If it feels like your social calendar is packed with birthdays at the end of summer and early fall, you aren’t imagining it. Take a look at US birth rates, and you’ll notice that more babies are born in July through September than in any other month. After some quick mental math, it appears that these babies were all conceived during the winter months. But why?
Sure, it seems easy to chalk it up to boozy holiday festivities that may lower inhibitions and lead to more sex. Or cold weather that makes us want to cuddle up or spend a bit more time in bed with a partner. And while these may all play a role, it turns out the answer may be a bit more complex. Reproductive seasonality, the academic term for changes in conception and birth rates throughout the year, is a well-documented phenomenon, at least in animals.
While it feels tempting to try to perfectly plan the so-called best time of year (or even the best time of day) to become pregnant, the truth is there’s still a lot we don’t know about reproductive seasonality. From biology to environment, here’s what the science says about why we tend to make more babies in the winter.
Animals follow reproductive seasonality patterns, but do we?
To understand reproductive seasonality, scientists first looked at animal behavior. It turns out that animals follow specific patterns because the season in which offspring is born can impact survival chances. Seasonal breeders, as they are called, experience changes in their hormones based on the amount of light they’re exposed to, so shorter days with less light tell the body it’s time to get busy.
Some species have also adapted to give birth during specific seasons when high-fat foods are more abundant, like spring or summer, as opposed to winter, where food may be limited and temps are too low for little ones.
But do humans follow the same patterns? Possibly, but not to the same extent. “Human reproduction doesn’t strictly adhere to the seasonal patterns seen in some animals, yet subtle variations in fertility rates across different seasons have been noted,” shares board-certified OB-GYN Dr. Rakhee Patel. “For instance, sperm quality may improve in colder months, potentially boosting conception chances.”
Heat may adversely impact sperm quality, while some research even suggests testosterone levels may be higher in the winter. One study found higher fertility rates in the late fall and early winter and lower fertility rates in the spring, particularly in the Southern US. Additionally, some research suggests a higher risk of early miscarriage in the hot summer months.
The influence of daylight on animals can also impact women, potentially affecting hormone levels that regulate ovulation and menstrual cycles. “The amount of light exposure and consequent hormonal changes are also crucial,” says Dr. Patel. “The longer nights in winter could alter melatonin production, which is believed to have a role in reproduction, though this connection is not fully established.”
Our behavior and environment also play a role in higher rates of winter conception.
Biology clearly plays a role, but our behavior and environment also contribute to human fertility patterns. As Dr. Patel explains, the winter season brings us holidays and more indoor activities, which can increase intimacy and contribute to increased conception rates. “Psychological factors linked to seasonal changes can affect mood and behavior as well. Colder months might encourage couples to spend more time indoors, leading to increased opportunities for conception.”
In other words, there doesn’t seem to be a single driving factor behind birth seasonality in people. Instead, various social, environmental and biological factors may play a role. “While these factors suggest a scientific basis for the idea of seasonal fertility, it’s important to recognize that human reproduction is influenced by a wide array of individual, environmental and physiological factors.”
To add even more complexity, human birth seasonality trends are changing slightly as the environment, access to family planning tools, and assisted reproductive technologies (ART) make it easier to conceive throughout the year.
While there may be a slight correlation between winter months and higher conception rates, it’s just one of many factors at play, and we don’t know enough to conclude that it’s a significant contributor to birth seasonality. If you’re TTC and feel more frisky in the winter, great! But fertility is a complex subject, and more research is needed.
Dr. Rakhee Patel, MD, FACOG, is a board-certified OB-GYN with Pinewood Family Care Co. Direct Care Network in NJ
Beltran-Frutos E, Casarini L, Santi D, Brigante G. Seasonal reproduction and gonadal function: a focus on humans starting from animal studies. Biol Reprod. 2022;106(1):47-57. doi:10.1093/biolre/ioab199
Lee JH, Lee SW. Monthly Variations in Serum Testosterone Levels: Results from Testosterone Screening of 8,367 Middle-Aged Men. J Urol. 2021;205(5):1438-1443. doi:10.1097/ju.0000000000001546
Wesselink AK, Wise LA, Hatch EE, et al. Seasonal patterns in fecundability in North America and Denmark: a preconception cohort study. Hum Reprod. 2020;35(3):565-572. doi:10.1093/humrep/dez265