Like quite a few people I know, I had my first child in my late thirties–39 to be exact. My maternal grandmother had a child at 39, too, but that girl was her eighth baby and her last.

This difference summed up for me the change that had occurred in two generations, when I started writing a book about the new later motherhood–its causes and effects, personal and social. Where 1 in 12 first babies these days is born to a mom 35 or over, it was 1 in 100 in 1970. Add in the adoptive moms, and you’ve got a big group.

Women today walk a very different road from all our ancestors in terms of education, work, and civic status. That’s in large part due to the arrival of hormonal birth control, which has enabled many to delay having kids, giving us time to finish our educations, establish ourselves at work, find the right partner, and just see the world a bit.

It’s definitely the case that later mothers are largely educated women, often in well-paid or professional jobs–which means middle or upper middle class generally, though not always. The women I interviewed for my book Ready: Why Women Are Embracing the New Later Motherhood included middle-class black, white, and Hispanic women, but the large majority were white. The intersection of class, race, and motherhood is a complicated one.

In any case, we’re not entirely off the old map of our grandmothers’ physical experience when we have kids in our late thirties and early forties today (while most women can get pregnant through their late thirties, only about half can do so at 41, and for all but a very few, fertility ends by 43–though increasing numbers of babies are born to women after that age with donor eggs).

Eight years after my elder daughter arrived, my husband and I adopted a one-year-old, putting me at the far end of the usual mother/child generational divide but still within shouting distance of my anomalous great-grandmother (Ma Dear, in the family parlance), who had a baby at 46. This was so unusual that they thought at first that the child (my paternal grandma) was a tumor.

With our 47-year gap, my family is not quite in the realm of the “how late is too late” debate that shows up regularly in the tabloids, when someone in her sixties or seventies has a baby via egg donation, but we’re definitely part of a growing group that’s pushing the age envelope past the point where parenthood used to end–both via IVF and adoption.

“Not that you look old,” said my 44-year-old friend on the phone last year. She was explaining why she wouldn’t be implanting that frozen embryo, though she’d just said that her four-year-old would love a sib and that she and her husband would both love a second child.

“I just don’t want to be one of those strange old lady moms you see in the park,” she sighed.

“I look my age,” I replied, by way of communicating to her both that I didn’t feel insulted and that I also wasn’t under any misapprehension that I looked younger than I was. She was distinguishing between her new-mom-at-40 self (entirely normal for many now) and what she would be if she had a baby when she was over 45.

Of course, in spite of her reassurance, we both knew that I’m exactly the kind of strange old lady mom she had in mind, though hair dye cuts the estrangement factor slightly (on the Granny Clampett to Diane Sawyer spectrum* of 65-year-old looks in my future, I aim for the middle). There’s a whole mess of confusion in her response that we all share about what age means these days.

So much has changed so fast–30 years added to the average lifespan within a century; birth control that’s decoupled sex from the inevitability of babies; fertility tech and adoption that’s expanded the family-timing options. All our assumptions about who will be doing what and when–and what they’re supposed to look like while they’re doing it–are in transit.

Or maybe my friend’s anxiety was really about work: She has a demanding, powerful job. In the absence of a national infrastructure to support families, delay has served many women as a form of shadow benefits system: giving them higher long-terms salaries and the clout to negotiate flexible schedules that younger workers cannot.

But where one child may be manageable for a two-career couple with demanding jobs, a second, no matter how much desired, might swamp their boat.

So far, I’m fine with phase-two later motherhood–or maybe just too busy to worry about it much. Occasionally it frightens me to think that I’ll be 65 when my youngest finishes high school. But then I remember that Ma Dear made it through, with a lot less technology to assist.

It’s possible my friend’s anxiety will fade in the coming years, and she’ll join the ranks of the even later moms. Or not–there are plenty of other ways to spend energy fruitfully. Among them, working to make the world better, and more manageable, for all families, at whatever point they arrive.

*NB: Neither of these ladies had kids.

Elizabeth Gregory is the author of Ready: Why Women Are Embracing the New Later Motherhood (Basic Books, 2008) and the director of the University of Houston Women’s Studies Program. Her current project explores the politics and economics of women’s work, and she blogs about that at Domestic Product. This piece originally appeared on WOMEN = BOOKS, the blog of Women’s Review of Books.