More Babies Are Conceived Over The Holiday Season

October 11, 2019

Have you been invited to many summer birthday parties? There’s a reason for that. In the United States, most births occur between June and early November. Count down the nine months and you’ll see that in the fall and winter where most conceptions take place.

What is the reason? Is it the cool fall air or the joy (or anxiety) of the holiday season that triggers more unprotected sex? Or is it something else entirely?

It turns out that reproduction is seasonal in all living organisms, from plants and insects to reptiles, birds and mammals – including humans. The ultimate explanation for this phenomenon is evolutionary.

The Earth’s environment is seasonal. Above or below the equator, the structure of the year consists of winter, spring, summer, and fall. At the equator, the wet and dry seasons punctuate the year. Organisms have evolved strategies to reproduce at times of the year that will maximize their reproductive success throughout their lives.

Humans are no exception, and have maintained this evolutionary result: birth seasonality. Researchers, including us, have recently been working to learn more about the causes of birth seasonality, as these patterns contribute to outbreaks of childhood diseases.

Tracking birth spikes around the world
The earliest studies demonstrating the seasonality of human birth date back to the early 19th century.

In some countries, local customs can explain the seasonality of births. For example, in the 1990s, researchers showed that the traditional July-August wedding season in Polish Catholic communities led to a large number of births in the spring. However, the wedding season does not drive the seasonality of births everywhere, and in most places there is only a small correlation between weddings and births 9 to 15 months later. Thus, the newlywed beds do not tell the whole story.

There is a clear pattern of births in different latitudes. In the U.S., northern states see a peak in births in early summer (June-July), while southern states see a peak a few months later (October-November).

Globally, popular birthdays follow a similar pattern, with peaks occurring earlier in the year the further north you go from the equator, such as in Finland at the end of April, and in Jamaica in November. And in the US, states further south, such as Texas and Florida, experience birth peaks that are not only later in the year, but more pronounced than those in the north.

So what affects conception?
Studies have shown that the seasonality of births is related to local variations in temperature and day length. In regions with extreme temperatures, there are usually two birth peaks per year. For example, data from the early 20th century show that West Greenland and Eastern Europe experience two distinct birth peaks each year.

Rural populations tend to have more pronounced seasonal birth pulses than urban populations, possibly because rural residents may be more affected by environmental conditions, including changes in temperature and day length. Environmental factors such as these may influence human sexual behavior.

In addition, as with other animals, these environmental changes may drive seasonal changes in fertility. This means that female and/or male fertility may change over the course of a year as an endogenous biological phenomenon, rather than just an increase in the frequency of intercourse, making it easier to conceive at certain times – provided, of course, that intercourse occurs.

Biologists know that the fertility of nonhuman mammals is affected by day length, which can resemble a reproductive calendar. Deer, for example, use the decreasing days of fall as a signal for when they are ready to reproduce. Females become pregnant in the fall and carry their pregnancies through the winter. The goal is to give birth at a time when there are plenty of resources available for newborns – it is evolutionarily advantageous to be born in the spring.

So animals with long gestations tend to be short-day breeders, meaning they only breed on the short days of fall and winter, they get pregnant through the winter, and give birth in the spring. Animals with short gestation periods, on the other hand, are long-day breeders; they get pregnant during the long days of spring or summer and, because of their short gestation periods, give birth to their young in the same spring or summer. Many species mate and are only able to get pregnant at certain times, such as those with long or short days, and the length of the day itself directs their hormones and ability to conceive.

Humans may be no different from other mammals. Day length has the potential to affect human fertility, and it does seem to explain the seasonal pattern of births in some places but not others. In addition to day length, researchers have shown that changes in social status and standard of living can also affect birth seasonality. There does not appear to be a single driver of human birth seasonality; a range of social, environmental and cultural factors all play a role.

What is the relationship between season of birth and disease?
Forest fires need fuel to burn. After a large fire, it must be replenished before another fire can spread.

Disease epidemics are no exception. Childhood infections require susceptible children for the pathogen to spread through the population. Once a child is infected with and recovers from polio, measles, chicken pox, etc., he or she is immune for life. Therefore, for a new epidemic to occur, there must be a new group of susceptible infants and children in the population. In the absence of vaccination, the birth rate of the population is an important factor in determining the frequency of childhood disease epidemics.

Babies are born with maternal immunity: antibodies from their mothers that help protect against infectious diseases such as measles, rubella, and chickenpox. This immunity is usually effective during the first 3 to 6 months of life. In the United States, many of the infectious diseases that strike infants tend to peak in the winter and spring. This makes infants born during the summer and fall birth seasons in the U.S. vulnerable after 3 to 6 months as maternal immunity wanes, which is when many infectious diseases strike in the winter and spring.

In humans, average birth rates are extremely important for understanding disease dynamics, and changes in birth rates influence whether epidemics occur annually or every few years, and how large they are. For example, the epidemic of polio in the first half of the 20th century resulted in thousands of children being paralyzed by polio every summer in the U.S. The size of the polio outbreak was determined by the birth rate. Because of this, after the World War II baby boom, birth rates rose and polio outbreaks became more extreme.

Likewise, the timing and intensity of birth spikes can affect the length of time between epidemics. Importantly, no matter how often an epidemic occurs – as with births – it is always seasonal. And births have been shown to directly alter the seasonal timing of viral outbreaks in children.

Does the number of children born in the summer drive the seasonal occurrence of childhood diseases? Does disrupting birth patterns change seasonal outbreak patterns? We know that changes in the average birth rate can alter the magnitude of childhood disease epidemics, such as polio during the baby boom years. Theoretical models suggest that seasonal changes in births can alter the size and frequency of childhood disease outbreaks. But whether seasonal changes in births over the past 50 years have actually altered childhood diseases remains an open question; more research is needed in this area.

Losing the seasonal link
There is one thing that all researchers in this field agree on. Across the northern hemisphere, people are starting to lose the seasonality of births (due to lack of data, it is not yet known what is happening in countries south of the equator, such as Latin America and Africa). (Due to lack of data, it is not yet known what happens in countries south of the equator, such as Latin America and Africa).

There are two pieces of evidence to support this. First, the intensity of the birth pulse – from June to November in the United States – has been declining for decades; and second, where there are two annual birth peaks there is now only one.

This loss of birth seasonality may be due in part to social factors, such as pregnancy planning and the increasing disconnect between humans and the natural environment, which leads to seasonal changes. The roots of this variation may be related to industrialization and its downstream social effects, including indoor work, less seasonal work, access to family planning, and the fact that modern housing and artificial light mask the natural day lengths that may affect fertility.

Whatever the causes of birth seasonality, at least in the United States, one thing is clear – this is still the prime time for conception.