Non-identical twins are born from two separate fertilised eggs
IVF twins face more health problems in early life than naturally conceived twins, experts suggest.
A study found these babies were far more likely to be admitted to neonatal intensive care and to be hospitalised in their first three years of life.
Other work in the same journal, Human Reproduction, provides reassurance on the outcomes of children born after embryos are frozen and stored, however.
A quarter of fertility treatment babies are now born after freezing.
Evidence from 21 controlled studies showed embryos that had been frozen shortly after they started to divide had a better, or at least as good, outcome in terms of premature birth and birth weight as children born from fresh cycles of IVF or another common assisted reproductive technique called ICSI (intracytoplasmic sperm injection).
The same cannot be said for twins born after assisted reproductive therapy (ART) when compared with naturally conceived twins.
It is known already that ART twins are at higher risk of problems such as low birth weight and premature delivery than singletons around the time of their birth, but, to a large extent, these risks exist as part of the problems associated with multiple births in general.
Until now there has been conflicting evidence about whether assisted reproduction itself is responsible for adding to the number of problems seen in ART twins.
As a precaution, experts have recommended transferring only one embryo per IVF cycle wherever possible.
To explore the risks, researchers in Australia and the UK looked at perinatal outcomes and hospital admissions for all twin children born in Western Australia between 1994 and 2000.
To make sure, as far as possible, they were comparing like with like, the investigators matched the ART twins with naturally conceived, non-identical twins of different sexes.
This is because all ART twins start from separate eggs where as some naturally conceived same-sex twins start from only one egg that divides.
They found that twins conceived following ART treatment had a greater risk of poor perinatal outcome, including preterm birth, low birthweight and death that the naturally conceived non-identical twins.
And ART twins had a nearly two-thirds higher risk of being admitted to neonatal intensive care and were more likely to be admitted to hospital during the first three years of their life.
Single embryo transfer
Researcher Michele Hansen, of the Telethon Institute for Child Health Research in Western Australia, said couples undergoing fertility treatment should be made aware of this and should consider the benefits of opting for single embryo transfer.
The reason for the increased risks to ART twins is unclear, but the underlying causes of parental infertility and components of the ART procedure have been mooted.
Dr Allan Pacey, fertility expert at Sheffield University and secretary of the British Fertility Society, said: "It is reassuring news about the embryo freezing.
"But the issue of IVF twins is concerning which is why we are trying to move towards single embryo transfer in as many women as possible to give these children the best start in life."
A spokesperson for the Human Fertilisation and Embryology Authority said: "Clinicians have a duty of care to ensure that patients fully understand what IVF treatment involves and what the risks are, for both themselves and their babies.
"This study is another piece of the jigsaw that women and their doctors need to consider before treatment.
"We know that multiple pregnancy and birth pose the biggest single risk to mothers and babies following fertility treatment."
The HFEA said that was why it was driving forward a national strategy to reduce the number of multiple births following IVF.