The medical examiner whose reevaluation of a 1987 infant death in Montgomery County helped lead to murder charges against the baby's father testified yesterday that advances in the understanding of sudden infant death syndrome changed his opinion.

Charles P. Kokes told a Montgomery County Circuit Court jury that he reconsidered his initial ruling last year, after police told him that the mother of Garrett Michael Wilson believed the infant had been murdered by his father.

Yesterday's testimony started the second week of the trial of Garrett Eldred Wilson, who is accused of smothering his two infant children to collect a total of $190,000 from life insurance policies. Wilson, 43, is being tried on a first-degree murder charge in the death of his 5-month-old son.

The Frostburg man is scheduled to stand trial in Prince George's County in September in the 1981 death of his 2-month-old daughter, Brandi Jean, allegedly to collect $40,000 in life insurance.

Kokes told the jury and Judge Ann S. Harrington that since he performed the autopsy on Garrett Michael in 1987, scientific understanding of SIDS has improved the ability of pathologists to more accurately pinpoint the cause of death in such cases.

Kokes said that improvements in technology have allowed pathologists to glean more evidence from the scene of death and the autopsy and to better interpret medical records and toxicology reports.

Pathologists now believe that the swelling of an infant's brain is a rare occurence in SIDS cases, Kokes said. He pointed out that Garrett Michael's autopsy noted that the brain was swollen at the time of death.

Kokes estimated that the probability of a child dying of SIDS is 1 per 2,000 live births and that the odds of two children in the same family dying of SIDS--with one child's brain being swollen--is about 1 in 100 million.

"That finding alone is very suspicious and causes great alarm for a knowledgeable pathologist," Kokes said. "It would cause any current pathologist to relook at the case. You'd be reluctant to classify that death as SIDS."

The prosecution's case against Wilson shifted yesterday from the emotional testimonies of the mothers of both dead children heard last week to the crux of its case: the validity of autopsy reports and the sale of expensive life insurance policies on two infants.

Daniel J. Sullivan, a Met Life insurance agent who sold Wilson $100,000 in insurance, testified that Wilson denied having purchased any other policies on his son, though he had purchased a $50,000 policy from Allstate agent Lee Smith a month earlier.

During cross-examination, Wilson's attorney, Barry Helfand, questioned the credentials of the medical examiners and the assistants who helped perform the autopsies on both children. He also raised repeated objections during State's Attorney Douglas F. Gansler's questioning of Alan L. Meltzer, an insurance expert. Meltzer outlined the different types of insurance available but was not allowed to address how often parents buy life insurance for infants.

Maryland's chief medical examiner, John Smialek, who changed the causes of death in both cases, is expected to testify today, along with Linda Norton, a Dallas-based forensic specialist widely considered among the nation's leading experts on SIDS.