HACKENSACK, N.J. (AP) _ A woman who hired a surrogate mother was correct in believing she could not safely bear her own child because she suffers multiple sclerosis, a doctor testified Thursday.

Dr. Stuart D. Cook, chairman of the department of neuroscience at the New Jersey School of Medicine, said that studies show that multiple sclerosis patients face a 20 percent to 40 percent chance of suffering additional symptoms of the disease after giving birth.

Because of the disease, Elizabeth Stern, and her husband, William, hired surrogate mother Mary Beth Whitehead, who later decided she did not want to give up the child, a move that sparked the court battle for the 9 1/2 -month- old girl.

The subject of Mrs. Stern's health has been key to attorneys for Mrs. Whitehead, who has testified that she wouldn't have entered into the $10,000 surrogate contract had she known Mrs. Stern was capable of bearing her own child.

Cook was the second doctor to testify he agreed with Mrs. Stern's decision not to bear a child. A third doctor, however, said the risk was minimal.

All the medical experts who have testified have agreed that multiple sclerosis doesn't make it impossible for a woman to become pregnant, but they have disagreed on the severity of the risks.

Mrs. Whitehead, 29, and her husband, Richard, testfied they changed their minds about the surrogate arrangement after the baby was born March 27.

Mrs. Whitehead turned down the $10,000 fee and fled with the child to Florida. Authorities later caught up with her and returned the baby to the temporary custody of the Sterns in Tenafly.

Mrs. Whitehead later was granted twice-weekly, two-hour visitation rights.

Cook was the first doctor to testify who has personally examined Mrs. Stern, who has said she feared she would become paralyzed if she attempted to have a child.

He said that in 1982, when Mrs. Stern ruled out pregnancy, medical literature suggested that multiple sclerosis could worsen in the months after giving birth.

''Had she come to me at that time I would have advised her against becoming pregnant,'' said Cook, who first examined Mrs. Stern in December.

He also disputed the accuracy of studies that show that multiple sclerosis sufferers face either a minuscule risk or none at all when becoming pregnant. No studies accurately show the disease's effects during a pregnancy, he said

But Dr. Gerard Lehrer, called by Mrs. Whitehead, testified Wednesday the chances of seeing symptoms of MS after pregnancy were less than 5 percent. Officials from the National Multiple Sclerosis Society in New York support this view.

Lehrer said that women with mild multiple sclerosis, such as Mrs. Stern, face about a 1 percent chance of becoming paralyzed because of pregnancy.

State Superior Court Judge Harvey R. Sorkow is hearing arguments on whether the contract is enforceable.