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After four days of poking and prodding, doctors examining British Columbia's conjoined twins say they can't yet determine if the four-month-old girls can be separated.

A crucial angiogram, which was to be performed yesterday at B.C. Children's Hospital, was postponed because the girls -- and their parents -- weren't up for yet another gruelling test.

The cerebral angiogram, which will provide a close-up look at the blood vessels and arteries in the girls' brains, is needed to determine if the twins, who are joined at the head, can be safely separated.

Apart from the battery of tests, Tatiana and Krista Hogan-Simms are thriving, pediatric neurosurgeon Douglas Cochrane said yesterday. The infants have charmed hospital staff and passersby, Dr. Cochrane added.

"I passed them in a public part of the hallway just a few minutes ago and they seemed absolutely happy and they seemed to be entertaining a number of folks," he said.

The girls are growing like weeds and their motor skills and cognitive abilities are normal. But doctors are still baffled by a bridge of cerebral tissue that connects the brains of the Vernon, B.C., twins.

Tatiana and Krista are joined at the head, just above the ear, an extremely rare phenomenon, which occurs in one of every 2.5 million births.

The angiogram would have shed some light on whether the tissue bridge has a function. Doctors also need a clearer picture of the blood vessels and arteries surrounding the girls' brains.

Dr. Cochrane said the brain "bridge" could provide the answer to whether separation is possible.

"It's key to understanding the vascular anatomy," Dr. Cochrane said. "What we don't know is . . . the nature of the blood vessels over the surface of the brain. We will find it out. We just have to wait until the kids are up to having the study done."

The neurosurgeon also warned of the high risks of separating craniopagus twins. The mortality rate for twins who undergo this kind of surgery is higher that 50 per cent, he said.

"Nature didn't make these two twins separate," he said. "She brought them together and to try and make them separate will likely occur at a cost of some sort."

Trying to determine "what that cost might be from a functional perspective of these two individuals is actually what this is all about," he added.

Doctors also performed a CT scan of the girls' heads and attempted a magnetic resonance imaging (MRI), but Tatiana wouldn't hold still for the test, despite her father's best effort to get her to sleep.

What doctors did learn this week was that the girls sleep at different times and have separate physical feelings and reactions.

Although they're identical twins, Tatiana is far smaller than Krista. She weighs 7 pounds, 4½ ounces compared to Krista, who is 10 pounds, 13 ounces. Neither girl can roll because of their condition and as a result, they spend a lot of time on their backs. Still, the infant girls continue to defy the odds, which they have been doing since before they were born last October.

Even though they were delivered early to avoid late-term complications, the two had a one-in-four chance of surviving for 24 hours.