Erin Striff sits on the couch with her lap divided between 5-year-old Megan and 22-month-old Ethan. A few feet away, Caitlin points a finger and protests.

"You want to be on my lap, too?" Striff asks Caitlin, who is Ethan's twin. "Come, there's room; you can be on my lap."

Sometimes Striff worries that, when her kids fight over the chance to sit on her lap, it's because they aren't with her enough.

"Maybe I haven't spent enough time with the little ones," she ponders. "Maybe there's not enough of me to go around ... to provide quality time for everyone."

It's the kind of worry that mothers seem to specialize in — no matter how much time they spend with their kids.

Research recently delivered some good news to today's mothers: They actually spend more so-called quality time with their children than mothers did 40 years ago, in the heyday of stay-at-home moms.

Fathers, too, are spending more time with their children, but mothers still do more of the primary childcare.

According to a University of Maryland study, mothers in 1965 spent 10.2 hours a week in focused time with their children — feeding them, reading to them, playing games with them.

That number declined in the '70s and '80s, rose in the 1990s, and now, at 14.1 hours per week, is higher than ever.

Even so, roughly half of mothers who work outside the home (and 18 percent of those who don't) believe they aren't spending enough time with their children.

There are no statistics that show whether mothers felt this way in the '60s and '70s, but many experts say that today's mother — no matter how much she does for her child — is more likely to feel there is more she could do.

Melissa Milkie, one of the authors of the Maryland study and of the book "Changing Rhythms of American Family Life," says mothers face "very strong cultural expectations that are impossible to achieve no matter how much you're doing."

That ideal of mothering has changed a great deal since the '50s and '60s, said Steven Mintz, a behavioral-sciences fellow at Stanford University.

The notion used to be, he says, that "if you just let a child out to play, they would grow up and mature almost automatically, and you really didn't need to do much intervention. ... I would call it kind of 'natural upbringing.' "

There was a feeling that intervention could lead to over-mothering, which could create psychological problems, says Mintz, who is co-chairman of the Council on Contemporary Families.

"We have the opposite idea now," he says.

"It's intensive cultivation. What this means is that from birth, even before birth, when the child is in the womb — that at every stage we need to cultivate our child if they are going to be successful.

"That idea makes it much harder to be a mother, especially with the huge demands on someone's time in the workplace as well as in the home."

Why this shift?

Mintz says it's because we live in a more competitive society and that "there is a great worry that my kid will get left behind unless I do something special. ... With housing costs rising out of hand, there is a real fear that kids won't be able to repeat their parents' class status.

"All of this has made parenting much more anxious," Mintz says.

Even mothers who stay home are not immune to worry and guilt.

Jeannie Newman of Middletown, Conn., doesn't worry about the number of hours she spends with her 3-year-old son, Gus. But she does worry about whether she is doing all she can for him.

She has friends who are also stay-at-home moms who fill their toddlers' lives with music class, dance, yoga.

"I feel pressure sometimes to fill up the day with activities. ... I think if I'm home with him, then I really better make it count — like a one-person daycare.

"But sometimes, I say enough is enough," Newman says. "If we are just playing with the truck in the backyard — that's enough. You don't have to do everything every day."

Striff says she tries to manage any anxiety she may have about whether she is doing the best she can for her children by thinking about balance: not living only for her children or only for herself.

"I always come to the same conclusion," Striff says.

"There is only so much you can do ... there is the idea of 'good enough' parenting. You'll never be the perfect parent, but you want to do more than make sure everyone has a pulse at the end of the day."