Statistical analysis
The pregnancy outcomes in our study included the clinical pregnancy rate (CPR), spontaneous abortion rate (SAR), and live birth rate (LBR). Clinical pregnancy was defined as detection of a gestational sac on ultrasound at 7 gestational weeks16. In China, spontaneous abortion is defined as the spontaneous loss of a clinical pregnancy < 28 gestational weeks, and live birth as the survival of delivery ≥ 28 gestational weeks.
The patients were divided into four groups according to the 1st, 2nd, and 3rd quartiles of the overall average distance distribution, which represent different uterine corpus inclinations (Group 1, ≤ 1.98 cm, n = 131; Group 2, 1.99-2.19 cm, n = 133; Group 3, 2.20-2.50 cm, n = 130; Group 4, ≥ 2.51 cm, n = 129).
Multivariable logistic regression models were calculated for the whole cohort, with the uterine corpus inclination as the main variable of interest. The potential risk factors entered into the multivariable regression model were those that showed clinical relevance or those with a P value < 0.2 on univariate analysis. The included variables were carefully selected based on the number of events available to ensure the stability of the regression equation. Adjusted odds ratios (OR) and their 95% confidence intervals (CI) were calculated to analyze the independent effect of the uterine corpus inclination on the pregnancy outcomes. As the migratory duration before implantation differed between cleavage-stage embryos and blastocysts, we analyzed the two stages separately. The number of cleavage ET cohort was 126, and the number of blastocyst transfer cohort was 400.
The second aim of the study was to determine the cut-off point for the average distance that would predict pregnancy outcomes. Receiver operating characteristic (ROC) curves were used to estimate the sensitivity and specificity of the average distance for predicting pregnancy outcomes. P < 0.05 was considered to indicate statistical significance. All statistical analyses were performed using IBM SPSS 26.0 (IBM Corp., Armonk, NY, USA).