STUDY QUESTIONDoes each millimeter decrease in endometrial thickness lead to lower pregnancy and live birth rates in fresh and frozen IVF cycles?SUMMARY ANSWERClinical pregnancy and live birth rates decline as the endometrial thickness decreases below 8 mm in fresh IVF-ET and below 7 mm in frozen–thaw embryo transfer (ET) cycles.WHAT IS KNOWN ALREADYPrevious studies have been heterogenous and have shown conflicting results on the impact of endometrial thickness on IVF outcomes. Most studies do not include many patients with an endometrial thickness below 6 mm, and there are few studies of frozen–thaw ET cycles.STUDY DESIGN, SIZE, DURATIONThis study is a retrospective cohort analysis of all Canadian IVF fresh and frozen–thaw ET cycles from the CARTR-BORN database for autologous and donor fresh and frozen–thaw IVF-ET cycles from 1 January 2013 to 31 December 2015. A total of 24 363 fresh and 20 114 frozen–thaw IVF-ET cycles were reported during this timeframe.PARTICIPANTS/MATERIALS, SETTING, METHODS33 Canadians clinics participated in voluntary reporting of IVF and pregnancy outcomes to the CARTR-BORN database. The impact of endometrial thickness on pregnancy, live birth and pregnancy loss rates were analyzed for fresh IVF-ET and frozen–thaw cycles.MAIN RESULTS AND THE ROLE OF CHANCEIn fresh IVF-ET cycles, clinical pregnancy and live birth rates decreased (P < 0.0001) and pregnancy loss rates increased (P = 0.01) with each millimeter decline in endometrial thickness below 8 mm. Live birth rates were 33.7, 25.5, 24.6 and 18.1% for endometrial thickness ≥8, 7–7.9, 6–6.9 and 5–5.9 mm, respectively. In frozen–thaw ET cycles, clinical pregnancy (P = 0.007) and live birth rates decreased (P = 0.002) with each millimeter decline in endometrial thickness below 7 mm, with no significant difference in pregnancy loss rates. Live birth rates were 28.4, 27.4, 23.7, 15 and 21.2% for endometrial thickness ≥8, 7–7.9, 6–6.9, 5–5.9 and 4–4.9 mm, respectively. The likelihood of achieving an endometrial thickness ≥8 mm decreased with age (89.7, 87.8 and 83.9% in women <35, 35–39 and ≥40, respectively) (P < 0.0001).LIMITATIONS, REASONS FOR CAUTIONThis study only included cycles which proceeded to ET, which may overestimate pregnancy outcomes. Approximately 8% of cycles could not be included in the analysis due to data irregularity related to data entry. Demographic data aside from age were unavailable but may be important as lower endometrial thickness may be associated with poor ovarian response.WIDER IMPLICATIONS OF THE FINDINGSAlthough pregnancy and live birth rates decrease with endometrial thickness, reasonable outcomes were obtained even with lower endometrial thickness measurements. These data provide valuable guidance for both physicians and patients when confronted with decisions related to a persistently thin endometrium.STUDY FUNDING/COMPETING INTEREST(S)This study was not funded. The authors do not have any conflicts of interests to declare.TRIAL REGISTRATION NUMBERN/A.
