A fertility doctor implanted a dozen embryos in the pregnancy that gave Nadya Suleman octuplets, a California state attorney said, a number that another doctor said was unheard of and surpasses Ms Suleman's assertion that only six embryos were implanted.

Dr Michael Kamrava's action endangered the mother and violated national standards of care, deputy attorney general Judith Alvarado said at the Medical Board of California's hearing to consider revoking or suspending the Beverly Hills physician's license.

Dr Kamrava "knew that a 12-embryo transfer was unsafe," Ms Alvarado said in her opening statement.

National guidelines issued by the American Society for Reproductive Medicine specify no more than two embryos are to be used in in vitro treatments for a healthy woman under 35.

The board set aside two weeks for the hearing to determine whether Dr Kamrava was negligent.

Dr Victor Y. Fujimoto, an expert witness for the medical board and director of the University of California San Francisco In Vitro Fertilisation Programme, testified that 12 embryos or blastocysts being transferred into a uterus is unheard of.

He reviewed hundreds of pages of medical records from Ms Suleman and other patients prior to giving testimony.

"I cannot imagine any colleague of mine transferring that many embryos," said Dr Fujimoto, adding he'd never transferred that many himself.

High-order multiple births can result in long-term developmental delays, cerebral palsy and various life-threatening ailments.

There are no hard-and-fast rules, but fertility specialists have criticised Dr Kamrava's methods, saying he endangered Ms Suleman's health and the long-term health of the babies. Ms Suleman's babies, born nine weeks premature in January 2009, are the world's longest-surviving set of octuplets.

Ms Suleman, a 33-year-old divorced single mother of 14, said Dr Kamrava implanted her with six embryos for each of her six pregnancies and two of them split when she had octuplets.

Dr Fujimoto said Ms Suleman, identified as "N.S." by the medical board, actually requested 12 blastocysts to be transferred into her, but it is the physician's job to make a decision not to transfer embryos, even when a patient insists.