When Queen Elizabeth II was 37, she gave birth — to her fourth child.

The queen's 37-year-old granddaughter-in-law, Meghan Markle, announced this week that she's pregnant — with her first.

What a difference two generations make.

The Duchess of Sussex is the latest celebrity poster woman for two clashing realities. One is that the ability to bear children declines so rapidly that age 35 is the threshold for "advanced maternal age." At that point, both mom and baby face significantly higher risks of complications.

But the other reality is that women are delaying childbearing anyway.

The latest federal data showing that women are having fewer babies and at later ages was released this week. Over the last decade, women in rural areas and small towns have bolstered this trend, although it is most pronounced in metropolitan counties like Philadelphia, where the average age of first-time moms rose by almost two years, to just shy of 28.

At the same time, birth rates have been falling for 20-somethings and climbing for women in their 30s and 40s.  Last year, 42 percent of the nation's 3.8 million births were to women in their 30s, according to federal data.

This seismic shift is seen in all Western countries. It is not driven by some environmental or political catastrophe a la The Handmaid's Tale. Mostly, it's because women can use birth control and legal abortion to postpone motherhood in favor of education and careers.

Still, the shift has inspired strident reactions. A lot of infertility specialists think women choose to ignore, or don't believe, that by age 35 they have only 3 percent of the eggs they were born with, and the supply and genetic quality go downhill fast after that. The experts blame the media for feeding willful ignorance by glorifying women like the duchess.

"The media portrayal of a youthful but older woman, able to schedule her reproductive needs and balance family and job, has fueled the myth that 'you can have it all,' rarely characterizing the perils inherent to advanced-age reproduction," Mark V Sauer, chair of obstetrics, gynecology and reproductive science at Rutgers Robert Wood Johnson Medical School, wrote in the journal Fertility and Sterility.

Women, meanwhile, rankle at medical terminology that they feel shames them for having biological clocks.

Laura Kenney, writing in Self magazine, decried the "litany of quasi-ageist terms knocked-up over-35s are blasted with: There was 'advanced maternal age,' 'geriatric pregnancy,' and the fact that I was considered 'high-risk.'"

How much of a dive does fertility take after 35? That can be hard to tease out, since married couples' friskiness also tends to decrease with age. A classic French study got around this by following women who were inseminated up to 12 times because their husbands were sterile. A little more than half of the women older than 35 got pregnant, compared with three-quarters of those younger than 31.

For women who use in vitro fertilization, age is still an issue. The chance of a baby after one IVF cycle was 42 percent before age 35, compared with 32 percent for ages 35 to 37, and 22 percent for ages 38 to 40, according to fertility clinic data.

The perils of childbearing – including miscarriage, stillbirth, even maternal death – also increase after 35.

Then again, the risks are small compared to the chances of success, especially if the woman is in good health – not obese, hypertensive, diabetic, or a smoker.

"In my opinion, the health of the woman is the most important factor," said Zaher Merhi, a reproductive endocrinologist at New Hope Fertility Center in New York City.

Now that older mothers have become normal, insensitive terminology is fading, he and other experts say.

"The word geriatric inherently has the connotation of disease," Mehri said. "If she's 36 and healthy, her pregnancy is not geriatric."

Echoed University of Pennsylvania infertility specialist Christos Coutifaris, who is the immediate past president of the American Society for Reproductive Medicine: "Geriatric puts on a label that is both inappropriate and inaccurate."

The challenge is finding a balance, said Thomas Jefferson University psychologist Andrea M. Braverman, who counsels infertile couples.

"On one hand, we don't want to pathologize delayed childbearing," she said. "On the other hand, we don't want to pretend women can do whatever they want whenever they want."

That goes even for royals. Markle, who married Prince Harry in May, is due in the spring.

"I'm glad she didn't wait," Braverman said.