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LYON, France - Scientists proposed a new theory this week for how identical twins are formed as embryos, in a discovery that may improve a broad range of artificial reproduction techniques.

Using specialized computer software to take photos every two minutes of 33 embryos growing in a laboratory, Dr. Dianna Payne, a visiting research fellow at the Mio Fertility Clinic in Japan, and colleagues documented for the first time the early days of twin development.

The research was presented Monday at a meeting of the European Society for Human Reproduction and Embryology in Lyon, France.

Payne and colleagues found that identical twins are formed after an embryo essentially collapses, splitting the progenitor cells - those that contain the body's fundamental genetic material - in half. That leaves the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop.

"This is very exciting because it explains the principle of how we get twins," said Dr. Soren Ziebe, an artificial reproduction expert at the University of Copenhagen, who was not connected to Payne's research.

"Until now, we have assumed that at some point when the embryo breaks out of its shell to adhere to the uterus, it somehow gets divided into two," Ziebe said. Payne's theory presents a plausible explanation of how that division occurs, according to Ziebe.

"This is an exciting paper because by watching the embryo growing continuously, it has given a new insight into the processes by which the embryo divides," said Dr. Alison Murdoch, a professor of reproductive medicine at Newcastle University's Fertility Centre. Murdoch was not connected to Payne's research.

While conducting the research, Payne also found a possible explanation for why in-vitro fertilization techniques are more likely to create twins. Only about three pairs of twins per thousand deliveries occur as a result of natural conception. But for IVF deliveries, there are nearly 21 pairs of twins for every thousand.

To date, scientists have been at a loss to explain the discrepancy. Payne suggests that the laboratory conditions in which embryos are grown - in solutions that attempt to reproduce the uterus environment - are different enough to somehow provoke the development of twins.

"There are a lot of very subtle effects in the body, like different gas concentrations, enzymes or salt solutions," said Payne. While scientists attempt to replicate an embryo's natural environment, they are still far off. "The difference between the lab conditions and what's present in the body is probably resulting in more twins," she said.

"We need to take careful account of this new information," Murdoch said, adding that growing embryos for longer periods in the laboratory may be adding to the likelihood of having twins.

Payne suggested that with more research, doctors could potentially devise a test to predict which embryos might be inclined to produce identical twins. Because carrying twins is more risky than carrying a single baby, experts think the success rates of artificial reproduction would increase if twins could be avoided.

In the photos from Payne's research, scientists were able to observe two distinct "inner cell masses" in the embryos that developed twins. Of the 26 embryos that developed enough for them to observe, two had these distinct internal cell masses that scientists think are characteristic of twins.

Experts think that Payne's discovery could be applied to a number of artificial reproduction techniques.

"If we can improve the laboratory conditions for embryos to minimize the development of twins, then we can optimize the conditions for all eggs and embryos in artificial reproduction," said Ziebe. "This research has the potential to help all IVF procedures."