Prospective observational comparison of arteria uterina blood flow between two frozen embryo transfer cycle regimens: natural cycle versus hormonal replacement cycle

Purpose Is there a difference in the blood flow of the Arteria uterina in frozen embryo transfer (FET) cycles between a Natural Cycle (NC) and a Hormonal Replacement Therapy (HRT) cycle? Methods Prospective observational study with measurement of the pulsatility index (PI) and resistance index (RI) throughout the ovarian stimulation cycle for IVF/ICSI, the FET cycle and at 12 weeks of gestation. Results A total of 124 ovarian stimulation cycles with preimplantation genetic testing for aneuploidy (PGT-A) and "freeze-all" strategy due to PGT-A were included. Mean patient's age was 31.4 years, mean BMI 26.47 kg/m2, mean AMH 3.62 ng/ml and a mean AFC of 13. FET cycles were performed in 77 patients (NC protocol: 37.7%, HRT protocol: 62.2%). The overall pregnancy rate was 75%, (NC group: 79%, HRT-group 73%; not significant). No significant change of PI and RI was seen during hormonal stimulation. In FET cycles, there was a significant increase between cycle day 2/3 and ovulation/P4-start in the HRT-cycle, followed by a significant decrease until 12 weeks of gestation. The slope of the decrease in patients with a pregnancy in an HRT-approach was a bit steeper than in the NC-approach for both PI and RI, however, without a significant difference. Conclusions Early measurements of the blood flow parameters during the FET cycle do not reveal a difference between the NC- and the HRT-approach for FET, which could be predictive for development of pre-eclampsia.