Fertility clinics should be allowed to transfer two embryos rather than just one to a woman during IVF treatment if she and her partner, together with the doctor, believe it is in their best interests, say the authors of a controversial new study .

The conclusions of the study, published in the Lancet medical journal, challenge current thinking, which is moving towards placing one embryo at a time in a bid to reduce the number of twin births.

The authors advocate more flexibility than some European countries now allow. Belgium and Sweden have brought in a single-embryo policy and the UK also has been moving in that direction of a presumption that just one embryo will normally be placed during fertility treatment, in order to reduce the risk of women bearing twins. There is a higher chance that twins will be born prematurely and at low birth weights, which mean they may suffer damage and the bill to the NHS for their care is high.

But Professor Debbie Lawlor, an epidemiologist at Bristol University, and Professor Scott Nelson, a researcher in reproductive medicine at Glasgow University, conclude from a large-scale study of the outcomes for women undergoing fertility treatment in the UK that one is not always best – although three should never be allowed. Women over 40 are sometimes implanted with three or occasionally four embryos. But more is not more successful, the scientists say. Those over 40 given three embryos were no more likely to have a live birth than those with two – and the chance of a poor outcome for the babies, such as prematurity and low weight, was higher.

The scientists looked at the results of more than 124,000 IVF attempts, which resulted in more than 33,500 babies being born. Success rates were much higher for the under-40s than for the over-40s, but in both groups, live births were more likely when two embryos were placed in the womb. In the over-40s, transferring two was less likely to lead to twins with the associated complications of small size and prematurity, mostly because older women are less successful in carrying twins to term.

Lawlor said their message was that, rather than legislate or bring in rules about single-embryo transfer, as has happened in some European countries, there should be flexibility. Some women may produce more eggs that become viable embryos in IVF than others. And women who are having to pay for treatment privately may hope to have twins because they cannot afford any further IVF bills.

In the UK at the moment, it is illegal to put more than three embryos into the uterus of a woman over the age of 40 and more than two for a woman under 40.

"In terms of UK policy, we would say you shouldn't transfer three to women of any age, but I think there should not be legislation to try to enforce transferring one to younger women and two to older women," Lawlor told the Guardian. "Guidance should be based on prognostic indicators."

The Human Fertilisation and Embryology Authority, which regulates the field and collected the data on which the study is based, has been trying to reduce the number of multiple pregnancies as a result of IVF from 24% in 2009-10 to 15% by April this year. It has encouraged clinics to opt for single embryo transfer, which it said is most appropriate for women under 37 with several good embryos available.

The authors warn that women needing fertility treatment should have it earlier rather than later. Money is a factor in the decision of couples to risk having or even to seek to have twins. In a commentary linked to the paper, Dr Liv Bente Romundstad of the Norwegian University of Science and Technology in Trondheim, writes that access to free fertility treatment helps to bring down the rate of multiple births by allowing couples to afford to have one child at a time. Fertility treatment has been one of the first areas cut by cash-strapped NHS managers, however. According to Pulse magazine, the average number of treatments in 2011-12 is running at 14% below the previous year's level.