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

Peter Tennant, Elizabeth Duxford Hook, Lauren Flynn, Kathrine Kershaw, Julie Goddard, Tomasina Stacey

Research output: Contribution to journalArticlepeer-review

Abstract

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; a quasi-experimental approach to estimating the effects of an intervention by comparing outcomes either side of a threshold for the intervention, such as that created by a diagnostic and/or treatment cut-off.
Setting
Two district general hospitals in West Yorkshire, UK
Population
7062 women with singleton pregnancies who were screened for GDM and gave birth to a baby ≥24 weeks’ gestation during 2017-2019 inclusive.
Methods
The causal effects of FPG and GDM diagnosis were estimated using the two-stage least square approach, around the diagnostic threshold of FPG≥5.6mmol/L recommended by the UK's National Institute for Health and Care Excellent (NICE), conditioning on ethnicity maternal age, parity, height, and weight. Main outcome measures: Birthweight (standardised for sex and gestational age) and large-for-gestational age (standardised birthweight above 90th centile).
Results
For each 1mmol/L increase in FPG the observed birthweight increased by Z=0.48 (95% CI: 0.39, 0.57) and the odds of LGA increased by OR=2.61 (95% CI: 1.86, 3.66). Conversely, GDM diagnosis reduced the observed birthweight by Z=-0.61 (95% CI: -0.94, -0.29), and lowered the odds of LGA by OR=0.33 (95% CI: 0.15, 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 (FPG between 5.1-5.5mmol/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.
Original languageEnglish
JournalBJOG: An International Journal of Obstetrics and Gynaecology
Publication statusAccepted/In press - 4 Aug 2021

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