Problem statement: Frozen embryo transfer (FET) is an increasingly popular method of assisted reproductive technology (ART) due to advances in cryopreservation and more favourable outcomes when compared to fresh embryo transfer (ET). Endometrial thickness (EMT) has previously been investigated as a determinant of pregnancy rates following ART, with many studies suggesting unfavourable outcomes below a threshold of 7 --8mm. However, there are limited studies investigating the role of EMT in FET cycles specifically. Hence, here we determine the relationship between EMT and outcomes following FET.
Methods: Data was retrospectively analysed from FET cycles at a single-centre over a one-year period. Seventy-five patients matched the inclusion criteria. Patients underwent a mix of GnRH agonist, GnRH antagonist and natural cycle protocols. Embryos were frozen on day 5 of development and EMT was assessed prior to implantation by transvaginal ultrasound. The relationship between EMT was compared to subsequent pregnancy rates, defined by a positive beta-HCG test result. Statistical analysis was performed via ANOVA, with results p<0.05 deemed to be significant.
Results: The overall pregnancy rate following FET cycles was 47.3%. EMT ranged from 6.2 to 16mm, with an EMT mean of 9.4mm and median of 8.9mm. One-way ANOVA showed that EMT did not influence pregnancy outcomes (p=0.982). Two-way ANOVA demonstrated that factors including age of the patient (range 23 – 45 years old), size of ovarian follicles prior to oocyte retrieval (range <10 - 21mm) or stimulation protocol used also did not influence pregnancy rates significantly (p=0.432, p=0.472, p=0.791 respectively). Using both ≤7 or ≤8mm as a ‘suboptimal’ EMT threshold (as commonly used in the literature), there were no differences in outcomes between groups above and below the threshold (p=0.56 and p=0.75 respectively).
Conclusion: EMT does not influence FET pregnancy outcomes. Other factors frequently considered in ART such as age of the patient, size of ovarian follicles and stimulation protocol used also do not influence the relationship between EMT and FET outcomes. Further, there does not appear to be a minimum EMT below which pregnancy outcomes are undesirable. Therefore, this study concludes that EMT is not a useful clinical indicator of FET outcomes.