Obesity during pregnancy increases the risk of serious morbidities and mortality for both the mother and child. Improving dietary intake and physical activity in obese pregnant women may reduce these health burdens. The UK Pregnancies Better Eating and Activity Trial (UPBEAT) is a lifestyle intervention principally aimed at reducing the incidence of gestational diabetes and macrosomia in obese pregnant women and their offspring. The aim of the present study was to assess overall diet quality, using the Alternative Health Eating Index for Pregnancy (AHEI-P), in participants of UPBEAT and to determine the effects of the intervention on diet quality. In brief, UPBEAT is a multicentre randomized controlled trial comparing the effects of a lifestyle intervention, aimed at reducing glycaemic load and saturated fat and increasing physical activity, with usual care in obese pregnant women. Dietary intake was assessed by structured 24-h recalls in 183 women participating in the UPBEAT pilot phase, and analysed using WISP dietary analysis software. The AHEI-P index of diet quality was applied to the dietary data at baseline (15+0-18+6 weeks gestation) and following the intervention (27+0-28+6 weeks gestation), and changes in diet quality assessed. AHEI-P is a 90-point scale based on intakes of fruit, vegetables, white and red meat, fibre, trans fats, polyunsaturated and saturated fats, folate, calcium and iron. Mann-Witney test of difference was used to compare intervention and control groups at baseline. Analysis of covariance was used to compare post intervention AHEI-P scores between groups, adjusting for baseline values. Relationships between diet quality and sociodemographic factors were explored using Pearson correlation and chi-squared tests.Mean AHEI-P scores were low prior to the UPBEAT intervention (intervention group 39.9 ± 8.4, control group 43.1 ± 9.5). Following the intervention, there was a significant difference in AHEI-P scores between the groups (intervention group 43.6 ± 8.5, control group 43.5 ± 9.3, p = 0.004). Significant group differences were observed for the vegetable (p = 0.001), white to red meat ratio (p = 0.005), fibre (p = 0.045), polyunsaturated to saturated fat ratio (p = 0.014), trans fat (p < 0.001), calcium (p = 0.017), iron (p = 0.014) and folate (p = 0.013), components. Baseline AHEI-P scores were positively correlated with age (r = 0.174, p = 0.019), and years in full-time education (r = 0.207, p = 0.005). There was a negative correlation between baseline AHEI-P scores and body mass index (r = -0.204, p = 0.006).In conclusion, we have found poor diet quality in a cohort of obese pregnant women. However, following the UPBEAT lifestyle intervention there were significant improvements in diet quality, which may be effective in improving pregnancy outcomes for this high risk group.