P–780 The increase of single embryo transfers does not impair pregnancy and live birth rates, although it lowers twin rate
Authors: Stimpfel M , Bacer-Kermavner L , Fevzer T , Petric P , Jancar N , Vratcnik-Bokal E 
Published: August 2021

Abstract

Study question
How does significant increase of the proportion of single embryo transfer over 10 years period affect pregnancy, live birth and twin rate.

Summary answer
Increase in single embryo transfer doesn’t change pregnancy and live birth rate, although it significantly lowers twin rate.

What is known already
Due to widely used approach in IVF of transferring multiple embryos to improve the pregnancy and birth rate, multiple pregnancies, mostly twin, are quite common. But because they are more often associated with adverse neonatal and perinatal outcomes as singleton pregnancies, they are not desirable. Therefore, more and more often the transfer of single embryo is encouraged. Furthermore, in the last years with improvements in cryopreservation techniques leading to effective cryopreservation of supernumerary embryos, there is more options for performing repeated single embryo transfers.

Study design, size, duration
We retrospectively collected the data of all fresh embryo transfers in couples treated in our centre from January 2010 to December 2019. We excluded embryo transfer where embryos were derived from cryopreserved oocytes and analysed the outcome of fresh embryo transfer regarding to the number of transferred embryos.

Participants/materials, setting, methods
In our analysis we included 10583 fresh embryo transfers. We tried to evaluate how the proportion of single embryo transfer has changed through analysed period of time and if this led to any differences in pregnancy, live birth, and twin rate. To determine the differences between the groups, the data were analysed with one-way ANOVA and Pearson’s chi-square, as appropriate. Statistical significance was set at P &lt; 0.05.

Main results and the role of chance
The analysis revealed that the proportion of single embryo transfers significantly increased from year 2010 to year 2019 (from 28% to 73%; P &lt; 0.001). The proportion increased every year, minimum increase was 1% whereas maximum increase was 16%. This increase over the years did not negatively affect the pregnancy (32% in 2010 vs. 34% in 2019: p = 0.317) and live birth rates (24% vs. 25%; p = 0.584), although it had favorable effect on twin rate (16% vs. 7%; p = 0.002). If we separately analyzed only single and double embryo transfer, we observed that pregnancy (24% in 2010 and 34% in 2019; p = 0.001) and live birth rates (17% vs. 26%, p = 0.001) significantly increased after single embryo transfers, but no difference was observed in double embryo transfers (pregnancy rate: 35% vs. 35%, p = 1; live birth rate: 27% vs. 22%, p = 0.097; twins rate: 20% vs. 27%, p = 0.244). Additionally, we observed that female mean age value significantly increased over analyzed period (34.2±4.5 years in 2010 vs. 35.7±4.7 years in 2019, p &lt; 0.001), although there was no difference in mean number of retrieved oocytes (8.2±5.4 vs. 8.1±4.9, p = 1) and obtained embryos (4.5±3.3 vs. 4.2±2.9; p = 0.684).

Limitations, reasons for caution
The limitation of the study is retrospective design, and not evaluating the influence of elective single embryo transfer. Also, the IVF laboratory methods and IVF culture media improved over the years meaning they could be partly responsible for observed differences.
Wider implications of the findings: Single embryo transfer could probably be performed in even higher proportion without lowering the chances for pregnancy.

Trial registration number
not applicable’