Ovarian stimulation will be performed by standard protocols. Stimulation medication dosage will be individualised prior to stimulation start according to the ovarian reserve parameters and during ovarian stimulation according to the ovarian response and the measured levels of E2 and progesterone (P4), in order to avoid progesterone elevation during late follicular phase. Final oocyte maturation will be achieved by administration of either 10.000 IU of hCG, 0.3 mg of GnRH agonist (Triptorelin) or dual trigger (hCG and GnRH-analogue), as soon as ≥ 3 follicles ≥ 17 mm are present. Oocyte retrieval will be carried out 36 hours after administration of the trigger. Embryos will undergo PGT-A at blastocyst stage and be vitrified thereafter.

Start of estradiol valerate 4mg on day 2 of the cycle for three days. Increase E2 to 6mg on day 4 of E2 treatment. E2 dose may be increased according to clinician discretion based on endometrial thickness. Maximum time of E2 exposure will be 14 days. Transvaginally scan to monitor endometrial development and to exclude the presence of a dominant follicle. Serial measurements of serum LH, estradiol and progesterone levels. Commence the initial progesterone dose of 100mg at 22hrs (vaginal suppository) after ≥ 7 days and ≤ 16 days of estradiol administration when the minimal endometrial thickness achieved is 6mm with a trilaminar appearance. Subsequently increase progesterone administration to 100mg vaginally three times daily. Continue estradiol administration 6mg (3 tablets daily).

Blastocyst transfer is scheduled on the 5th full day of progesterone administration, following the initial initiation of progesterone.

Ultrasound scans to monitor follicular growth and serial measurements of serum LH, estradiol and progesterone levels to determine the timing of ovulation. The LH surge will be considered to have begun when the concentration rises by 180% above the most recent serum value and continues to rise thereafter.

Day 1 after the LH rise, a decrease in estradiol concentration is identified. Twenty four hours later progesterone concentrations rise with a level of greater than or equal to 1.5ng/ml confirming ovulation (day 0). This is considered as day 0 with initiation of vaginal progesterone 100mg (vaginal suppository) at 2200H. The following day (day 1) increases progesterone administration to 100mg vaginally three times daily (8 hourly) and continues this regime until 7 weeks gestation as per clinic protocol.

Embryo transfer is scheduled 5 days (day 5) following confirmation of ovulation (day 0).

Mesurement of E2, P4, LH, FSH hormones

Ongoing pregnancy rate (≥ 12 weeks) in patients with endometrial compaction compared to patients without endometrial compaction after frozen embryo transfer of 1 or 2 euploid blastocysts

Positive hCG, but at 5 gestational weeks no ultrasonographic visible gestational sac seen after embryo transfer in HRT-FET cycles with one or two euploid embryos, depending on the degree of compaction.