There is a code phrase for this, popularized in the last century but still very recognizable today: career woman. From Oxford Dictionaries:
1.4 [as modifier] (of a woman) interested in pursuing a profession rather than devoting all her time to childcare and housekeeping.
Nowadays we don't think of a career and motherhood as mutually exclusive, but the phrase was coined to make exactly this distinction. Some examples of use:
[M]y wife had an aunt whose daughter was a career woman; she did not want children, and had never had a child . . . . —Fr. Anthony Zimmerman, "Interview: Lloyd J. Duplantis, Ph.D", The Linacre Quarterly, 1999
“She was a career woman. No time for children.” —Vivienne Wallington, The Last Time I Saw Venice, 2011
“Prospective employers can't legally ask if you want children in the near future, but you know for damn sure they'd like to,” she said. “So I say it up front. I'm a career woman with no time for children.” —Cathy Kelly, It Started With Paris, 2015
And a discussion of the anachronism of the phrase, which nonetheless acknowledges its underlying meaning:
The problem with the term “career woman” is that it's anachronistic; it's from a generation ago when a woman who worked was an outlier, a rebel, a feminist.
It's really not relevant to today . . . .
Yet even the Boston University Department of Economics couldn't resist the title “Are Career Women Good for Marriage?” for a 2008 report. The authors explained that by “career women,” they meant any woman, married or single, who works. Which only begs the question: Why are there no “career men”? And while there are certainly women who are child-free by choice (and sometimes that choice is made in order to have more career freedom), most of the women I know want children. —Melanie Notkin, Otherhood: Modern Women Finding A New Kind of Happiness, 2014
I would recommend using this phrase with caution, as it does not generally reflect today's reality and carries more than a whiff of sexism. However, if it works for your audience and story-line you could potentially take advantage of this relic of the battle of the sexes, with something like
When a unique post-apocalyptic baby is born, a former career woman becomes its guardian...
I would substitute former for retired here, just to avoid confusion about her age (unless she is actually in her seventies).
(Of course this phrase will work best if her back-story includes some kind of career that took up much of her time; if she was more a lay-about who happened to have a child phobia, you probably should look for something else.)