Intervention fidelity and behaviour change mechanisms in a health promotion intervention for women following gestational diabetes mellitus: Secondary analysis of the Face-it RCT.

Aims: We evaluated the fidelity of the Face-it intervention and its impact on behaviour change mechanisms among women with recent gestational diabetes mellitus (GDM).

Methods: In this randomised controlled trial, 277 women were allocated to usual care or an intervention comprising three home visits, digital platform health coaching and cross-sectoral communication to support health behaviour change during the first year after delivery. Behaviour change mechanisms included social support, motivation, self-efficacy, risk perception and health literacy. High fidelity was defined as completing three home visits and ≥ 9 coaching contacts.

Results: Within the intervention group, 86.4 % completed ≥ 2 home visits, 88.6 % registered digitally with a median (IQR) of 10.0 (3.0-20.0) contacts. At one-year after delivery, the high-fidelity group (n = 73; 39.7 %) had higher odds of perceiving moderate/high diabetes risk (OR 2.42; 95 % CI 1.06-5.51) and higher health literacy (adjusted difference 0.20; 95 % CI 0.04-0.35), whereas no difference was found for social support, motivation and self-efficacy compared with usual care. No difference was observed between the low fidelity and the usual care group in behaviour change mechanisms.

Conclusions: The Face-it intervention achieved acceptable fidelity. High fidelity appears essential for improving risk perception, health literacy and supporting behaviour change mechanisms among women with recent GDM.