This pilot study evaluates how progesterone levels change after hCG-triggered ovulation in modified natural cycles. Forty women preparing for frozen embryo transfer will have blood tests over 6 days to monitor hormone levels. The goal is to understand whether hCG affects the timing of the luteal phase and embryo implantation.

Inclusion Criteria:

• At least 1 follicle of 14 mm or more
• Endometrium of 6 mm or more
• Plasma luteinizing hormone (LH) less than 2 times basal level (measured at the beginning of the cycle between D1 and D4) and progesteronemia less than 1.5 ng/mL
• For whom ovulation induction with recombinant hCG (Ovitrelle) is then proposed (standard care).

Exclusion Criteria:

• Patient whose venous capital contraindicates repeated blood sampling over a short period of time over a short period.
• Patient treated with exogenous natural progesterone (vaginal or injectable) which may interfere or synthetic progesterone, which may interfere with endogenous progesterone secretion.
• Patient who has not given written consent to participate in the study.
• Patient not fluent in French.
• Patient under guardianship, curators or without social security coverage.
• Patient participating in another study with an ongoing exclusion period or other interventional research with no ongoing exclusion period involves the use of a drug or procedure capable of altering progesterone levels .