When to test: defining the earliest reliable time for pregnancy detection and its prognostic value after frozen embryo transfer

BACKGROUND: Serum human chorionic gonadotropin (hCG) testing performed 12 to 15 days after embryo transfer (ET) is the standard method to confirm pregnancy, and hCG levels are widely used to predict subsequent pregnancy outcomes. However, the earliest reliable time for pregnancy detection using urinary hCG testing after frozen embryo transfer (FET) and its prognostic value remain undefined. OBJECTIVE: To determine the earliest time that pregnancy could be reliably detected by urinary hCG test following FET and to identify its influencing factors as well as the predictive value for pregnancy outcomes. STUDY DESIGN: This was a prospective cohort study conducted in an academic center for reproductive medicine between September 2024 and May 2025. A total of 120 patients undergoing frozen single blastocyst transfer were recruited. All participants performed daily urinary hCG tests from the third day after FET until the 11th to 14th days when serum hCG levels were measured. RESULTS: In patients without exogenous hCG administration before FET, the earliest positive urinary hCG results could be detected on the fourth day after FET. Among patients with pregnancy demonstrated by serum hCG measurement, 100% could be detected on the ninth day after FET by urinary hCG test. In patients with exogenous hCG administration, 28.3% had a positive urinary hCG test on the third day after FET, and all pregnancies were detected on the ninth day after FET. Compared with the late positive group, the early positive group exhibited a higher ongoing pregnancy rate (92.6% vs 68.4%, P =.015) and a lower risk of pregnancy loss (7.4% vs 31.6%, P =.015). CONCLUSION: Urinary hCG test performed on the ninth day after FET was reliable for the earliest detection of pregnancy. An earlier positive urinary hCG result was associated with a higher ongoing pregnancy rate compared with a later positive result.