Naomi Silverstone talks in superlatives these days.

"Pure joy" are two of her favorites words.Doctors might opt for "pure luck."

After suffering two miscarriages in her 30s, Silverstone stopped trying to get pregnant until a year ago, when she gave motherhood one last shot. Silverstone gave birth to healthy twin girls last summer - at the seasoned age of 45.

"After two miscarriages, having twins felt almost magical," she said. "I feel so incredibly blessed. This has been the most magnificent thing in my life. I have to believe this was how it was meant to be. I have two absolutely beautiful, wonderful, good-natured little girls."

Women in their 30s and 40s still eager to have children savor stories like Silverstone's. Such stories assure them they have more time.

But doctors caution that other people's miracles do not deliver all women a baby in their later years.

Infertility seldom announces its encroachment with symptoms. Take two women who are the same age with similar medical histories: One could be fertile well into her 40s, the other infertile by 35. Without tests, neither would know how fertile she was.

Single women who haven't tried to get pregnant are particularly vulnerable to fears and misinformation about their chances of having a baby. Here, four infertility specialists provide reassurance, caution and - above all - solid information.

- Older women have a tougher time getting pregnant just because they are older. A woman's fertility typically begins declining around age 30. In her 20s, a woman trying to conceive has a 20 percent chance of getting pregnant each month, said Dr. C. Matthew Peterson, an infertility specialist on the faculty at the University of Utah Medical School. "After age 35, you have a 5 to 10 percent chance of getting pregnant each cycle. After 40, you have a 0 to 5 percent chance of getting pregnant. At that point, it gets very difficult for women to get pregnant. That doesn't mean we don't try and aren't successful, but the odds are much less," he said.

Peterson noted that those percentages are for healthy women of those ages. Add to a woman's age any problems with blocked tubes, endometriosis and the decreasing sperm quality often seen in men in their 40s and 50s, and pregnancy becomes even more of a gamble.

Dr. Stephen Terry, an infertility specialist in private practice, often has patients ask him how long they can safely wait to get pregnant. "I tell them to start making some definite plans to get pregnant by their early to mid-30s. To delay much beyond that increases the difficulty in getting pregnant."

- Fertility can be measured. Dr. Kirtly Parker Jones, director of the fertility center at the University of Utah, often tests women's "ovarian reserve" to determine if fertility therapy can help them.

"Many women who come to the center have regular periods and no symptoms of a fertility problem. Fertility problems are usually without symptoms," she said.

So Jones performs some simple tests, costing about $100, to see how well a woman's ovaries work. The tests indicate nothing about a woman's tubes or other problems that may exist, but they do tell Jones how well the ovaries are producing eggs - a critical starting point in fertility.

- Women who suffer severe pain from endometriosis - the development of lesions on the reproductive organs - should not assume they can't get pregnant.

The American Fertility Society established a rating system for endometriosis that tells a woman and her doctor what her chances of getting pregnant are in a particular stage of endometriosis. Without going through that evaluation, women with severe pain from endometriosis often assume the disease is advanced and they can't get pregnant.

That's not true, Peterson said. "Women who have the least amount of disease may actually have the most pain. Women must understand there is no relationship between the level of pain she suffers from endometriosis and the stage of the disease." Only medical tests can determine how advanced the disease is and what it does to a woman's fertility.

Pregnancy rates for women with endometriosis run between 24 and 57 percent, Peterson said.

- Women who get pregnant for the first time in their late 30s and 40s often have wonderful pregnancies. Silverstone could not believe how easy her pregnancy was. Despite her age, history of miscarriages and the added strain of carrying twins, Silverstone led the busy life of a college professor until the last seven weeks of her pregnancy. She was hospitalized for those seven weeks to prevent early labor. "That was a small price to pay for my magnificent babies," she said.

Terry said his pregnant patients in their late 30s and 40s usually "do very, very well." He describes such patients as dynamic professional women who are often in better physical shape than some of his younger patients.

Despite the horror stories about older women and difficult labors, Terry said his older first-time mothers experience labor "about like any first-time labor. Their labors aren't much different from those of women 10 years younger."

Terry has had about half a dozen patients deliver babies for their first time in their 40s. "I've been very reassured. They've all done fine."

- However, older mothers do have a greater chance of giving birth to small babies or babies with chromosomal defects. However, new ultrasound techniques make those problems easy to detect early in a pregnancy.

Doctors used to be able to detect such problems only through amniocentesis: using a long needle to withdraw fluid from the amniotic sac surrounding the baby. The procedure triggers miscarriages in one out of every 100 women, said Dr. Greggory DeVore, a perinatologist who has pioneered the use of ultrasound to detect chromosomal damage.

For women like Silverstone who have a history of miscarriages, the threat of another miscarriage is often more frightening than not knowing of a child's defects.

Highly sophisticated ultrasound (not those typically done by most obstetricians) can now detect whether a child suffers chromosomal damage, DeVore said.

If a 38-year-old woman's ultrasound is normal, "her risk of having a baby withchromosome defects is as low as that of a woman in her mid-20s."

An ultrasound later in pregnancy can also determine if an unborn child has stopped growing - another problem older mothers face.

Devore scans his older patients once at 18 weeks and again at 34 weeks. "If the baby isn't growing like it should, then I put her to bed rest."

Medical breakthroughs make a safe pregnancy for women in their late 30s and 40s more possible each year.

"I don't think that all 40-year-olds should give up. On the other hand, women in their early 30s should not take their fertility in their late 30s for granted," Jones said.