessive versus adequate). Pregnancy outcome was analyzed according to the GWG during pregnancy for each BMI group through multivariable multinomial logistic regression. Relationships between maternal BMI, GWG, and offspring weight were examined. The analysis of covariance (ANCOVA) was used with adjusting the baseline values. The risks for adverse birth weight outcomes in women with different pre-pregnancy BMIs and GWGs were tested using the multivariable multinomial logistic regression analysis.

Results: The Mean±SD of maternal age was 28.36±5.60 years. The Mean±SD birth weight was 3271.37±486.57 g. About 4.5%, 89.3%, and 6.2% of newborns were low birth weight, normal weight, and macrosomia, respectively. Also, 46.4% of women had weight gain above the guidelines. The increased risk for large for gestational age in overweight mothers (odds ratio [OR] =3.18, 95%CI; 0.45-7.29, P=0.007) and an increased risk for small for gestational age in those mothers with gestational weight gain below the guidelines (OR=2.9, 95%CI; 1.16-7.45, P=0.02). An increased risk of large for gestational age, low birth weight, and macrosomia were observed in overweight mothers with gestational weight gain out of the guidelines. An increased association was found between the maternal pre-pregnancy BMI and fasting blood sugar in 24 to 28 weeks of gestation. Hence, hyperglycemia is related to the incidence of macrosomia (OR=3.58, 95%CI; 1.70-7.66, P=0.0001). 

Conclusion: Managing maternal weight with respect to reproductive health care is required for all women in childbearing age (before and during pregnancy) to reduce the adverse pregnancy outcome.