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 per the site director), within
   one year of study enrollment and repeated at the discretion of the investigator. Use
   of thyroid medication is permitted.

Exclusion criteria

To be eligible, subjects must not meet any one of these criteria:

   - Medical contraindication to pregnancy

   - Embryos created using donor oocytes

   - Embryo donation

   - Gestational carrier

   - Reciprocal IVF (one female partner carrying pregnancy, other female partner as source
   of eggs)

   - Fresh embryos created from frozen oocytes. Patients with frozen embryos created from
   frozen oocytes can be included.

   - Recurrent implantation failure defined as no clinical pregnancy with ≥ 2 prior
   consecutive embryo transfers unless patient also had successful live birth between or
   after the failed embryo transfers

   - Anti-phospholipid antibody syndrome or rheumatologic disease requiring chronic
   systemic medications

   - Uncontrolled diabetes mellitus

   - History of >1 pregnancy loss in the second or third trimester

   - Uncontrolled hypertension

   - Untreated hydrosalpinx (i.e., hydrosalpinx in situ which has not been ligated or
   removed)

   - Mullerian uterine anomaly, if not correctable

   - Physician recommendation to perform the embryo transfer outside of the timing
   specified by the protocol

   - Contraindication to any medication which must be used in preparation for the frozen
   embryo transfer (i.e., estradiol, progesterone, hCG)