Uterine Contractions and Mode of Progesterone in Frozen Embryo Transfer (FET) Cycles: Randomized Clinical Trial

When patients undergo in vitro fertilization (IVF), they often make more embryos than it is safe to transfer back in the womb all at once. These excess embryos are frozen and stored for future use. Frozen embryo transfer (FET) is therefore an important option of IVF where there are extra embryos. FET should take place under ideal conditions when uterine receptivity and uterine endometrial lining thickness are at the appropriate developmental stage as the embryo. A common way of preparing the uterus for FET is by using medications (Estrogen and Progesterone) that imitate a normal menstrual cycle while monitoring the endometrial lining development by transvaginal ultrasound. Progesterone (P4) may be administered vaginally or intra-muscularly (IM). The mode of Progesterone administration is based upon local conventions, experts' opinion and patients' preferences. All the mentioned modalities are common in the daily practice of IVF clinics across the world.

The current medical literature does not present high quality evidence for the superiority of one form of Progesterone administration over the other for FET. The investigators know that in fresh embryo transfer cycles where natural follicles continue to function and produce Progesterone as corpora lutea, there were no reported differences in pregnancy rates when luteal vaginal P4 was compared with IM P4 support, but the investigators do not have the same reassurance regarding frozen embryo transfers. The aim of this study is to compare vaginal versus IM route of administration of P4 for FET cycle in a well-designed trial. Since many outcome variables are possible, the investigators have chosen to concentrate in the one that might shed light on a possible biological difference between the two modes of Progesterone administration; uterine contractions. Uterine contractions have been previously recognized as a possible factor that compromises success rates in patients undergoing IVF treatments. Progesterone is considered a uterine relaxant and its levels in the blood versus the levels in the uterus differ by its mode of administration: blood levels are higher when given IM whereas uterine P4 concentrations are higher when given vaginally. The investigators are interested to see whether IM progesterone in frozen embryo transfers presents a different uterine contraction pattern than the vaginal administration.