Why the Birth Spacing Discussion Matters

It's not just about our ancient ancestors

A couple of weeks ago I published a piece about the evolutionary history of human birth spacing. In it, I explain how the natural interbirth interval (the number of years between live births) for human hunter-gatherers is 3-4 years, which is actually shorter than any other great ape. Orangutans have their babies 8 years apart and chimpanzees, our closest living relatives, have their babies 6 years apart. This data suggests that, in our evolutionary past, the interbirth interval was probably closer to 4 years, until the advent of agriculture roughly 10,000 years ago (when we were already 95% of the way along our evolutionary timeline). In fact, the interbirth intervals of the earliest hominids, if we rewind back 5 million years ago to when our lineage split from chimpanzees, may have been closer to 6 years and then evolved to become shorter as our species became more socially intelligent and cooperative started rearing children collectively.

But what does it matter what our hominid ancestors did? Today, many women are perfectly capable of having babies two years part (or less) and many even feel that it is ideal. In fact, I would argue that most of our society considers a two-year gap between siblings to be “ideal,” and that is precisely why this discussion matters. There are many valid reasons for spacing births closer together in the modern context, but I believe it’s time for a narrative shift around what is “normal,” “natural” and healthy. Women should be making informed choices about how to plan their families based on a clear understanding of the risks and benefits, not based on social media motherhood propaganda or pushy relatives’ advice.

When A Woman’s Body Refuses

Although a four-year gap between births may well have been the norm for some 95% of our history as a species, many contemporary women feel that two years is the ideal age gap between siblings and often feel immense pressure to conform to this ideal. Whether external or internalized, the influence is often so strong that it overrides a woman’s intuitive sense of what is right for her and her body or for her existing children. In many cases, a woman’s body will simply refuse to cooperate. Secondary infertility (the inability to get pregnant again after having a first child) affects about 11% of Americans, and while there are many possible causes, I suspect that one reason is simply that couples try too soon after a previous birth. Indeed, miscarriage is more common among women with shorter interbirth intervals than among those who wait at least 18 months after the previous birth to try again (source).

Amara, a school teacher from Ontario, told me that, even though no one was putting explicit pressure on her, she had internalized the idea that two years was really the “ideal” age gap and felt terrible when she kept having miscarriages.