Comparison of Vaginal and Intramuscular Progesterone in Vitrified-warmed Blastocyst Transfer Cycles

Inclusion Criteria: * Signed informed consent * Patient age at time of embryo freezing: 18 to 40.9 years * Patient age at embryo transfer: 18 to 50 years (i.e. women currently ages 18-50 years who are transferring embryos created using eggs retrieved from women aged 18-40.9 years) * Blastocysts frozen by vitrification at Brigham and Women's Hospital * Standard eligibility criteria for blastocyst transfer at Brigham and Women's Hospital Exclusion Criteria: * Fresh or cleavage-stage embryo transfer planned * Gestational carrier cycles * Natural and modified natural cycles * Embryos frozen more than once or derived from thawed oocytes * Embryos frozen at centers other than Brigham and Women's Hospital * Embryos frozen using techniques other than vitrification (i.e. slow frozen) * Patients with recurrent pregnancy loss, defined as failure of 3 or more clinical pregnancies * Uterine factor infertility: Asherman's, submucosal fibroids, polyp greater than 1 cm at time of embryo transfer, uninterrupted hydrosalpinx * Patients with three prior failed embryo transfers (fresh or frozen) * BMI\<18 or \>40 kg/m2 at screening * Currently breast feeding or pregnant * Embryo biopsy performed * Current smoking, alcohol or illicit drug use * Allergy to study drugs * Refusal or inability to adhere to study protocol * Participation in other experimental drug trials concurrently within the past 60 days