Statistical analysis
Statistical analysis of the data was performed using SPSS 22.0 (IBM, Armonk, NY, USA). For maternal demographic and lifestyle characteristics, continuous variables were described as mean ± standard deviation. Categorical variables were described as the sample number and frequency of each category. Step wise linear regression was performed across the four visits on the following variables: PSQI, sleep latency, sleep duration and sleep efficiency to establish the relationship between sleep quality and blood pressure throughout pregnancy, after considering potential confounding factors. Data from each individual visit was analysed separately. Mixed model analysis was employed to study the overall correlation between sleep and BP as well as sleep and uterine artery resistance and pulsatility index, combining the information from all 4 antenatal visits. Analysis was adjusted for possible confounders.