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Single embryo transfer - a new understanding of factors for success

June 30, 2004

Berlin, Germany: Transferring a single embryo to a woman can result in a similar number of pregnancies as double embryo transfer, while at the same time reducing the risk of multiple births and the complications due to twin pregnancies, a scientist said today at the 20th annual conference of the European Society of Human Reproduction and Embryology. Dr. Ann Thurin, from Sahlgrenska University Hospital, Gùteborg, Sweden, told the conference that her research had in a large study shown that single embryo transfer (SET) could produce an acceptable pregnancy rate while lowering the incidence of multiple pregnancies.

Dr.Thurin and her team undertook a randomised double-blind study in 11 centres - five in Sweden, two in Norway, and four in Denmark. 661 patients under 36 years of age, who were undergoing a first or second IVF cycle, and who had at least two good quality embryos available for transfer or freezing were included. The patients were randomised into two groups; the first to receive a fresh SET and if there was no live birth, a subsequent frozen SET, and the second to a double embryo transfer (DET)

"The concept of 1+1 compared with a 2-embryo transfer has not been tried before", said Dr. Thurin, "and we were pleased to see that the women who had SET had an ongoing pregnancy rate nearly as high as those who had DET (39.7% versus 43.5%)."

There were no significant differences for other variables - the average age of the women at fresh transfer was 30.9 in the SET group and 30.8 in the DET group, and the number of good quality embryos available 4.6 in both groups.

"The only problem is that SET patients might need to go through one additional frozen cycle in order to have the same chance of pregnancy, but that is offset by the lower risks for mothers and children by avoiding multiple births", said Dr. Thurin. "However, the selection of the right embryo for SET is essential and a careful assessment needs to be undertaken before a choice is made. It is also important to have a well organised and functioning freezing programme, so that embryos that are surplus after SET can be properly preserved", she said.

In another study presented at the conference, scientists said that they had identified factors which independently of other factors affected the success of IVF or ICSI. The number of blastomeres - the cells formed in the first stages of embryonic development, just after the splitting of the fertilised egg - have a clear impact on whether fertilisation will lead to 'ongoing implantation' - an implantation which is not directly aborted by the body, they said. It was found that 4-cells embryos on day 2 were superior to faster cleaved embryos.

Dr. Christina Bergh, also from the Sahlgrenska Hospital, Gùteborg, told the conference that she and her team had looked at the same 661 patients studied by Dr. Thurin. They then analysed the IVF and ICSI cycles where implantation was either entirely successful or unsuccessful in its first stage i.e cycles where all transferred embryos implanted or did not implant.

Researchers had suspected for some time that the number of blastomeres could play a key role in the success of artificial reproduction techniques, said Dr. Bergh. "But until now, the only trials in this area were on a small scale and did not take confounding factors such as the age of the women and the number of good embryos into account."

Although the patients in the study were all under 36, researchers think the finding can be generalized to women of other ages. "This research is an important step towards understanding how and why implantation can be most successful, and therefore towards reducing multiple births by improving the effectiveness of single embryo transfer", said Dr. Bergh.

In a third presentation, Dr. Jan Olofsson, from the Fertility Centre Scandinavia, Gùteborg, Sweden, told the conference that he believed that SET, properly carried out, was sufficiently successful for it to be introduced on a wide scale without compromising results. His team studied the outcomes of all stimulated fresh embryo IVF and ICSI cycles at the Fertility Centre in 2002, and compared them with outcomes of cycles performed in 2003, when a new Swedish law forbade the transfer of more than one embryo at a time, aside from exceptional cases where two embryos are transferred.

"In Sweden we estimate that about 70% of all stimulated IVF-treatment cycles are performed with single embryo transfer", said Dr. Olofsson, and our data show that their chances of success are almost as high as with multiple transfer, but without the additional cost, both physical and financial, of multiple pregnancies."

Dr. Olofsson's team found little difference between the number of pregnancies and implantation rate between the two years. But the number of multiple births declined considerably - by 29% in 2003. "We will follow these results by looking at all deliveries in 2003", he said, "and are optimistic that, when we add in the results from the frozen and thawed cycles, the pregnancy rate will rise yet higher. We have shown that SET works and believe that the Swedish model could be extrapolated to other countries with similar results."

European Society for Human Reproduction and Embryology