Background
Maternal obesity is a key risk factor for morbidity in pregnancy. Accurate data on trends in obesity are required in high‐risk populations such as in western Sydney to implement effective policy.

Aims
This study examines multi‐site public hospital data on maternal ethnicity, body mass index (BMI), gestational diabetes mellitus (GDM) and hypertension across 20 years in Western Sydney Local Health District (WSLHD).

Materials and methods
This is a retrospective cohort study of all women who delivered a live birth beyond 20 weeks at Westmead, Blacktown and Auburn Hospitals (WSLHD) between 1 January 1997 and 31 December 2016.

Results
There were 112 308 pregnant women included. Between 1997 and 2006, mean booking‐in BMI climbed from 24.9 (median 23.9) to peak at 26.2 (24.9). It then fell to 25.3 (24.1) in 2012 before rising to 25.6 (24.4) in 2016. Rates of hypertensive disorders changed little over the period, with a small fall in pre‐eclampsia. In contrast, there was a progressive upward trend in the prevalence of GDM, accelerating considerably after 2010. These trends were associated with a shifting ethnic profile with proportions of Australia/New Zealand‐born women falling from 56.9% to 36.8%, while those from South Asia increased from 4.5% to 26.3%.

Conclusions
Western Sydney booking‐in BMI fluctuated between 1997 and 2016, reaching its peak in 2006. Despite this, rates of GDM progressively rose, with one in six mothers in western Sydney now diagnosed with some form of the condition. Both patterns are associated with a notable shift in the ethnic profile of patients booking‐in to antenatal care in the region.