A comparison of South Asian specific and established BMI thresholds for determining obesity prevalence in pregnancy and predicting pregnancy complications: Findings from the Born in Bradford cohort

OBJECTIVE: To describe how maternal obesity prevalence varies by established international and South Asian specific BMI cut-offs in women of Pakistani origin and investigate whether different BMI thresholds can help to identify women at risk of adverse pregnancy and birth outcomes. DESIGN: Prospective bi-ethnic birth cohort study (The Born in Bradford Cohort). SETTING: Bradford, a deprived city in the North of the UK. PARTICIPANTS: 8,478 South Asian and White British pregnant women participating in the Born in Bradford cohort study MAIN OUTCOME MEASURES: Maternal obesity prevalence; prevalence of known obesity related adverse pregnancy outcomes: mode of birth, hypertensive disorders of pregnancy (HDP), gestational diabetes, macrosomia, pre-term births. RESULTS: Application of South Asian BMI cut-offs increased prevalence of obesity in Pakistani women from 18.8% (95% CI 17.6 to 19.9) to 30.9% (95% CI 29.5 to 32.2). With the exception of pre-term births, there was a positive linear relationship between BMI and prevalence of adverse pregnancy and birth outcomes, across almost the whole BMI distribution. Risk of gestational diabetes and HDP increased more sharply in Pakistani women after a BMI threshold of at least 30kg/m(2), but there was no evidence of a sharp increase in any risk factors at the new, lower thresholds suggested for use in South Asian women. BMI was a good single predictor of outcomes (Area Under the Receiver Operating Curve: 0.596 to 0.685 for different outcomes); prediction was more discriminatory and accurate with BMI as a continuous variable than as a binary variable for any possible cut-point. CONCLUSION: Applying the new South Asian threshold to pregnant women would markedly increase those referred for monitoring and lifestyle advice. However, our results suggest that lowering the BMI threshold in South Asian women would not improve the predictive ability for identifying those at risk of adverse pregnancy outcomes.