Fasting plasma glucose, diagnosis of gestational diabetes and the risk of large for gestational age: a regression discontinuity analysis of routine data.

OBJECTIVE

To estimate the causal effects of fasting plasma glucose (FPG) and diagnosis of gestational diabetes (GDM) on birthweight and the risks of large for gestational age (LGA).

DESIGN

Regression discontinuity analysis of routine data.

SETTING

Two district general hospitals in West Yorkshire, UK.

POPULATION: A cohort of 7062 women with singleton pregnancies who were screened for GDM and gave birth to a baby at ≥24 weeks of gestation in 2017-2019, inclusive.

METHODS

The causal effects of FPG and GDM diagnosis were estimated using the two-stage least-squares approach, around the diagnostic threshold of FPG ≥ 5.6 mmol/l recommended by the UK's National Institute for Health and Care Excellent (NICE), controlling for ethnicity, maternal age, parity, height and weight.

MAIN OUTCOME MEASURES: Birthweight (standardised for sex and gestational age) and large for gestational age (standardised as birthweight above the 90th centile).

RESULTS

For each 1 mmol/l increase in FPG the observed birthweight increased by Z-score = 0.48 standard deviations (95% CI 0.39 to 0.57) and the odds of LGA increased by OR = 2.61 (95% CI 1.86 to 3.66). Conversely, GDM diagnosis reduced the observed birthweight by Z = -0.61 (95% CI -0.94 to -0.29) and lowered the odds of LGA by OR = 0.33 (95% CI 0.15 to 0.74). Similar, but less certain, patterns were observed for caesarean section, shoulder dystocia and perinatal death.

CONCLUSIONS

The relationship between FPG and LGA is potent but is dramatically reduced by GDM diagnosis (and all the consequences thereof). Women with mild hyperglycaemia (with an FPG of 5.1-5.5 mmol/l) who fall below the current NICE threshold for GDM diagnosis have the highest risks of adverse outcomes, suggesting a need to reconsider their current care.

TWEETABLE ABSTRACT: Regression discontinuity analysis shows that untreated mild hyperglycaemia increases the odds of large for gestational age, but that a diagnosis of gestational #diabetes lowers the odds by three times.