A 32-year-old Northwest Washington woman who told the police her nine-month-old adopted son died after falling down a staircase was charged with second-degree murder yesterday in D.C. Superior Court.

Homicide detectives said the woman, Lillie Mae Caldwell Gasaway, of 5917 4th St. NW, changed her account of the incident several times during the past week. The D.C. medical examiner's office said after an autopsy that the boy died of multiple internal injuries.

An official of the D.C. Department of Human Resources said Gasaway had been receiving counseling from the department since last October after "a potential neglect case" was reported to the agency involving a six-year-old girl the woman adopted in 1973.

The department official also said Gasaway told the caseworker that the baby boy who died was her natural child.

The incident has prompted an investigation by the U.S. attorney's office here, the D.C. Department of Human Resources and the corporation counsel's office into the District's adoption regulations.

"I have some pretty serious concerns about this case," Albert Russo, director of the Human Resources Department, said yesterday.

The department operates a public service adoption agency and licenses private firms in the city through the department's Licensing, Certification and Consultation Division of the Office of State-Agency Affairs.

Gasaway, a housewife, told police she called a private adoption agency, the Family and Child Service Agency of Washington, on Sept. 17 and reported that her son was unconscious, according to a police report.

The private agency, one of seven licensed by the city, called an ambulance and D.C. police. The medical examiner's report said the child, Hildred Thomas Gasaway II, had died at least eight to 12 hours before Gasaway's call to the agency, policy said.

Gasaway's six-year-old adopted daughter was taken from the home last Friday in the custody of the D.C. police youth division and placed in a privately run orphanage.

Gassway's husband was not home when the baby was hurt, police said. They said he had fallen down the couple's backporch steps on Sept. 14 and was admitted with a broken leg to the Veterans' Administration Hospital.

Police yesterday were investigating a report that two other children had lived with the Gasaway family during recent years, but declined to discuss where they were or when they lived at the home. A Department of Human Resources official said the children were not in custody but were accounted for by department caseworkers.

John Theban, director of the Private Family and Child Service Agency, declined to discuss the case yesterday, citing juvenile and adoption case confidentiallty laws. An official of the Lutheran Social Service Agency, which police said placed the six-year-old girl in the Gasaway home, also declined comment.

Betty Queen, chief of the DHR Bureau of Family Services and Social Rehabilitation, said Gasaway "came to our attention" in October 1978. She said Gasaway told the caseworker that she was pregnant and later told the caseworker that the baby boy had been delivered at home in December.

Queen declined to discuss other details of the case.

Richard Caplan, assistant U.S. attorney assigned to D.C. Superior Court, said yesterday that this office is investigating "all aspects" of the adoptions.

After her court hearing yesterday, Gasaway was released into custody of the DHR Bureau of Family Services.