To determine whether the of vaginal progesterone replacement for frozen embryo transfer results in equivalent live birth rates to intramuscular injection progesterone replacement.

The purpose of this ongoing study is to look at whether Endometrin® (vaginal micronized progesterone tablets) supplemented by intramuscular injection of progesterone in oil (PIO) work as well as PIO alone for women undergoing transfer of frozen-thawed blastocyst(s). Another goal of the study is to determine whether patients prefer Endometrin or PIO. Endometrin® has been approved by the United States Food and Drug Administration, or FDA, "to support embryo implantation and early pregnancy by supplementation of corpus luteal function as part of an Assisted Reproductive Technology (ART) treatment program for infertile women." (FDA New Drug Approval Letter, Endometrin®, 2007). The use of Endometrin® (vaginal micronized progesterone tablets) in this study is investigational. An investigational use is one that is not approved by the U.S. Food and Drug Administration (FDA).

Approximately 1170 women between the ages of 18-48 who are having difficulty becoming pregnant and wish to undergo frozen embryo transfer will be asked to participate. The participants will be recruited from among patients of Shady Grove Fertility.

One-half of the participants who qualify and wish to take part in the ongoing study will be randomized (assigned by chance, like the flip of a coin) to receive Endometrin® and an intramuscular injection of PIO every third day. One-half will be randomized to receive an intramuscular injection of PIO every day. This study is a type of study called an "open label," assessor-blind study. This means that you and your doctor will know which treatment you are assigned and receive; however, the person analyzing the information obtained from the study will not know which patients received which study treatments.

Patients enrolling in the study will receive the medications for their frozen embryo transfer cycle free of charge.

Inclusion Criteria:

1. Signed informed consent
2. Female age between 18 and 48 years
3. Having available blastocyst(s) frozen by vitrification method at our center (Shady Grove Fertility).
4. Standard eligibility criteria to undergo frozen blastocyst transfer at Shady Grove Fertility.

Exclusion Criteria:

1. Requires fresh embryos or surrogate carrier
2. Embryos from frozen oocytes and embryos frozen more than once
3. Any embryo cryopreserved by slow freeze method and/or prior to blastocyst stage
4. Presence of any clinically relevant systemic disease contraindicated for ART
5. History of more than 3 failed cycles in previous ART attempts and/or more than 3 recurrent pregnancy losses after ET
6. Surgical or medical condition or requirement for medication, which may interfere with absorption, distribution, metabolism, or excretion of the drugs to be used
7. Subjects with a body mass index (BMI) of <18 or >38 kg/m2 at screening
8. Current or recent substance abuse, including alcohol and tobacco. (Note: Subjects who stopped tobacco usage at least 3 months prior to screening visit will be allowed)
9. Currently breast feeding, pregnant, or having (a) contraindication(s) to pregnancy
10. Refusal or inability to comply with the requirements of the protocol for any reason, including scheduled clinic visits and laboratory tests
11. Trophectoderm or blastomere biopsy of the blastocyst(s) to be transferred.
12. Documented intolerance or allergy to any of the medications used, including the study medication
13. Participation in any experimental drug study within 60 days prior to screening
14. If a subject undergoes more than two frozen blastocyst transfers meeting study criteria, she will only be eligible to enroll in the study for two of these.