PRogrammed Versus Modified Natural Cycle After Euploid Failed Embryo Transfer

Major Inclusion Criteria: The following are major inclusion criteria:

1. Patients planning to undergo frozen embryo transfer cycle with a single euploidblastocyst.

2. Patients who have previously undergone their first unsuccessful frozen embryotransfer cycle, defined as either failure of implantation with negative pregnancytest or biochemical pregnancy, using a programmed cycle protocol.

3. Preimplantation genetic testing for aneuploidy (PGT-A) of the embryo used in failedprogrammed embryo transfer cycle must have occurred after January 1, 2017.

4. Patients ages 18 to 53 years old as per practice guidelines.

5. Patients with BMI between 16-45 kg/m2.

6. Patients with at least one embryo remaining in storage, from either the same or aseparate cohort.

7. Patients with proven ovulatory function, as defined by the presence of regularmenstrual cycles or detection of luteinizing hormone surge on serum or urinary testkits.

8. Patients with ≥ 7 mm endometrial thickness prior to progesterone start in priortransfer cycle.

9. Normal uterine cavity as evidenced by recent (within 1 year) saline sonogram orhysteroscopy.

Major Exclusion Criteria: The following are exclusion criteria:

1. More than 1 prior unsuccessful frozen embryo transfer cycle.

2. The prior FET failure having had resulted in a clinical loss or ectopic pregnancy

3. Previously cancelled frozen embryo transfer cycle for inadequate endometrialresponse.

4. Patients who required a different route of estrogen administration in the priorprogrammed cycle (vaginal, transdermal, intramuscular).

5. PGT-A analysis of the available embryo for transfer performed prior to January 1,

7. Anovulatory or oligo-ovulatory patients unable to undergo modified naturalendometrial preparation.

8. Patients with an endometrial thickness < 7 mm prior to progesterone start in priorcycle.

9. History of hypertensive disorders of pregnancy, including gestational hypertension,preeclampsia, and eclampsia.

10. Mullerian anomalies, excluding arcuate uterus and repaired septum.

11. No euploid embryos available for transfer.

12. Concurrent submucosal fibroids, unrepaired uterine defects or present indication foruterine surgery.

13. Communicating hydrosalpinx without plans for surgical correction prior to studyenrollment.

14. Failure of patient to agree to enrollment in study with written consent.

15. Concurrent pregnancy.

16. Concomitant use of adjunctive therapies for endometrial proliferation orreceptivity, including anticoagulation and antihistamines. These must bediscontinued upon enrollment.

17. Embryo planned to be used for transfer generated from surgically obtained sperm.

18. Recurrent/persistent endometrial fluid in prior cycles visualized on the majority oftransvaginal ultrasound monitoring.

19. Third party reproduction patients (donor sperm sources can be included).