“Oops, I’m pregnant.” Even in today’s age of safe and effective modern contraception, women in every society get pregnant when that wasn’t the plan. It’s a simple point I explore in More: Population, Nature, and What Women Want. In the wealthy and health-obsessed United States, for example, 49 percent of conceptions result in “oops” pregnancies. The figure for the world as a whole is estimated at around 38 percent. I suspect that women in many countries under-report unintended pregnancies and that the real proportion is even higher. Interestingly, the estimated number of annual unintended pregnancies worldwide is almost the same as the annual added population—around 80 million in the first case, 78 million in the second. The two numbers actually aren’t fully comparable, since many unintended pregnancies result in abortions and others simply occurred earlier than a woman intended. Nonetheless, it’s clear that much of the world’s population growth is the outcome of unintentional or at least ill-timed reproduction. Unintentional pregnancy is common even among rich, well-educated, and influential women. You could hardly find a better example of this than the curious case of Cherie Blair, wife of former British Prime Minister Tony Blair. In her new tell-all autobiography, Speaking for Myself, Ms. Blair reveals that her fourth pregnancy came about because she was too embarrassed to bring her “contraceptive equipment” with her on a royal visit to Balmoral Castle, Queen Elizabeth’s Scottish residence. Apparently the Royal Unpackers at the residence carefully remove and put away all the contents of their guests’ luggage. When the prime minister and his wife first visited the castle, Ms. Blair writes, she had been annoyed that all her possessions had been unpacked, down to “my distinctly ancient toilet bag with its range of unmentionables.” (Interesting noun.) So, on the next visit, she “had been a little more circumspect” and left the said unmentionables at home. I’ll spare you her description of what happened later in the “bitterly cold” castle, but the result was the Blair’s fourth child, who is considerably younger than the other three. Incredibly, though Cherie Blair was only 45 years old at the time, this accomplished barrister and judge believed she was “too old” to become pregnant. I wouldn’t spotlight this example of an “oops” pregnancy if Ms. Blair hadn’t published it. I’ve heard similar stories from friends for years. Such stories support important points in my book about women’s lives and population. Many people—including some prominent economists—seem to believe that sexually active partners simply decide how many children to have and then set about having their “desired family size.” But sex happens even when couples don’t want to conceive. Preventing conception takes effort, a willingness to risk embarrassment (whether at shop counters or in royal residences), and some kind of “contraceptive equipment.” This is just as true among the wealthy as it is among the poor. The wealthy contribute a lot more on a per-capita basis to human-induced climate change and many of the world’s other environmental problems. Yet a significant proportion of their own population growth results from “oops” pregnancies. For anyone who cares about the environment and the influence of population size on it, it’s not enough to support access to family planning in developing countries, important as that is. We also need much better contraceptive access and options in industrialized countries as well. And we need to figure out how to make contraception less of an “unmentionable” for every woman and man, right up to the level of prime ministers and their spouses.