The results of the study, involving 1472 embryos, were presented at the annual conference of the British Fertility Society in Edinburgh last week.
'This research shows the importance of quality over quantity,' said Dr Nick Raine-Fenning, medical director and research lead at the Nurture Fertility clinic in Nottingham. 'Most patients understandably expect adding a second embryo will increase or even double their chances, but this has never been the case.'
All the embryo transfers were carried out at the clinic between 2009 and 2013, and were at the blastocyst stage (five days after fertilisation). The embryos were graded using a standardised IVF grading system.
When a good quality embryo was transferred at the same time as a poorer quality one, the chance of pregnancy was reduced by 27 percent compared with transferring a single good quality embryo. Transferring two good quality embryos was no better in terms of achieving a pregnancy than transferring just one. However, when two poorer quality embryos were transferred at the same time, there was a higher chance of achieving a pregnancy.
'The current feeling is that a good embryo will be recognised by the body and will be captured for implantation. But a poor quality embryo should be rejected by your body – your endometrium will reject it. What our research suggests is that if you put a poorer quality embryo back with a good one, it's more likely to compromise the chance of the good one implanting,' said Dr Raine-Fenning.
'In their first cycle I would absolutely recommend that women – even older women – have one embryo transferred and the second embryo is frozen for future transfer,' he said.
The HFEA recommends that women opt for a single-embryo transfer when having IVF treatment to help reduce the number of multiple births, which carry risks to the mother and babies. However, around half of women – around 26,000 each year – currently opt to have two embryos transferred in hopes of increasing their chance of getting pregnant. This latest research suggests that hope is misplaced.
'We hope this research will help clinics further reduce the number of multiple births, whilst crucially keeping their success rates high,' said Dr Raine-Fenning.