Natural vs. programmed cycles for frozen embryo transfer: study protocol for an investigator-initiated, randomized, controlled, multicenter clinical trial oleh Sheriza Baksh, Anne Casper, Mindy S. Christianson, Kate Devine, Kevin J. Doody, Stephan Ehrhardt, Karl R. Hansen, Ruth B. Lathi, Fatmata Timbo, Rebecca Usadi, Wendy Vitek, David M. Shade, James Segars, Valerie L. Baker, NatPro Study Group