The objective of this study was to assess the impact on the clinical pregnancy rate of luteal phase progesterone treatment in patients being prepared for natural cycle frozen embryo transfer (FET) with induced ovulation.

This retrospective cohort study collect all the FET protocols over a 6-month period at Strasbourg University Hospital fertility unit between December 2016 and May 2017. In total 293 consecutive patients with regular menstrual cycles were prepared for natural cycle FET during this period. All patients had an embryo cryopreservation secondary to in vitro fertilisation (IVF) or by intracytoplasmic sperm injection (ICSI). There were 2 protocols during this period and patients either received or did not received progesterone. Ovulation was routinely triggered in all patients by injection of choriogonadotrophin alfa. Patients in the treated group received vaginal natural micronized progesterone treatment of 400mg daily, starting on the day of ovulation. The principal assessment criterion was the occurrence of pregnancy.

In total, 231 patients were analysed: 108 in the group not receiving progesterone and 123 in the group receiving progesterone. Patient characteristics were comparable between groups. A higher clinical pregnancy rate (39% vs. 24.1%, p=0.02; 95CI [1.10; 3.74]) was recorded in the treated group.

Our results suggest that luteal phase support with vaginal progesterone statistically increases the clinical pregnancy rate following hCG-triggered natural cycle FET and that it should be used more widely.