What was shown
Frozen embryo transfer is now the dominant first-line procedure in ART. The talk compares HRT, modified natural and true natural cycles, examines E2 duration, day of progesterone before transfer, day-of-transfer progesterone monitoring, and the NPP protocol — a natural-cycle scheduling tool that uses vaginal progesterone instead of an HCG trigger.
Key findings
HRT cycles are associated with significantly more preeclampsia, hypertensive disorders, postpartum haemorrhage and preterm birth than natural cycles. E2 preparation should not exceed 28 days; under 8-10 days is associated with worse outcomes. Day 5, 6 and 7 of progesterone before blastocyst transfer all give comparable pregnancy rates, with day 6 now adopted as standard. Day-of-transfer serum progesterone below 8.8-9.2 ng/mL is associated with significantly lower live birth, and adding a second route (subcutaneous progesterone or oral dydrogesterone) normalises outcomes.
What this means in practice
Shift to natural or modified natural cycles where feasible, especially in patients with cardiovascular or obstetric risk. Measure serum progesterone on transfer day and rescue with a second route below the cut-off. The NPP protocol — starting vaginal progesterone when an adequate follicle and endometrium are seen, without HCG trigger — is a way to avoid weekend retrievals while keeping a corpus-luteum-supported luteal phase.
Caveats
Dydrogesterone serum levels are not routinely measurable, so it is positioned as a second-line add-on. The 7 mm endometrial-thickness threshold is now disputed (Atta review): it was arbitrary, retrospective, and inter-observer variability exceeds 2 mm. The first-line role of progesterone monitoring in natural cycles is still debated, with conflicting data on whether routine luteal-phase support is needed.
