Sample size estimation The ongoing pregnancy rate of IVF-ET in 2010 was about 30.0%. Assuming a 10% increase following use of protective ET catheters as significant, 356 patients in each arm was required at a power of 80% and a significance level of 5% (Sigmastat, Jandel Scientific, San Rafael, CA, USA). A total of 720 patients will be recruited into the study.