Frozen-Thawed Embryo Transfers: a Comparison Between Natural Cycles With Spontaneous or Induced Ovulation

Compared to repeated oocyte retrieval procedure, frozen-thawed embryo transfer (FET) has been widely used to increase the cumulative pregnancy rate per IVF-cycle, with demonstrated superiority in preventing ovarian hyperstimulation syndrome and improving cost-efficiency and time to pregnancy.

It is controversial whether triggering ovulation of the dominant follicle using human chorionic gonadotrophin (hCG) may benefit or reduce embryo implantation, when compared with a natural cycle environment. Unfavourable clinical outcomes of controlled ovarian stimulation have been reported by recently published studies, compared to the spontaneous LH surge.

This study aimed to compare the effectiveness in terms of better clinical pregnancy rates (CPR) of pure natural cycle frozen-thawed embryo transfer (NC-FET) versus natural cycle frozen-thawed embryo transfer modified by HCG administration\with hCG-triggered ovulation (mNC-FET).

A retrospective analysis was conducted between 2012-2017. In patients with regular ovulatory cycles, the timing of embryo thawing and transferring was based on spontaneous LH surge (NC-FET). Patients attended for ultrasound evaluation of the dominant follicle from Day 8 to 10 of their menstrual cycle (depending on cycle length), detecting luteinizing hormone (LH) surge in urine/ taking an ovulation test for urinary LH measurement. In selected cases, a serum assays of LH, progesterone and estradioI has been further obtained. When the endometrial thickness reached 8 mm and dominant follicle 16-20 mm in diameter, hCG was administered in absence of urinary LH surge. Embryo thawing and transfer was planned 7 days after LH surge or HCG administration, whether G5 or G6 blastocyst. Exogenous progesterone supplementation started 2 days after hCG administration versus the same day of embryo transfer procedure in NC- ET. To limit potential confounders, only single blastocyst transfer cycles were included, vitrified on Days 5 or 6, excluding PGT-a (Pre Gestational Test for aneuploydia) cycles and cleavage stage embryo transfers. A unilevel and multi level logistic regression analysis was conducted using Stata Software versione15.