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High-risk pregnancies are on the rise in the United States and may be more common now than at any other time since modern obstetric care became available.

Why? More fortysomething moms are having babies, and epidemics of diabetes, obesity and high blood pressure are causing pregnancy and birth complications.

But in this otherwise troubling trend is also some good news: A small but growing number of women are successfully having children despite life-threatening conditions that once made a safe pregnancy almost inconceivable.

Exact numbers are not available, but doctors say that tens of thousands of organ transplant recipients, breast cancer survivors, women with heart defects, and even women with the AIDS virus have decided to risk childbearing in the last several years.

Not all of these stories have happy endings, and many people worry that some of these women will not live long enough to raise their children, or that they will pass on their medical problems.

But most results have been so surprisingly good that they are overturning decades of gloomy dogma about who is medically fit to have a child.

"These people define a whole new era of pregnancy for us," said Temple University's Dr. Vincent Armenti, who runs a registry that tracks births to transplant patients.

"We have to change our mindset about the perfect pregnancy," he said. Women should be given advice based on solid research "instead of an emotional feeling that some people just shouldn't have a baby."

No one knows precisely how many high-risk pregnancies there are. It is a catchall term that in the past has meant the mom is over 35 or has a condition like high blood pressure that is dangerous for her or her fetus. But as these situations have become more common, even this loose definition is changing.

Births to women 35 and older are soaring. And many complications are becoming so routine that nurse midwives can manage them instead of sending women to high-risk care specialists. Doctors around the country say they are treating far more of these risky cases than they did a decade ago.

"Patients are getting older, so by definition, they're higher-risk. The diabetes rate is going through the roof, so that's high-risk. More people with high blood pressure are getting pregnant. The list goes on and on," leaving fewer women considered low-risk, said Dr. Jacques Moritz, director of gynecology at Roosevelt Hospital in New York City.

Dr. Mary D'Alton, the Columbia University Medical Center obstetrics chief who recently delivered twins to a 59-year-old woman, has replaced the term "high-risk" with a 1-to-10 scale.

Birth outcomes for older women and those with medical problems "have been better than we would expect," although complications are more common, she said. For example, about half of organ transplant recipients give birth prematurely, although often by just a couple of weeks. "I don't mean to paint a rosy picture, but I would want to paint an optimistic picture," D'Alton said.

Dr. Gabrielle Fish felt optimistic when she decided to try pregnancy after her kidney transplant. Fish, of Cherry Hill, N.J., was in her mid-30s and had been stable on her anti-rejection drugs for two years. After researching pregnancies among transplant patients -- there have been more than 1,000 in the last decade -- she asked her doctors' advice.

"They pretty much encouraged me. They said, 'You're a healthy woman and you don't want to wait till you're high-risk because of your age,'" Fish said.

The result: a healthy daughter, Madelyn, now 6. Fish said she feels lucky to have her, but would not try to have a second child.

Carla Taylor also feels lucky. A delivery room nurse from New York, she was 42 when she discovered she was pregnant after having artery embolization to treat uterine fibroids. Such women are advised not to try pregnancy because the treatment plugs blood vessels that feed fibroids, and that might keep a fetus from growing properly.

Taylor had a healthy daughter, Bailey, now 6. Moritz, the gynecology director who works with Taylor at Roosevelt Hospital, said 16 of his patients have unexpectedly had children after the fibroid treatment and most have done well.

"Usually if they were able to get pregnant after the embolization, it means that blood supply to the uterus is pretty good," he said. Doctors expect more of these cases as embolization becomes a popular option to hysterectomy for fibroids, which plague millions of women.

Other reasons high-risk pregnancies are increasing:

• Older moms. In 2005, there were more than 104,000 births in the United States to women ages 40 through 44, and over 6,500 to women 45 and older. Advanced age raises the risk of birth defects.
• Chronic health problems. Obesity among women ages 18 to 44 rose from under 9 percent in 1990 to almost 22 percent in 2005.
• Hospital admissions for diabetes or related conditions among pregnant women and new mothers rose from 175,655 in 1993 to 269,861 in 2004, says the March of Dimes. Admissions for high blood pressure rose from 260,222 to 403,271.
• Multiple births, often the result of infertility treatment. In 2004, they made up more than 3 percent of all live births, up from about 2 percent in 1980. Such babies are more likely to be born prematurely and to have health problems.
• More cancer survivors. Hundreds of thousands are in their prime childbearing years.
• "Pregnancy raises enormous concerns for women with a history of breast cancer because we know that many breast cancers are hormonally driven," and high estrogen levels during pregnancy might raise the risk of relapse, said Julia Rowland, director of the National Cancer Institute's Office of Cancer Survivorship. "They may also be concerned that they may not be here for a child."
• Better medical care. Women with congenital heart defects used to die young. Those who lived were urged not to get pregnant. Now many such defects can be fixed, and children of women with heart defects are having their own children.
• More drugs. Nearly two-thirds of women who gave birth from 1996-2000 took a medication during pregnancy, a large federally funded study found. Of those, nearly 40 percent took a drug whose safety in pregnancy is not established, and nearly 5 percent took a drug potentially risky to the fetus. More pregnant women have taken new medicines for cancer, depression and other problems. More than 30 registries now track outcomes of pregnant women on various drugs.
• Growing prevalence of HIV. Each year, about 6,000 to 7,000 HIV-infected American women give birth. Such pregnancies have been controversial because of the risk of spreading the virus to their babies. But modern AIDS drugs are so effective at protecting babies from the virus' spread that more doctors are accepting these women's choice to have children.

"I view HIV disease now as basically a chronic illness," said Dr. Nancy Chescheir, obstetrics chief at Vanderbilt Medical Center. Despite several hundred births to infected moms at Vanderbilt, "we haven't had an HIV transmission in six years," she said.

D'Alton, the Columbia obstetrics chief, agreed.

"I just feel that these are very personal choices," she said. D'Alton sees her job as giving good care and supporting those choices "as much as possible."