Effect of Intramuscular Progesterone Supplementation on Clinical and Ongoing Pregnancy Rates in Patients With Low Serum Progesterone Levels on the Day of Embryo Transfer in Artificial Frozen Cycles

Cryopreservation of frozen-thawed embryos has become a cardinal procedure in assisted reproductive therapies. This can be attributed to the development of the vitrification process and the improvement of embryo survival rates after thawing, leading to the progressive increase in the use of frozen embryo transfer (FET).

The need for FET has also increased greatly due to the implementation of a single embryo transfer policy in many areas of the world to reduce multiple pregnancies. This has led to an increase in surplus embryos derived from ovarian stimulation cycles. Moreover, although the 'freeze all' technique or elective embryo cryopreservation was mainly developed for patients with an increased risk of developing ovarian hyperstimulation syndrome, its use has now been extended to cover other indications, such as cycles involving pre-implantation genetic diagnosis, late-follicular progesterone elevation and embryo-endometrial asynchrony.

Despite this increased need for FET, there still exists debate over the best method to implement the practice. The two most common used treatment regimens to prepare the endometrium for embryo transfer are the so-called natural cycle (NC) and the hormone replacement therapy (HRT) cycle.

In a NC FET, there is no medical intervention apart from ultrasound monitoring during the proliferative phase, to schedule the transfer when the endometrium is synchronized to the developmental stage of the embryo. Although the advantage is the absence of estrogen supplementation, this protocol entails more frequent visits to the clinic and less cycle control. Despite this, there is still an ongoing debate whether frozen embryos transferred in a 'more physiologic' non-stimulated endometrium, may not only result in higher pregnancy rates, but also potentially decrease maternal and neonatal morbidity.

On the other hand, in the HRT cycle, also referred to as an artificial cycle, exogenous administration of estrogens and progesterone is used to mimic a natural cycle and therefore endometrial development. Although originally developed to allow embryo transfers in recipients of donated oocytes, the HRT protocol has proven to be successful in the general population as well.