Women and medicine are pushing limits of childbirth beyond 50

When Brigitte Nielsen announced that she is expecting her fifth child at 54, she joined an elite club of women who are giving birth after 50.

That club is growing, as more women push back the age at which they have children and science and technology offer more middle-aged women the option to carry a pregnancy well into their 40s and beyond.

"The most important point is to realize that these are all cases of donor egg or frozen embryos," Dr. Jennifer Ashton, ABC News' chief health and medical editor, said. "The chances of spontaneous natural pregnancy at, say age 48, is less than 1 percent."

That is because the quantity and quality of a woman's eggs are greatly reduced after age 42.

"The egg quality is the main component to consider." Dr. Reza Radjabi, a fertility specialist and founder of New York City's Ferny Medical Clinic, told ABC News. "Being able to have successful pregnancy relies on the egg quality."

Once the egg issue is sorted out, then it's a matter of the woman's physical ability to carry a pregnancy.

"Not everybody is aging the same way," Radjabi said, adding that some women are healthier than others.

Good health is especially important for pregnant women in their 50s, who face a higher risk of gestational diabetes, preeclampsia and preterm delivery, according to Ashton.

"It's also physically harder for the woman, regardless of how fit she is," she added.

Both doctors agree, having a baby after 50 is a very individual decision.

"If there's longevity and you feel young and energetic, go for it," Frieda Birnbaum, who gave birth to twin boys at 60, told ABC News. "It's not judging yourself the way others do."

Birnbaum believes that seeing celebrities like Nielsen and Janet Jackson giving birth in their 50s will give more women permission to rewrite the rules of aging.

"They are projecting a message that it's okay -- you don't have to lie about your age anymore," said Birnbaum, adding that she believes there are more women in their 50s having babies than we know.

Cyma Shapiro, who writes and speaks about midlife mothers, has seen the change even in just the last decade.

"Motherhood over 40 or midlife mothers is no longer a secret society or rarity," she said.

Once they get past the hurdles of conceiving and giving birth, midlife mothers face other challenges as their kids grow older.

"There's also a biological reality on the back end that can really become problematic as children grow older," Shapiro said, adding that women may find they have less energy as their kids grow.

Birnbaum said her age is less a concern than the way parenting has changed.

"The parents are more involved with the schools and homework, and sports are more prominent than ever," she said.

Birnbaum, author of "Life Begins at 60,” added that she has more in common with mothers of her children's classmates and friends than she does women her own age.

She wouldn't trade having kids at this stage in her life.

"It's definitely much easier now than when I was younger, because I have the opportunity to have perspective, to know that time passes quickly and a lot of angst is forgotten," she said.

"There are pros and cons," Dr. Ashton said. "This is an individual decision, but technology and science and medicine is at the point where we may be seeing more and more of this."