A nurse once employed by former Washington abortionist Dr. Robert J. Sherman testified yesterday that Sherman performed abortions on women past the 12th week of pregnancy, that he charged higher fees based on the length of pregnancy and that he reused instruments so many times that they were broken or their marking faded.

Sherman told the father of a 13-year-old girl who was five months pregnant that he would perform an abortion for $1,000, the nurse testified. She said she later observed the girl return to Sherman's clinic for the abortion.

Alexis Morman, 30, told a D.C. Superior Court jury that instruments used in one type of abortion procedure were cleaned in a "boiler box" filled with hot water. The box was lined with a deposit from continuous use "like an old pot," Morman said.

Those instruments were reused so often that their tips became broken and bent and color markings faded, Morman testified. She also testified that Sherman had failed to follow procedures designed to determine if an abortion had been successfully completed.

Earlier in the trial, a prosecution expert on abortion procedures had testified that medical standards call for abortions past the 12th week of pregnancy to be conducted in a hospital. Sterilization of instruments is critical to avoiding infection, the expert said.

Sherman, 65, is charged with the second-degree murder of Rita McDowell, a 16-year-old high school student who died at D.C. General Hospital four days after Sherman performed an incomplete abortion on her at his clinic in March 1975.

He also is charged with 26 counts of perjury in connection with procedures used at the clinic, known as the Columbia Family Planning Clinic, at 1835 I St. NW. Sherman, whose license to practice medicine in Washington was revoked a year ago, has denied all the charges.

The prosecution contends it will prove that Sherman deliberately performed incomplete abortions in order to realize additional income when his patients returned for follow-up treatment.

Other practices, including the use of unclean instruments and unqualified personnel also were aimed at maximizing Sherman's profits, the government has charged.

Morman, now a research assistant at the George Washington University Medical Center, worked for Sherman from Sept. 12, 1974, to Oct. 25, 1974.

While she was employed at the clinic, Morman testified, Sherman saw five to 15 abortion patients a day. Sherman told her that abortions for women up to six weeks pregnant would cost $125, said Morman, and that $25 extra would be charged for each additional week.

When a woman entered the clinic, she would state the estimated term of her pregnancy, according to Morman. If Sherman discovered by examination that the woman's pregnancy was further along, he would tell the patient the additional fee would have to be paid before the procedure could continue, Morman testified.

In some cases, when the pregnancy exceeded 12 weeks, Sherman informed the patient he would terminate the pregnancy but that a follow-up visit for additional cleansing would be required, Morman told the jury. That second visit would cost $150 in the clinic or $200 if performed in a hospital, Morman said.

Sherman told her he preferred to conduct that procedure in the clinic because it took longer "to get anything done" at the hospital, Morman said during questioning by Assistant United States Attorney Whitney Adams.

Morman testified that Sherman once reprimanded her for "wasting money" when she attempted to dispose of a used syringe, or opened a new instrument. The label on that instrument said "use once and destroy," Morman testified.

Morman testified that a nurse's aide performed surgical procedures on some patients, which the government contends also violated medical standards.

Morman is expected to resume her testimony today, and faces cross-examination from defense attorneys who repeatedly objected to her testimony.

Judge Fred B. Ugast cautioned the jury that Sherman is not charged with any crime related to the acts Morman described. He told them the testimony was admitted into evidence in connection with the perjury charges, and to assist the jury n evaluating the government's contention that Sherman acted in malicious disregard for the life and safely of his patients.