Natural Versus Programmed Frozen Embryo Transfer (NatPro)

Inclusion criteria To be eligible, subjects must meet all these criteria: * Age 18-39 years at the time that embryos were created if no PGT testing was performed. If PGT testing was performed and indicates that an embryo is euploid, the patient can be included if she was age 18-41 years at the time that the embryo was created. * Age 18-41 years at the time of randomization as advancing age itself becomes a significant risk for preeclampsia * Normal uterine cavity as assessed by saline infusion sonohysterogram, hysterosalpingogram, or hysteroscopy within one year of the enrollment visit, and repeated at the discretion of the investigator * Regular menstrual cycle length (approximately 24-35 days) indicative of ovulatory cycles. * Willing to undergo elective single embryo transfer * Body Mass Index \<=40 * If Body Mass Index is over 30 or individual has other risk factors for diabetes, normal hemoglobin A1C * Prior to enrollment, participant will have at least one vitrified blastocyst with euploid result by pre-implantation genetic testing (PGT-A) or at least one vitrified blastocyst of fair or better morphologic quality if no PGT-A results are available. * Willingness to be randomized to either a modified natural or programmed cycle, with a willingness to administer intramuscular progesterone in oil if assigned to the programmed cycle. * Normal thyroid stimulating hormone (TSH), according to local laboratory standards (or TSH level with a clinically insignificant abnormality…