ALEXANDRIA, Va. (AP) _ An infertility doctor says he doesn't know how many children he fathered by his artificial insemination patients, but he disputes the 15 cases alleged by prosecutors in his fraud and perjury trial.

The defense rested its case Wednesday in the trial of Dr. Cecil Jacobson, who also is accused of using hormone treatments to trick women into believing they were pregnant. He faces 52 counts of fraud and perjury.

Ultrasound expert Dr. Roger Sanders, testifying today in rebuttal for the prosecution, disputed contentions by an earlier defense witness who said ultrasound photographs had shown definite pregnancies among women who now say they were tricked.

In one case involving patient Susan Dippel, Sanders said he was ''baffled'' that the defense expert had found a definite pregnancy.

The last witness, Dr. Gary Stuhlmiller, defended genetic testing by his company, Roche Biomedical Laboratories, saying that minor clerical errors were corrected properly. The defense has contended the tests were flawed and unreliable in determining whether Jacobson fathered his patients' children.

Jacobson, who spent eight hours on the witness stand over two days, told a packed courtroom he used his own sperm on some of his artificial insemination patients without telling them.

''I do not know the number of children that I was the donor in where pregnancy resulted,'' Jacobson testified. He said he did not donate his sperm frequently but did it in cases where another donor could not be lined up.

In any case, he said, ''It was not true that I fathered 15 children,'' as the prosecution says genetic tests showed.

''I do not accept those results,'' Jacobson said.

A paternity testing expert testified for the prosecution that the tests indicated a 99.99 percent probability that Jacobson was the biological father. A defense witness contended the tests were flawed.

Prosecutor Randy Bellows has said that Jacobson may have fathered up to 75 children by his patients while claiming to use an anonymous-donor program. But defense lawyer James Tate insisted it is not illegal for a doctor to donate sperm to his patients.

Jacobson also said he did not commit perjury in 1989 when he said in a sworn statement he never donated sperm to his patients.

''That was done to protect the anonymity of my patients,'' the doctor said. ''That was a correct, truthful answer at that time.''

And he said it was ridiculous to suggest, as the prosecution has, that there is a risk that the children he fathered could unwittingly meet, marry and wind up having a deformed child.

Jacobson also insisted Wednesday he ''did not tell women they were pregnant when they were not pregnant.''

He said he did not realize the pregnancy test he used was sensitive enough to show a false positive result after injections of the hormone HCG. Other doctors have testified that they warned Jacobson of that possibility years ago.

Jacobson acknowledged he told one patient she had become pregnant, miscarried and her body had ''reabsorbed'' the fetus 10 times over a five-year period.

And he admitted he made mistakes in reading ultrasound images and telling some patients their pregnancies were progressing well up to 23 weeks. The defense has contended those pregnancies miscarried and the tissue were reabsorbed, but several of the women testified they saw other doctors who told them they had not been pregnant at all.

Former patient Teresa Gibson said Wednesday that after Jacobson told her she was pregnant she went to her obstetrician, who told her she was not pregnant.

She said she went back to Jacobson, who told her the fetus was in good shape. When she told him what her other doctor had told her, she said, ''Dr. Jacobson got really upset with me and told me he had not given me permission to see my regular ob-gyn.''

Jacobson testified earlier that he did not discourage women from seeing their regular obstetricians.