Two embryos are better than one for maximising the chances of having an IVF baby at any age, a study has found.

However, a woman's age does have a bearing on whether transferring two embryos into her womb will lead to birth complications, said researchers.

In older women, the transfer of two embryos rather than one was less likely to lead to multiple births, low birthweight or premature birth than in younger women.

Decisions about how many embryos to implant during a single in-vitro fertilisation cycle are controversial.

Some fertility clinics have been accused in the past of taking risks with women's health by implanting multiple embryos to increase the chances of a live birth.

The Human Fertilisation and Embryology Authority, which regulates IVF clinics, has issued guidance that generally no more than one embryo should be implanted per cycle.

Older women or those with poor quality embryos are considered exceptions to the rule who may be suitable for double embryo transfers.

The new research, published in an early online edition of The Lancet medical journal, found that three or more embryos should not be implanted at any age because of the risks involved.

However, it was less clear cut about the question of transferring one or two embryos.

Authors Professor Debbie Lawlor, from the University of Bristol, and Professor Scott Nelson, from the University of Glasgow, said clinicians and patients should be given "greater freedom" to decide whether one or two embryos was the right number to use.