Summary of hCG and progesterone levels from the time of hCG trigger until early pregnancy during ART. After hCG trigger (day -2), hCG levels rapidly rise to around 200 IU/L at the time of oocyte retrieval (day 0) and are then cleared by around day 5 after retrieval (Beckers et al., 2003). Progesterone levels follow more slowly, as granulosa cells become luteinized, and peak around day 5 after retrieval and rapidly drop there after (Beckers et al., 2000). This creates several days during which endogenous progesterone levels lack hCG stimulation and require supplementation to remain over the threshold of 80–100 nmol/L to maintain pregnancy (Andersen and Andersen, 2014).