Scientists may have solved a puzzle baffling fertility experts - why test tube babies do not do as well as other infants in early life.

Babies conceived through in-vitro fertilisation (IVF) are usually born earlier than naturally conceived babies. They also have a lower birth weight and spend more time in hospital after the birth. But doctors could not explain why.

“Now we have two reports that might answer the question,” said Dr. Arne Sunde, chairman of the European Society of Human Reproduction and Embryology (ESHRE).

Researchers suggest the answer can be found in the number of embryos transferred into the woman’s womb during IVF treatment.

“We believe that our work shows clearly that single embryo transfer is best for both the mother and child,” Dr. Diane De Neubourg, of the Centre for Reproductive Medicine in Antwerp, Belgium, told an ESHRE meeting on Tuesday.

“These babies do very well in their outcomes compared to the outcomes in spontaneous pregnancies.”

In some countries, two or more embryos are transferred into the womb to increase the odds of a pregnancy. But that practice also raises the risk of multiple births, which can be dangerous for both the mother and babies.

De Neubourg and her team showed that IVF babies resulting from the transfer of a single embryo are as healthy as babies conceived normally.

“We analysed the outcomes of 251 SET (single embryo transfer) pregnancies and births and found very little difference between SET babies and those naturally conceived,” she added.

Dr. Anja Pinborg, of the University of Copenhagen, believes the difference in the health of babies following single or multiple embryo transfers could be competition in the womb.

Multiple embryos compete for the same nutrients and blood supply in the mother, so even if only one of two embryos survives it will not have had the same early advantage as a child from a single embryo transfer.

“They might have identified one of the puzzles of why IVF singletons are not exactly the same as naturally conceived singletons,” Sunde said.

“The answer seems to be that there may be too many embryos in the womb - the one that died affected the one that survived,” he added.

The average number of embryos transferred during IVF varies from one to four or more depending on the country. The average number of single embryo transfers is 12 percent in Europe, but dual embryo transfers rose from 46.7 percent in 2000 to 51.7 percent in 2001, according to the ESHRE.

The number of triple embryo transfers dropped from 33.3 percent in 2000 to 30.8 percent during the same period, while four-embryo transfers fell from 6.7 percent to 5.5.