The purpose of this pilot study is to evaluate whether a decrease in endometrial thickness (compaction) prior to progesterone exposure (endogenous or exogenous) to the day before a single frozen embryo transfer has an impact on the clinical pregnancy rate with euploid embryos in exogenous hormone and modified natural thaw cycles. Information derived from this study may give insight on ways to improve pregnancy rates in cryo-thaw cycles.

Methods: Study participation will not influence any element of IVF treatment and all participants will be receiving the standard of care at Boston IVF. Potential participants will be identified by a member of the research team, who will review the appointment log and medical records to ensure eligibility. Eligible participants interested in participating in the study will have an abdominal ultrasound endometrial thickness measurement on the day of the participants embryo transfer (T2). This abdominal ultrasound is part of routine clinical care necessary for the embryo transfer. The eligible participant will provide verbal consent to a member of the research team prior to the ultrasound. Baseline demographic data will be obtained from the electronic medical record including: age, race, education level, partner status, height, weight, gravidity, and parity.

Two groups of thaw cycles will be included: (1) exogenous hormone replacement thaw cycles and (2) modified natural thaw cycle. Both groups will follow the standard of care for cycle preparation at Boston IVF. Ultrasound measurements of the endometrial lining (T1) will only be used if they were within 1 day of the initiation of progesterone (group 1) or ovulation trigger injection (group 2). At T1, multiple endometrial lining measurements will be made per standard protocol, but only the optimal image and measurement will be saved. At T2, only a single endometrial thickness measurement will be measured by the sonographer and will be recorded on a study log. Images should include a longitudinal view of the endometrium and cervix. Measurements from the T2 ultrasound will be blinded from the participant and they will not be used to make any clinical decisions. Both groups will have a comparison of the difference in endometrial thickness measurement between T1 and T2.

Sample Size Justification: Given the limited data on this specific topic, the study will be an observational pilot study. Given the available resources, the researchers will aim to have 100 participants per cycle preparation group, for a total of 200 participants.

Data Analysis: Descriptive data will be presented as the proportion, mean with standard deviation, or median with interquartile range. Comparisons will be made using the chi-square or Fisher's exact test for categorical variables and parametric or non-parametric tests for continuous variables based on data distribution. Logistic regression analysis will be performed for multivariate analysis. All data will be analyzed with SAS 9.4 (SAS Institute Inc., Cary, NC, USA). All tests will be two sided and a P value < 0.05 will be required to confer significance.

Inclusion Criteria:

• Single euploid frozen embryo transfer
• ≥ 7 mm endometrial lining on transvaginal ultrasound
• 1st or 2nd frozen embryo transfer

Exclusion Criteria:

• Obesity with body mass index > 40
• Underweight with body mass index < 18.5
• Utilization of a gestational carrier
• History of recurrent pregnancy loss (≥ 2 spontaneous abortions)
• Uterine factors (history of uterine adhesions, uterine surgery, fibroids, polyps)
• History of an endometrial receptivity assay (ERA) biopsy

Participants that are having a single frozen embryo transfer with a euploid embryo.