In April of last year, Rachel Brown’s oncologist called with bad news — at age 36, she had an aggressive form of breast cancer. The very next day, she found out she was pregnant after nearly a year of trying with her fiancé to have a baby.

She had always said she would never have an abortion. But the choices she faced were wrenching. If she had the chemotherapy that she needed to prevent the spread of her cancer, she could harm her baby. If she didn’t have it, the cancer could spread and kill her. She had two children, ages 2 and 11, who could lose their mother.

For Ms. Brown and others in the unlucky sorority of women who receive a cancer diagnosis when they are pregnant, the Supreme Court decision in June, ending the constitutional right to an abortion, can seem like a slap in the face. If the life of a fetus is paramount, a pregnancy can mean a woman cannot get effective treatment for her cancer. One in a thousand women who gets pregnant each year is diagnosed with cancer, meaning thousands of women are facing a serious and possibly fatal disease while they are expecting a baby.

Before the Supreme Court decision, a pregnant woman with cancer was already “entering a world with tremendous unknowns,” said Dr. Clifford Hudis, the chief executive officer at the American Society of Clinical Oncology. Now, patients as well as the doctors and hospitals that treat them, are caught up in the added complications of abortion bans.

“If a doctor can’t give a drug without fear of damaging a fetus, is that going to compromise outcomes?” Dr. Hudis asked. “It’s a whole new world.”

Cancer drugs are dangerous for fetuses in the first trimester. Although older chemotherapy drugs are safe in the second and third trimesters, the safety of the newer and more effective drugs is unknown and doctors are reluctant to give them to pregnant women.

About 40 percent of women who are pregnant and have cancer have breast cancer. But other cancers also occur in pregnant women, including blood cancers, cervical and ovarian cancer, gastrointestinal cancer, melanoma, brain cancer, thyroid cancer and pancreatic cancer.

Women with some types of cancer, like acute leukemia, often can’t continue with a pregnancy if the cancer is diagnosed in the first trimester. They need to be treated immediately, within days, and the necessary drugs are toxic to a fetus.

“In my view, the only medically acceptable option is termination of the pregnancy so that lifesaving treatment can be administered to the mother,” said Dr. Eric Winer, the director of the Yale Cancer Center.

Some oncologists say they are not sure what is allowed if a woman lives in a state like Michigan, which has criminalized most abortions but permits them to save the life of the mother. Does leukemia qualify as a reason for an abortion to save her life?

“It’s so early we don’t know the answer,” said Dr. N. Lynn Henry, an oncologist at the University of Michigan. “We can’t prove that the drugs caused a problem for the baby, and we can’t prove that withholding the drugs would have a negative outcome.”

In other words, doctors say, complications from a pregnancy — a miscarriage, a premature birth, birth defects or death — can occur whether or not a woman with cancer takes the drugs. If she is not treated and her cancer gallops into a malignancy that kills her, that too might have happened even if she had been given the cancer drugs.

Administrators of the University of Michigan’s medical system are not intervening in cancer treatment decisions about how to treat cancers in pregnant women, saying “medical decision making and management is between doctors and patients.”

I. Glenn Cohen, a law professor and bioethicist at Harvard, is gravely concerned.

“We are putting physicians in a terrible position,” Mr. Cohen said. “I don’t think signing up to be a physician should mean signing up to do jail time,” he added.

Oncologists usually are part of a hospital system, Mr. Cohen said, which adds a further complication for doctors who treat cancers in states that ban abortions. “Whatever their personal feelings,” he asked, “what are the risks the hospital system is going to face?”

“I don’t think oncologists ever thought this day was coming for them,” Mr. Cohen said.

Behind the confusion and concern from doctors are the stories of women like Ms. Brown.

She had a large tumor in her left breast and cancer cells in her underarm lymph nodes. The cancer was HER2 positive. Such cancers can spread quickly without treatment. About 15 years ago, the prognosis for women with HER2 positive cancers was among the worst breast cancer prognosis. Then a targeted treatment, trastuzumab, or Herceptin, completely changed the picture. Now women with HER2 tumors have among the best prognoses compared with other breast cancers.

But trastuzumab cannot be given during pregnancy.

Ms. Brown’s first visit was with a surgical oncologist who, she said, “made it clear that my life would be in danger if I kept my pregnancy because I wouldn’t be able to be treated until the second trimester.” He told her that if she waited for those months passed, her cancer could spread to distant organs and would become fatal.

Her treatment in the second trimester would be a mastectomy with removal of all of the lymph nodes in her left armpit, which would have raised her risk of lymphedema, an incurable fluid buildup in her arm. She could start chemotherapy in her second trimester but could not have trastuzumab or radiation treatment.

Her next consult was with Dr. Lisa Carey, a breast cancer specialist at the University of North Carolina, who told her that while she could have a mastectomy in the first trimester, before chemotherapy, it was not optimal. Ordinarily, oncologists would give cancer drugs before a mastectomy to shrink the tumor, allowing for a less invasive surgery. If the treatment did not eradicate the tumor, oncologists would try a more aggressive drug treatment after the operation.

But if she had a mastectomy before having chemotherapy, it would be impossible to know if the treatment was helping. And what if the drugs were not working? She worried that her cancer could become fatal without her knowing it.

She feared that if she tried to keep her pregnancy, she might sacrifice her own life and destroy the lives of her children. And if she delayed making her decision and then had an abortion later in the pregnancy, she feared that the fetus might feel pain.

She and her fiancé discussed her options. This pregnancy would be his first biological child.

With enormous sadness, they made their decision — she would have a medication abortion. She took the pills one morning when she was six weeks and one day pregnant, and cried all day. She wrote a eulogy for the baby who might have been. She was convinced the baby was going to be a girl, and had named her Hope. She saved the ultrasound of Hope’s heartbeat.

“I don’t take that little life lightly,” Ms. Brown said.

After she terminated her pregnancy, Ms. Brown was able to start treatment with trastuzumab, along with a cocktail of chemotherapy drugs and radiation. She had a mastectomy, and there was no evidence of cancer at the time of her surgery — a great prognostic sign, Dr. Carey said. She did not need to have all of her lymph nodes removed and did not develop lymphedema.

“I feel like it has taken a lot of courage to do what I did,” Ms. Brown said. “As a mother your first instinct is to protect the baby.”

But having gone through that grueling treatment, she also wondered how she could ever have handled having a newborn baby and her two other children to care for.

“My bones ached. I couldn’t walk more than a few steps without being out of breath. It was hard to get nutrients because of the nausea and vomiting,” she said.

The Supreme Court decision hit her hard.

“I felt like the reason I did what I did didn’t matter,” she said. “My life didn’t matter, and my children’s lives didn’t matter.”

“It didn’t matter if I lost my life because I was being forced to be pregnant,” she said.