The global increase in frozen-thawed embryo transfer (FET) cycles has led to a critical evaluation of endometrial preparation methods. While various approaches such as natural or modified natural cycle FET, stimulated FET by use of letrozole (LTZ) and/or gonadotrophins, and artificial cycle (AC) FET, are currently in clinical use, the optimal regimen remains unclear, particularly for women with oligo-anovulation or polycystic ovarian syndrome (PCOS). This systematic review and meta-analysis compares LTZ FET with AC FET regarding reproductive, obstetric, and neonatal outcomes in these populations.

The search identified 74 studies, and included 15 observational studies and two randomized controlled trials (RCTs) meeting the inclusion criteria; the studies encompassed a total of 8307 women treated with LTZ FET (±additional gonadotropin) and 16,940 women treated with AC FET. The meta-analysis comparing LTZ FET to AC FET demonstrated a modest yet statistically significant increase in the odds of LB (OR 1.37, 95% CI 1.21–1.56), corresponding to an 8% risk difference (95% CI 4%–11%). The one RCT that reported on LB yielded a similar LBR for LTZ FET and AC FET, thus did not support a better outcome after LTZ FET. Pregnancy losses, defined either as a loss following a positive serum hCG or following a clinical pregnancy, were compared between LTZ FET and AC FET. The meta-analysis indicated a reduction in the odds of PL with LTZ FET (OR 0.63, 95% CI 0.51–0.78). However, the two RCTs reporting this outcome exhibited high heterogeneity, introducing uncertainty of the result. LTZ FET was associated with lower risks of HDP (OR 0.70, 95% CI 0.58–0.84) and LGA (OR 0.75, 95% CI 0.67–0.85), but no significant differences were observed for GDM or SGA. For all outcomes, the certainty of evidence was low. LTZ FET may offer a modest improvement in reproductive outcomes and a lower risk of some obstetric complications compared to AC FET, particularly in women with oligo-anovulation. However, the quality of evidence remains low, and more well-designed RCTs are needed to confirm these findings. While awaiting further data, LTZ FET may be recommended as a viable alternative to AC FET for women with ovulatory disorders.

Letrozole FET appears to be superior to artificial cycle FET in terms of increasing live birth rates and reducing hypertensive disorders of pregnancy. FET, frozen-thawed embryo transfer; PCOS, polycystic ovarian syndrome; BP, blood pressure; RCT, randomized controlled trial.