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New method of testing eggs for abnormalities could solve problems of embryo freezing

June 19, 2006

Prague, Czech Republic: Italian researchers have shown for the first time that it is possible to test a woman's egg, before fertilisation, for chromosomal abnormalities that might make an embryo less likely to implant successfully or more likely to miscarry at a later stage.

The technique involves analysing the first polar body, a small membrane-bound cellular structure that is expelled from the mature egg (oocyte) before fertilisation, and which mirrors the chromosomal status of the egg.

Dr Anna Pia Ferraretti told the 22nd annual conference of the European Society of Human Reproduction that, since the change in the Italian law in 2004, doctors were banned from discarding or freezing surplus embryos and only three embryos could be created at one time, all of which had to be transferred.

"As a consequence, a maximum of three oocytes have to be selected for insemination in order to avoid the development of more than three embryos. However, the three chosen might not be the best oocytes and, especially in women over the age of 35, there is a very high chance of choosing aneuploid oocytes — oocytes where one or two chromosomes have been lost or gained and which, consequently, will develop into embryos that either fail to implant or are more likely to miscarry at a later stage," said Dr Ferraretti, who is scientific director of the Società Italiana Studi di Medicina della Riproduzione (SISMER), in Bologna, Italy.

Dr Ferraretti and her team decided to see whether analysis of the first polar body could be a safe and effective tool for choosing viable eggs for fertilisation. Previously, no other researcher had attempted to use this technique in "real time" before insemination. "It involves a great team effort and sophisticated technology," said Dr Ferraretti.

They performed 510 egg retrievals between March 2004 and July 2005, and in 266 cases they tested the first polar body for five chromosomes among the eight that were known to be most frequently responsible for the aneuploidies detected in miscarriages: chromosomes 13, 16, 18, 21 and 22. The analysis for each retrieval was completed within three hours and a maximum of three apparently normal eggs were inseminated by intra-cytoplasmic sperm injection (ICSI) before being cultured and transferred to the women. In the other 244 cases, the researchers used the conventional criteria of choosing eggs that appeared, on close external inspection, to be normal, before insemination by ICSI.

There was little difference in the average number of embryos transferred and the implantation rates between the two groups, but the early miscarriage rate was significantly lower in the group that had had the first polar body (PB1) analysis (11.5% compared to 28.6%).

In women aged 34 or younger there was one miscarriage (6%) in the PB1 group compared to seven (21%) in the control group; in women aged between 35 and 37, the PB1 group had two miscarriages (12%) compared to five (50%); and in women aged 38 to 43, there were three miscarriages (18%) in the PB1 group compared to four (33%) in the control group.

Dr Ferraretti said: "These preliminary data show for the first time that early PB1 biopsy performed with the aim of having results before insemination does not affect fertilisation, embryo development and implantation potential. The PB1 pre-selection significantly decreased the risk of transferring normally cleaving aneuploid embryos with implantation potential, and therefore decreased the miscarriage rate in all age groups.

"At present, because of the limited number of chromosomes that can be analysed, the benefits of the technique are confined to reducing the risk of miscarriages. Therefore, it is indicated for use mainly in women who have a higher risk of aneuploidies because of age, previous miscarriages and repeated IVF failure.

"However, in the future, it could become an important tool for all women undergoing assisted reproduction, not just in Italy, but worldwide, and also to reduce the surplus of those frozen embryos that are carriers of aneuploidy."

Next, the researchers plan to carry out more tests to check the safety of the procedure, to test different chromosomes that could be responsible for failure in early embryo development or implantation independently from age, and to use the technique to classify and array all the chromosomes in the first polar body.

"If and when we are able to have the full information on an oocyte's chromosome competence, assisted reproductive technology will become more efficient. The technique, designed to overcome the most restrictive law regulating infertility treatments, could turn out to be a new predictive tool for assessing the reproductive potential of each patient," she concluded.

European Society for Human Reproduction and Embryology