Validity of birth certificate-derived maternal weight data in twin pregnancies

Birth certificates are an important source of pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) data for surveillance and aetiologic studies, but little is known about their validity in twin pregnancies. Twins experience high rates of adverse perinatal outcomes that have been associated with BMI and GWG in singletons. Our objective was to evaluate the accuracy of birth certificate-derived pre-pregnancy BMI and GWG compared with medical record-derived data in a sample of 186 twin pregnancies at a teaching hospital in Pennsylvania (2003-2010). Twelve strata were created by simultaneous stratification on pre-pregnancy BMI (underweight, normal weight/overweight, obese class 1, obese classes 2 and 3) and GWG (<20th, 20-80th, >80th percentile). The agreement of birth certificate-derived pre-pregnancy BMI category with medical record BMI category was lowest among underweight mothers [75% (95% confidence interval 51-91%) ] and highest among normal/overweight [97% (90-99%) ] and obese classes 2 and 3 mothers [97% (85-99%) ]. Agreement for GWG category from the birth certificate varied from 57% (41-70%) for GWG >80th percentile to 80% (65-91%) and 82% (72-89%) for GWG <20th and 20th-80th percentiles, respectively. The misclassification of BMI and GWG was primarily due to error in pre-pregnancy weight rather than weight at delivery or height. Agreement proportions for twins were not meaningfully different from the proportions in a comparable sample of singleton pregnancies. These data suggest that birth certificate-based BMI and GWG data are prone to error in twin pregnancies. Those who use these data should conduct internal validation studies and adjust their results using bias analyses.