O-296 The benefits of higher concentrations of synthetic serum substitutes in post-warming embryo culture and transfer media: a retrospective study of 864 frozen blastocyst transfer cycles
Authors: D’Alessandro G , Bartolacci A , De Girolamo S , Barbagallo B  M , Solano Narduche L , Marzanati D , Papaleo E , Pagliardini L , Alteri A 
Published: July 2024

Study question

Does a higher concentration of synthetic serum substitutes (SSS) in post-warming embryo culture and transfer media improve clinical outcomes in frozen blastocyst transfer (FBT) cycles?

Summary answer

Increased SSS concentration in post-warming embryo culture and transfer media reduces miscarriage rate in FBT cycles and improves ongoing pregnancy rate in euploid blastocyst transfer.

What is known already

Several studies showed increased clinical pregnancy rates with the use of complex protein supplementation such as SSS in embryo culture media. Nevertheless, there is a lack of standardization on their concentrations. The range of concentrations employed is strikingly wide, spanning from 5% to 50%. Moreover, no studies investigating different SSS supplement concentrations in post-warming embryo culture and transfer media in FBT cycles are available. These uncertainties warrant further investigation in this field.

Study design, size, duration

This retrospective study included 864 patients undergoing vitrified-warmed euploid (n = 102) and untested (n = 762) blastocyst transfers between September 2022 and August 2023. The cohort was divided based on the concentration of SSS used in post-warming embryo culture and transfer media: 10% (n = 415) and 20% (n = 449). The exclusion criteria were ICSI-cycles performed with surgically retrieved or frozen sperm, gamete donation and vitrified-warmed oocyte cycles. Prior to embryo transfer, warmed blastocysts were cultured for 2-3 hours.

Participants/materials, setting, methods

Treatment allocation to 10% or 20% SSS groups followed an alternating regimen. To reduce any pre-existing differences between the two groups, a propensity-score (PS) analysis was performed, targeting unbiased treatment effect estimates. Variables included in the PS were: maternal age, maternal BMI, infertility type, transfer day, endometrial preparation, blastocyst quality, and the operators for biopsy, warming, and transfer.

Main results and the role of chance

Patient baseline characteristics were evenly distributed in the two study groups. After the PS-based analysis, no statistically significant differences in ongoing pregnancy rates for both overall and untested blastocyst transfers were observed between 10% and 20% SSS groups. However, the 20% SSS group exhibited a significantly lower miscarriage rate in overall FBTs [OR = 0.61, 95% CI (0.37-1.00), p-value=0.050] and a higher ongoing pregnancy rate in euploid blastocyst transfers [OR = 3.48, 95% CI (1.30-9.35), p-value=0.013]. A mild trend of an increased miscarriage rate was also observed in both euploid and untested blastocyst transfers in the 10% SSS group compared to the 20% SSS group [OR = 2.834, 95% CI (0.651-12.337), p-value=0.165; OR = 1.558, 95% CI (0.897-2.705), p-value=0.115 respectively].

Limitations, reasons for caution

The study is limited by its retrospective design and a lack of randomization, although a PS-based analysis may mitigate such a weakness. Not considering live birth rate as outcome and protein concentrations higher than 20% and lower than 10% might represent other study limitations.

Wider implications of the findings

To our knowledge, this is the first study investigating the impact of varying protein supplementation concentrations in post-warming embryo culture and transfer media on clinical outcomes in FBTs. Our findings suggest that higher serum concentrations improve the likelihood of ongoing pregnancy, especially in blastocysts undergoing stressful manipulations such as biopsy.

Trial registration number

not applicable