An expert in abortion procedures, called as a government witness in the second-degree murder trial of Dr. Robert J. Sherman, testified during defense questioning yesterday that some doctors have performed abortions in out-patient clinics on women past the 12th week of prenancy, contrary to recommended medical practice.

Earlier in his testimony at D.C. Superior Court, Dr. Marc Jerome said that in the later stages of pregnancy, it is "preferable" that such procedures be carried out in a hospital. This is because of the increased risk of complications following an abortion attempt past the 12th week, he said.

Sherman, 65, is charged in the death of 16-year-old Rita McDowell following an incomplete abortion he performed on her in his northwest Washington abortion clinic in March 1975. He also is charged with 26 counts of perjury.

Assistant U.S. Attorney Carl S. Rauh told the jury at the start of Sherman's trial last week that the government would prove that McDowell was 16 weeks pregnant at the time of the procedure and that she died of severe blood poisoning, kidney failure and shock as a result of the incomplete abortion.

Sherman's defense attorneys contend that their evidence will show pregnant and that her death was caused by gross mistreatment at D.C. General Hospital.

The government also contends that it will prove that Sherman deliberately performed incomplete abortions - necessitating a second office visit - and used unclean instruments in order to make money and cut costs at his office, once known as the Columbia Family Planning Clinic, at 1835 1 St. NW.

Jerome began his testimony Friday, with a detailed explanation of abortion procedures at reterm, and narrated a video tape of an actual abortion procedure for the jury.

Yesterday, while not directly referring to Sherman's practice, Jerome testified that in his opinion it would be a "major" violation of basic medical standards for abortionists to use unclean instruments, to send a patient home after an incomplete abortion or to fail to conduct the necessary examination following an abortion to determine whether the procedure was complete.