The standard logic goes as follows. Woman gets pregnant. Woman takes time off for pregnancy. Woman finds out how high the cost of childcare is, or how hard it is to leave a child, and drops out of her job within the first year of her child’s life. This logic assumed that very young children pushed women out of the labor force. And it was right—that is, up until recently.

Last week I wrote about some new evidence that some women do in fact opt out of the labor force. The study shows that a significant percentage of college-educated women whose husbands make more money have the urge to leave their careers. I speculated that while this is clearly about the gender wage gap, it may also be about childcare duties. After all, many couples still assume that only a wife’s, and not a husband’s, income goes toward paying for someone to watch the little one.

But that picture seems to be far more complicated than we may have once assumed. In fact, the tug of childcare duties may have completely changed since the 1970s and ’80s. As Stefania Albanesi, senior economist at the Federal Reserve Bank of New York and author of the study, told me in a conversation about the childcare implications in her study, “It doesn’t seem that children play the role that they played in the past. The flattening out of labor force participation occurs both for women with no children and women with children; this suggests that having children per se is not driving it.” In the past, giving birth to a child was the key factor in whether a woman left her job. Now both mothers and women without children are those dropping out.

There’s evidence of this in Albanesi’s study. The percentage of women in the work force whose husbands are high earners has been leveling out since the late ’80s and ’90s for both women with children and women without. It also holds true across most age groups—from 21- through 53-year-olds—which suggests that young children aren’t a big factor.

There are some other studies that also point in this direction. A recent Census Bureau analysis looked at women and childcare arrangements from 1961 to 2003. In the past, the study’s findings show, childbirth seemed to push women out of the labor force in droves. From 1971–75, only about 20 percent of women were working again six months after the birth of a child. That trend has changed. From 2000–02, over 50 percent of women had returned to their jobs. Women also take less time off during pregnancy: from 1971–75, only 43 percent of women took time off the month before their birth. About 45 percent took time two to five months out, and an amazing 13 percent left a full six months ahead of time. These days it’s almost all in the last month: from 2001–03, nearly 80 percent of women left with just a month to go.

So if childbirth doesn’t push women out the way it used to, does it have any effect? Yes, but it’s delayed. A study of MBA graduates of the Booth School of Business at the University of Chicago, both male and female, from 1990 to 2006 showed that women weren’t leaving their careers at the first sign of pregnancy. Interestingly, the study found (my emphasis), “The careers of MBA mothers slow down substantially within a few years following their first birth.” The effects of a child don’t hit right away. The study found a large decline in women staying in their jobs in the first year after birth, but that trend continued over the next four. In fact, it found, “A woman’s likelihood of not working in a year is about 13 percentage points higher in the two years immediately following her first birth,” and that number “increase[es] to 18 to 19 percentage points higher at three years following the birth.” The further out from a birth, the more likely a woman is to have dropped out, not less.

The study also found that most weren’t actually leaving their careers. Instead, they started to slow them down, decreasing hours, shifting to part-time work, or becoming self-employed. The study observes that “almost no decline in labor-force participation is noted, and only a modest decline in hours worked are apparent in the two years before the first birth.” Those hours keep falling off in the four years after a woman has her child. In conjunction, there is a “large shift” into part-time or self-employment. That represents a huge change in the way we normally think of mothers: these days, rather than leaving work to go full-time mommy, many seem to be trying to do both.

So why are women taking time to care for children so much later on, once they’re likely in school all day? A Brookings paper in 2010 noted that there was a sharp rise in the time parents were spending on childcare in the 1990s, but it wasn’t spent on infants and toddlers; it was going to teenagers. The paper argued that this was a response from college-educated parents to the double whammy of a rise in the perceived value of getting a degree coupled with an increase in competition for getting into college in the first place. Parents were focusing so heavily on their children out of the desire to help them get into a good school.

That’s not likely to be the whole story, but the overall trend seems to have changed significantly from earlier decades. We still tend to assume that women feel the pressures of full-time care of very young children, leading them to jump off the career track right away, but the real story is much more complex.

And it also must be said: these trends hold for middle- to upper-middle-class college-educated women with spouses. The story is very, very different for low-income women, and particularly for single mothers. They rarely even have the option to drop out of work. While these new trends should change some of the ways we think about working mothers, it’s not an across-the-board shift.