Nowadays, frozen-thawed embryo transfers (FET) are expending. This practice avoids risk of ovarian hyperstimulation syndrome (OHSS), as well as allowing better synchronization between endometrium and embryo, which is fundamental for pregnancy.

There are several FET protocols, including hormonal replacement therapy cycle (HRT), which enable clinicians to adapt the day of embryo transfer.

However, increase in spontaneous miscarriages was observed with this latter protocol compared to fresh embryo transfers and the other endometrial preparations (natural and stimulated), in relation with the lack of physiological corpus luteum.

Then, Clinicians interrogate about measuring serum progesterone in order to adjust their treatment and/or transfer date. Various studies have shown thresholds below and/or above which pregnancy or live birth rate were lowered.

The main objective is to find a serum progesterone threshold on the day of embryo transfer above which live birth rate is increased. The secondary objectives are to analyze the factors associated with increased serum progesterone on the day of transfer, to analyze the miscarriage rate, and impact of change on luteal phase support on day 12.

Inclusion Criteria:

• women between 18 and 43,
• having frozen-thawed single embryo transfer of a day-5 blastocyst with hormonal replacement therapy, with measurement of serum progesterone on the day of transfer

Exclusion Criteria:

• frozen-thawed transfer of a day-2 or -3 or -6 embryo,
• double embryo transfer,
• other protocol than HRT,
• patient without serum progesterone measurement on the day of transfer

patient having frozen-thawed single embryo transfer of a day-5 blastocyst with hormonal replacement therapy