1. Commence estradiol tablets (E2) 4mg from day 2 or 3 of period for 3 days
2. Increase E2 to 6mg on day 4 of E2 treatment, according to clinician discretion based on endometrial thickness.
3. Transvaginal scan throughout the HRT cycle to not only monitor endometrial development but to also exclude the presence of a dominant follicle on the ovaries.
4. Serial measurements of serum LH (luteinizing hormone), estradiol and progesterone levels.
5. Initial progesterone dose of 100mg at 22hrs (vaginal suppository) after ≥ 10 days and ≤ 16 days of estradiol administration when the minimal endometrial thickness achieved is 6mm with a trilaminar appearance.
6. Subsequently increase progesterone administration to 100mg vaginally three times daily. Continue E2 administration 6mg (3 tablets daily). Embryo transfer is scheduled 5 days following the initial initiation of progesterone

1. Day 2 of menses and throughout patients' natural cycle scans to monitor follicular growth.
2. Measurements of serum LH, estradiol and progesterone levels to determine ovulation.
3. 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 (Irani et al. 2017, Fatemi et al., 2010).
4. 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 1.5nmol /L confirming ovulation (day 0) (Irani et al., 2017; Speroff et al.). This is considered as day 0 with initiation of vaginal progesterone 100mg at 22hrs that night. The following day (day 1) the patient increases progesterone administration to 100mg vaginally 8 hourly and continues until 7 weeks gestation as per clinic protocol. Embryo transfer is scheduled 5 days (day 5) following confirmation of ovulation (day 0).

On the day of progesterone administration / progesterone rise. Quantitative continuous variable

On the day of cycle start and the day of ovulation induction. Continuous quantitative variable measured in millimeters (mm)