PRogrammed Versus Modified Natural Cycle After Euploid Failed Embryo Transfer

The goal of this randomized clinical trial is to compare frozen embryo transfer protocols in patients undergoing a second frozen embryo transfer (FET) after a unsuccessful first programmed FET cycle as a possible treatment for people undergoing infertility treatment. The purpose of this research study is to: * Determine if there is a difference between FET protocols in patients who require a second FET cycle. * Investigate if switching the FET protocol after a failed programmed cycle is beneficial for patients undergoing a second FET cycle. * Examine pregnancy outcomes including obstetrical and neonatal outcomes (if applicable) * Obtain uterine flexibility/stiffness measurements via transvaginal ultrasound prior to the embryo transfer procedure. This is called shear wave elastography. Participants will be randomized in their second FET transfer attempt to either another programmed protocol or a modified natural protocol.