What is the most common birth date in the U.S.? According to data from the U.S. National Center for Health Statistics, it’s Sept. 9. In fact, September as a whole is a very busy month in U.S. delivery rooms. And, for those of you doing the math, that makes December a very busy month for, uh, other reasons.

Is there something in particular that makes December special, though? Could that possibly be our so-called “mating season,” even though we don’t, say, roar, pump out citrus scents or bash antlers to cue desire like other species do? 

For the most part, humans and other large primates likely aren’t driven to reproduce because of specific seasonal cues, says Melissa Emery Thompson, an anthropologist at the University of New Mexico. “[But] we are very strange creatures,” she adds.

But humans also approach pregnancy and child rearing differently than do other large primates. And all the resources at our disposal — from nannies to extreme sports to air conditioning — might help dictate how we reproduce, too.

Perfect Timing

In the rest of the animal kingdom, those that raise their young often time their births so that their babies appear when parents have enough resources to feed and foster them. For some, this means giving birth when the mother has the most energy she can possibly muster. Seals that feed their young exclusively off of stored blubber, says Emery Thompson, have their babies when mom has a lot of fat packed away.

Other animals plan child rearing for a time when the rest of their environment provides the resources they will need. Baby blue tits, for example, hatch from their eggs right when the birds’ preferred meals — caterpillars — are plentiful.

But when it comes to primates, these breeding rules start to shift. Most can’t put on and store fat, which means that’s not a reliable breeding strategy for them. And some of the smaller species, like lemurs, don’t sink a whole lot of time and energy into their young, so their birth seasons might revolve around when a particular resource shows up. Infant mortality rates shoot up when a drought or other environmental factors deprive the species of the seasonal foods they planned on having for their offspring, says Emery Thompson.

And big primates like great apes and humans are, well, big. Raising these babies takes a lot more energy for a prolonged period — like breast-feeding for years at a time. In these cases, it's too hard to stick to just one of the typical approaches to rearing offspring. “You can’t rely on one period of food abundance, and you can’t store up enough body fat,” says Emery Thompson.

Humans — and fellow large primates — are more likely to tie pregnancy to our personal ability to support a child, as well as the abundance of available resources. Research with hunter-gatherer communities has found that births swell in times of high food availability. Sure, those births might become “seasonal” in that they might happen at predictable times, Emery Thompson says. But the rhythms have more to do with when mothers can nurture their infants best — rather than an innate drive to reproduce at that time. 

Additionally, humans live all around the world in a huge range of climates and nourish themselves with a wide variety of foods. Pegging pregnancies to a single environmental factor or resource doesn’t make as much sense when you have as much flexibility as our species does. “It’s better to cue [pregnancy] on yourself and how you’re doing, rather than the external environment,” Emery Thompson says.

Our Sensitive Systems

So when and why we reproduce probably isn’t just dictated by environmental cues. But decoding what internal biological processes set us up for child rearing is challenging to figure out. On the whole, human reproductive systems are very sensitive to change. Both obesity and being underweight can tamper with fertility, and intense exercise can inhibit periods or ovulation, too.

Beyond that, we generally struggle to get pregnant compared with other primates. For those species, it might take two to three menstrual cycles for conception — in humans, the average is six, Emery Thompson says. 

Even some of the biological factors that determine how often other primates get pregnant don’t apply to us. Our societal structures let us have kids in succession faster than our other relatives. Energy-dense foods, like those with lots of oils and fats, give us the calories we need to produce breast milk all in a few bites. Getting that much nutrition that fast is near impossible for other primates foraging on plants.

And while other great ape mothers breast-feed their infants until the baby can feed itself, we cut that process short and rely on other foods — or other people — to help us nourish our offspring until they can get their own food. “By weaning, we speed things up,” Emery Thompson adds.

This is not to say that biological undercurrents have no influence on when humans have kids. Some research has linked sperm quality to the seasons, for example, with lower-quality specimen appearing in the summer months. But the way humans can choose to live our lives has significant power, too.

After all, a lot of babies may be born in the summer, but that's not necessarily when they’re conceived. Between 1931 and 2010, researchers found that days above 80 degrees Fahrenheit led to lower birth rates eight to 10 months later. These heat wave-induced changes became less dramatic in the 1960s — right around the time that air conditioning became widespread.