Association between oocyte donors' or recipients' BMI with clinical outcomes after first single blastocyst transfers. The uterus is the mostly affected.

To assess if high BMI in either oocyte donors or recipients is associated with poorer outcomes after first single blastocyst transfers. Retrospective study including 1394 first blastocyst single embryo transfers (SETs) conducted by 1394 recipients during oocyte donation cycles with the gametes retrieved from 1394 women (Jan2019-July2021). Four BMI clusters were defined for both donors and recipients (underweight: <18.5; normal weight: 18.5-24.9; over-weight: 25-29.9; obese: ≥30). 1394 recipients, 42.4±4.0 years old and with a BMI 23.2±3.8, and 1394 donors, 26.1±4.2 years old and with a BMI 21.9±2.5. All oocytes were vitrified at two egg banks and warmed at eight IVF clinics part of the same network. ICSI, blastocyst culture and either fresh or vitrified-warmed SETs were conducted. Putative confounders were investigated, and the data adjusted through regression analyses. The primary outcome was the live birth rate (LBR) per SET according to donors' and/or recipients' BMI. The main secondary outcome was the miscarriage rate (<22 The LBR per blastocyst SET showed no significant association with donors' BMI. Regarding recipients' BMI, instead, the multivariate-OR was significant in obese versus normal weight recipients (0.58, 95%CI 0.37-0.91, p=0.019). The miscarriage rate per clinical pregnancy was also significantly associated with recipients' obesity with a multivariate-OR of 2.31 (95%CI 1.18-4.51, p=0.014) versus normal weight patients. A Generalized Additive Model (GAM) method was used to represent the relationship between predicted LBR or miscarriage rates with donors' or recipients' BMI; it pictured a scenario where the former outcome moderately but continuously decreases with increasing recipients' BMI to then sharply decline in the BMI range 25-35. The miscarriage rate, instead, increases almost linearly with respect to both donors' and recipients' increasing BMI. Obesity mostly effects the uterus, especially due to higher miscarriage rates. Yet, poorer outcomes can be appreciated already with a BMI of 25 in both oocyte donors and recipients. Finer markers of nutritional homeostasis are therefore desirable, recipients should be counselled about poorer expected outcomes in case of overweight/obesity, and oocyte banks should avoid assigning oocytes from overweight donors to overweight/obese recipients.

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