Fresh vs. frozen embryo transfer: evidence from registry data vs. randomized trials

We welcome the article by Weiss et al. (1), who compared fresh vs. frozen single embryo transfer using national surveillance data. They compared the results of their study with those of our randomized trial (2) by applying "similar" eligibility criteria. However, there were differences in the included patients between the 2 studies. We screened patients on day 3 of embryo culture, and those with 4 or more cleavage-stage embryos were randomly assigned to minimize the risk of failure of blastocyst formation.