To investigate whether pregestational exposure to oral combined hormonal contraception (CHC) is associated with a rise in the risk of gestational diabetes (GDM).
Prevailing GDM was assessed for all pregnancies that occurred in Tuscany, Italy, from years 2010 to 2018, using administrative data coupled with information about CHC prescriptions in the year prior to pregnancy retrieved from the regional registry of drug prescription claims. The relation between exposure to CHC and risk of GDM, expressed as Odds Ratio: OR (95% Confidence Intervals, CI), was calculated separately based on citizenship of mothers using multiple logistic regression analysis models, after adjusting for confounders.
Among 210,791 pregnancies from 170,126 mothers, GDM was present in 22,166 (10.5%) pregnancies. CHC prescription within 12 months before the index pregnancy was present in 9065 (4.3%) mothers. The risk of GDM was weakly but significantly higher in pregnancies exposed to pregestational CHC only in pregnancies of mothers of Italian citizenship: OR:1.11 (95% CI 1.02-1.21); p = 0.02, after adjusting for age, parity, calendar year and pregestational body-mass index. The CHC-mediated effect was no longer present in pregnancies of mothers at higher risk of GDM, such as pregestational obesity, migrating from countries at higher GDM risk or after adjusting for the entire panel of confounders including employment status, prior spontaneous abortions, and education degree.
CHC had a modest effect on GDM risk, which became insignificant when added to basal prevailing risk factors for impaired glucose metabolism in pregnancy, such as pregestational obesity or originating from countries at high GDM risk.

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