Women who try to double their chances of getting pregnant through IVF by using two embryos could actually cut their odds of having a baby by a quarter, a study has found.
About half of women in Britain undergoing fertility treatment use more than one embryo, with the latest figures showing more than 26,000 did so in 2013.
Many couples hope this will increase the probability of conception.
But if one of these embryos is poor quality, experts have found, it can dramatically cut the odds of a woman falling pregnant.
This is thought to be because the womb will focus on and reject the poor quality embryo, potentially along with any others.
Transferring two embryos where one was good quality and one was poor resulted in a 27 per cent lower chance of achieving a pregnancy, a study of almost 1,500 embryos found. Dr Nick Raine-Fenning, medical director and research lead at Nurture Fertility, which conducted the research, said: ‘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.
‘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.’
The embryo transfers studied were carried out between June 2009 and December 2013 at the Nurture Fertility clinic in clinic in Nottingham.
Transferring two embryos of good quality was also no better than one for achieving a pregnancy. Women who opt to have two embryos placed inside them following fertility treatment are more likely to have twins or other multiple births, which are more likely to be born prematurely.
But clinics can recommend the procedure for women over the age of 35 or who have been previously unsuccessful in IVF cycles.
It has led regulator the Human Fertilisation and Embryology Authority to campaign to persuade women of the benefits of having embryos transferred one at a time during IVF.
Dr Raine-Fenning said: ‘Most patients understandably expect adding a second embryo will increase or even double their chances but this has never been the case.’
He added: ‘This research shows the importance of quality over quantity.
‘We hope it will help clinics further reduce the number of multiple births, while crucially keeping their success rates high.’
The only exception to the rule appears to be if a woman has two poorer quality embryos, where they were still found to have a higher chance of falling pregnant than if just one embryo was put back.
The HFEA states that multiple births, including twins, are six times more likely to be born prematurely than single babies.
Long-term health problems of prematurity include difficulty breathing, cerebral palsy and other physical and learning difficulties.
The study is due to be presented at the British Fertility Society conference in Edinburgh today.