The government moved toward completion of its case in the second-degree murder trial of Dr. Robert J. Sherman yesterday with testimony from a medical expert who stressed the critical need for sterile instruments and complete abortion procedures.

Dr. Cleveland Smith, chief of gynecological services at the Columbia Hospital for Women here, told a D.C. Superior Court jury that a physician who used dirty instruments during an abortion would be "looking for trouble."

Smith, who said he has performed 5,000 abortions since 1971, also testified that it would be "barbaric" for a doctor to fail to make sure that an abortion was completed, a condition that could lead to infection and death.

Smith was the 25th government witness called at Sherman's trial - now in its third week - before Superior Court Judge Fred B. Ugast. Sherman, 65, is charged with murder and perjury in connection with the death in March 1975 of Rita McDowell, then a 16-year-old high school student.

McDowell died at D.C. General Hospital four days after she went to Sherman's abortion clinic, once located at 1835 I St. NW. The government contends McDowell's death was caused by massive blood poisoning and shock as a result of an incomplete abortion. According to the government's case, Sherman then recreated McDowell's medical records and made false statements and included employes to lie for him in an attempt to cover up the circumstances of McDowell's death.

The jury is expected today to hear key testimony for the government from the chief deputy city medical examiner, Dr. Brian D. Blackbourne, who performed an autopsy on McDowell and received medical records from Sherman three days after her death.

Principal Assistant U.S. Attorney Carl S. Rauh told Ugast yesterday that the government intends to finish presenting evidence tomorrow Defense attorneys Robert F. Muse and Constance O'Bryant of the city's Public Defender Service are expected to take at least two weeks to present their case.

Much of Smith's expert testimony yesterday centered on medical standards for abortion procedures. The government contends Sherman routinely violated those standards in order to cut costs and increase profits at his Columbia Family Planning Clinic. Sherman has denied all the charges.

Smith is expected to complete his testimony today.