LOS ANGELES — As Sandra Spath glanced around the yoga class for soon-to-be mothers, the thought that had nagged her that day evaporated.
“I wasn’t even the oldest one,” said Ms. Spath of Santa Monica, Calif.
Just a few weeks before she turned 40, Ms. Spath gave birth to a boy — and joined the growing ranks of women becoming mothers later in life.
Federal data show that women in their 40s are more likely to have babies now than at any time in more than four decades. Among American women ages 40 to 44, birthrates have hit their highest point since 1967, data recently released by the National Center for Health Statistics reveal.
Births have also become increasingly common among women in their late 30s.
As the number of older mothers has risen, younger women have become less likely to bear children. Younger women are still much more likely to have babies than older women, but birthrates sank to record lows among teens and women in their early 20s in 2011, the data show.
Some older mothers say they were too restless to have children as young women. Others said they hadn’t found Mr. Right or were absorbed in careers that allowed little time for family.
Older motherhood is hardly unprecedented: Fortysomethings were more likely to bear children in the 1940s and ’50s, as many women had the last of their three, four, or more children in middle age.
As the baby boom petered out and birth control expanded in the late 1960s and ’70s, the numbers plummeted.
But motherhood among fortysomethings has rebounded, this time driven by women who chose to put off having children.
An older mother today is more likely than in decades past to be having her first child, national statistics show.
Immigration has also affected the numbers, as immigrant women are more likely than those born in the United States to have babies at older ages.
Pushing childbirth to later in life has raised fears that women could be frittering away their fertility or risking disorders more common among children of older parents.
A number of older mothers have turned to modern technology for help: Attempts at in vitro fertilization rose by half among women 41 and older between 2003 and 2011, Society for Assisted Reproductive Technologies statistics show.
Although fertility doctors warn about the risks of pushing parenthood too late, there are plenty of reasons to wait: Older parents are more confident and less stressed, studies have shown. Their children do better on math and reading tests.
In Britain, where the average age of women first becoming a mother is 28, one study published last year found that children of older women were less prone to injuries or emotional troubles — even after researchers controlled for the higher education and wealth of the mothers.
Then there’s the bottom line. At the University of Virginia, economist Amalia Miller calculated that for every year a woman delays motherhood, she makes about 9 percent more in lifetime earnings. A decade of delay could mean nearly doubling her income, Ms. Miller has extrapolated — a windfall that makes it much easier to buy all those diapers.
For most women, having their first child later in their careers means they’ll earn higher wages, economist Jane Leber Herr found.
In some fields, women feel it isn’t just a question of when to have children, but whether they can have them at all.
Lorri Herman, who used to run a $30 million business in the fashion industry, remembered waking up at 5:30 to call her New York office and phoning in lunch orders beforehand to save time.
“If you want to have the success that I had, you will sacrifice your family,” Ms. Herman said. It wasn’t until after she left her business that she adopted a child, at 47.
Ms. Spath, the Santa Monica mother, was also in the fashion industry, jetting from New York to Miami to Los Angeles while running her own showroom.
Two weeks was the longest vacation she took in a decade.
“If I’d stayed in Europe, I definitely would have had kids earlier,” said Ms. Spath, who grew up in France. “But the lifestyle here — I didn’t have time for it.”
Her French friends and family are guaranteed time off, Ms. Spath pointed out.
The United States is one of the few countries in the world where new mothers are not legally guaranteed paid leave, according to the World Legal Rights Data Center — even though both parents working is now the norm.
Elizabeth Gregory, a University of Houston professor who wrote Ready: Why Women Are Embracing the New Later Motherhood, said that instead of asking why women put off motherhood, “you could ask, ‘Why do people even have children in this family-unfriendly environment?’”
Yet even women who relish becoming mothers later in life talk about other trade-offs. Some worry about juggling the needs of elderly parents against toddlers, or fret about growing feeble before their children are grown.
In Ventura, Calif., Deanna Scott imagines the day she can claim senior discounts — while her twins are still in elementary school. She hopes her children won’t see her as “an old fuddy-duddy.”
“I get scared about that,” said Ms. Scott, 45, just weeks after her twins were born through surrogacy. “But am I scared enough that I wish we weren’t in this position? Hell, no.”