Is there a “right” time of year to make a baby? Obviously, every couple has to decide that for themselves. But Mother Nature has her own agenda, too.
Animal babies are everywhere in the spring. Farms are filled with adorable calves, chicks, and colts, and the birds in your backyard lay eggs. Animals have babies in spring because temperatures are warmer and food sources more abundant. So why don’t humans have a huge crop of babies each spring, too?
Jay Schinfeld, MD, director of the division of reproductive endocrinology and infertility at Abington Memorial Hospital in Abington, Pennsylvania, and clinical associate professor at Jefferson Medical College, explains that “traditionally, humans were seasonal breeders like other animals. Having babies in the winter without sufficient food supplies and the cold would not have been an effective way to preserve the species. When fire and clothes were incorporated, as we became better hunters and could store food, these factors changed.” Humans moved away from being seasonal breeders once they began to control their environment.
Although humans have evolved to the point where the seasons no longer absolutely control reproduction, there is evidence that our fertility is still influenced by the seasons . . . and that paying attention to these influences just might help the chance of conception.
Temperature and Fertility
One of the most noticeable differences between the seasons, at least in most of the United States, is temperature. As temperatures rise throughout the spring and summer, the increased heat can have a negative effect on sperm count. Reproductive endocrinologist Stephen Somkuti, M.D., Ph.D., associate professor in the department of obstetrics, gynecology, and reproductive sciences at the Temple University School of Medicine in Philadelphia, and medical director of the In Vitro Fertilization Program at Abington Memorial Hospital’s Toll Center for Reproductive Sciences, explains: “The male reproductive system is designed to allow sperm production to occur at approximately one degree cooler than body core temperature. The scrotum is essentially an air-conditioning unit for the testes, where sperm is produced. If there is an increase in temperature, there is a corresponding decrease in sperm production, quality, and count.”
A 1994 study in the medical journal Annals of the New York Academy of Sciences found that there is a definite decrease in conceptions in the United States in July and August, especially in the southern part of the country, and medical experts believe this is linked to higher temperatures decreasing sperm count. According to Scott Slayden, MD, reproductive endocrinologist with Reproductive Biology Associates in Atlanta, men who already have low sperm counts can experience a worsening of their condition during the summer if they spend a lot of time outdoors and are exposed to prolonged periods of high heat without access to air conditioning.
Light and Fertility
Light is another seasonal variable that may have an impact on fertility—in this case, female fertility. Light affects the pineal gland, which produces the hormone melatonin, responsible for regulating the body’s daily rhythms. When there’s less natural light, there’s an increase in melatonin, which may suppress ovulation. The more natural light available (such as during summer), the more fertile a woman may be. Geoffrey Sher, MD, executive medical director for the Sher Institutes for Reproductive Medicine and member of the InterNational Council on Infertility Information Dissemination (INCIID) Advisory Board, says a good example of this is studies showing that native Alaskan Inuit women do not ovulate in winter when there is almost no natural light. He points out that women’s bodies are more sensitive to the environment than men’s.
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Birth Rates, Birth Dates and Babies
According to U.S. Census data, the most popular birth month in the country for the past decade has been July, with August a close second. Most of these pregnancies would have been conceived in the fall: October, November, December. Catherine Racowsky, Ph.D., director of the Assisted Reproductive Technology Lab at Brigham & Women’s Hospital in Boston and secretary of the American Society for Reproductive Medicine (ASRM), says there’s no biological reason for Americans to favor these months. In fact, in the fall, when the most babies are conceived, there’s less light, which should suppress ovulation. So, biologically speaking, it’s not the ideal time for conception. On the other hand, for the male half of the equation, the cool fall weather is a good time for sperm production.
So it’s possible that higher temperatures in the summer may decrease male fertility, while increased light may increase female fertility, and these factors may cancel each other out. Since more babies are conceived in late fall, it may be that even though male and female fertility is not at its peak, it’s also not suppressed.
There may be other reasons, in addition to heat and light, why seasons may affect fertility. Some experts believe the summer birth rate bump is tied to fall holidays and family vacations, when couples spend more time together. And cooler fall weather may make couples frisky after a hot summer.
In addition to the national statistics, Racowsky says birth rates and dates vary by state. According to state data, as you move south and west across the country, high birth rates shift from fall to summer. The farther west or south the state is, the greater number of summer births (with autumn conceptions). Northern and eastern states show higher birth rates in the fall, meaning most conceptions took place in the spring.
Finally, international birth data may also show evidence of the seasonal effect on birth rates. Several studies have examined fertility rates in different countries and concluded that the lower the latitude of the country (the closer it is to the equator), the less likely seasons are to affect conception. In these countries, temperature and light vary little throughout the year. But the farther a country is from the equator, the more likely it is to have decreases in fertility in the winter months.
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