Factors affecting success rates in two concurrent clinical IVF trials: an examination of potential explanations for the difference in pregnancy rates between the United States and Europe

ObjectiveTo compare a US clinical trial of gonadotropin therapy for IVF with a similar European trial to determine what factors may explain the higher clinical pregnancy rate in the US trial.DesignComparison of baseline, treatment, and outcome variables in the United States (US) and European trials.SettingIVF practices in the US (n=4) and Europe (n=6).Patient(s)297 women undergoing IVF.Intervention(s)None.Main Outcome Measure(s)Clinical pregnancy rate.Result(s)Clinical pregnancy rates were 43.4% in the US compared with 29.7% in Europe (p=0.016), with a live birth rate of 38.2% versus 27.6% (p=0.064). This difference in clinical pregnancy rate could not be explained by differences in the US versus Europe for number of embryos transferred (2.3 vs. 2.6) or female age (34.6 vs. 30.4). Although the starting dose of gonadotropin was higher in the US trial compared with the European trial (300 versus 225 IU), the total dose of gonadotropin was only slightly higher in the US. In multiple logistic regression analysis of 81 pretransfer variables on clinical pregnancy, the only two found to be significant predictors of outcome were baseline endometrial thickness following down-regulation and number of days of gonadotropin treatment.Conclusion(s)This study suggests the possibility that US pregnancy rates may be higher in part because of differences in down-regulation or gonadotropin dosing. Other factors not assessed in these studies or in national datasets likely also contribute to the difference in pregnancy rates.