Statistical analysis
The study was initially designed to detect an 8% absolute difference in PE rate with 80% power and a 5% type I error rate.18 We assume the incidence of PE was 4% in the LDA group, while in the control group the rate was 12%. Accounting for a potential 20% loss to follow-up, therefore, a sample size of 424 (212 in the LDA group, 212 in the Control group) was needed. However, the actual loss to follow-up (including 47 subjects met twin-to-twin transfusion syndrome) of 12.90% (n=138) was less than the 20% expected and we finally followed up 932 patients (277 in the LDA group and 655 in the control group).
To adjust the unmatched baseline characteristics, a 1:1 propensity score matching (PSM) was applied between the LDA group and the control group to simulate a randomized controlled trial (RCT) of the variables. The matching algorithm was the nearest-neighbor matching and the caliper was set at 0.1. The standardized mean difference (SMD) was calculated by using R (version 2.15.3, the R Foundation for Statistical Computing, Tsinghua University, China; downloaded at http://www.R-project.org/). The value of SMD≤0.1 was considered to be a negligible difference. We also performed a sensitivity analysis to evaluate the biases caused by propensity score matching. SPSS version 22.0 software (SPSS Inc, Chicago, USA) was used for all the statistical analyses.
We dealt with data via an independent t-test and described as mean ± standard deviation if variables were in accordance with a normal distribution. Otherwise, variables were described as mean ± quartile and examined by the Kruskal–Wallis test. Differences in the classified variables were presented by percentage and evaluated by the McNemar’s test. P < 0.05 or SMD>0.1 was considered as significant. A conditional logistic regression model was used in paired data (after PSM), while an unconditional logistic model was applied in the sensitivity analysis. Confounding factors including age, BMI, and those which were previously reported to be connected with preeclampsia or were unmatched between LDA and normal groups.