By Caroline Parkinson Health reporter, BBC News, Barcelona
Frozen is better than fresh when it comes to transplanting embryos in IVF treatment, a study shows.
Danish scientists found babies born after a frozen embryo was thawed and implanted had higher birth weights than those born from fresh embryos.
The study of over 19,000 babies also found no added risk of birth defects.
A European fertility conference heard frozen embryo babies did better because only the most robust embryos survived the freezing and thawing process.
Freezing embryos allows couples to have several cycles of treatment from one egg collection.
That means it cuts the amount of times women have to take ovarian stimulation drugs.
As doctors want to avoid multiple pregnancies, it is common for just one embryo - which has been fertilised in the lab - to be transferred into the womb, and the rest frozen.
In later cycles, a frozen embryo is thawed and implanted three to five days after ovulation, exactly the same way as fresh embryos are used.
While single embryo transfers are becoming increasingly common, the researchers said there was little data on the results of using frozen embryos.
But earlier mouse studies had shown a higher rate of behavioural and development problems in animals born from frozen embryos.
In this study, presented to the European Society for Human Reproduction and Embryology meeting in Barcelona, all 1,200 babies who had been born from frozen embryos between 1995 and 2006 in Denmark were compared to the 17,800 babies born from fresh embryos.
The data showed no increase in the rate of congenital malformations - which include conditions such as spina bifida and cleft palate.
Fewer frozen embryo babies were admitted to neonatal care units, but the researchers said this was probably because there was a higher rate of multiple births in the fresh embryo group.
In addition, pregnancies lasted slightly longer in the frozen embryo group, and babies were an average around 200 grams bigger.
There was also a lower proportion of low-birth weight babies weighing under 2,500 grams (5.5lbs) and fewer premature births, before 37 weeks.
Dr Anja Pinborg, who led the research, said: "We think the reason for the differences is probably positive selection of the embryos for frozen embryo replacement.
"Only the very top quality embryos survive the freezing and thawing process.
"And you only get pregnancies in patients with lots of good embryos to freeze."
She added that by the ovarian stimulation patients have to go through in order to get fresh embryos could negatively influence a consequent pregnancy - something women using frozen embryos would not be affected by.
Dr Pinborg said: "The findings are reassuring.
"If our results continue to be positive, it can be accepted as a completely safe procedure, which can be used more frequently than it is currently."