Climate Change Could Spark A Decline In Conception Rates

A study published this year in the Journal of Population Economics suggests climate change may result in lower rates of human conception. However, it’s not due to less sex, and it doesn’t necessarily mean fewer babies born. Instead, society may see fewer unplanned pregnancies as human fertility declines.

The authors of the study, Tamás Hajdu and Gábor Hajdu of Hungary, took advantage of random variation in temperatures to examine the effects of heat shocks on conception rates, including looking at lagged effects from these shocks over time. Instances of human conception are counted in the study based on three outcomes: the number of live births, the number of abortions, and the number of clinically identified fetal losses (i.e., miscarriages).

The study uses data from Hungary, so the authors are careful to note that their results may not extend everywhere. That said, Hungary is a modern European nation with a climate not unlike much of Europe and parts of the U.S., so the findings could have important implications.

First, the authors predict an overall decline in conception rates as a result of warming of the planet due to climate change. At first, one imagines fewer cold winter nights cozying up by the fireplace. However, the authors suggest temperature is not associated with sexual activity according to prior research. More likely, heat tends to impair sperm quality and reduce sperm formation, and may also have implications for female reproduction. By extension, heat will tend to lower the overall probability of pregnancy by lowering fertility.

In the study, there is an observed decline in conception rates following periods of high temperatures. However, the authors also observe the potential for shifts in pregnancies, with a corresponding increase in some conceptions in cooler periods—potentially leading to greater seasonal differences in the future.

In other words, the authors find a dip in conceptions in the initial five weeks after a heat shock, but for pregnancies resulting in live births there is an increase 6 to 25 weeks after the shock. For those trying to get pregnant, the pregnancy might just occur a little bit later when excess heat is present. But this “rebound” effect is not present with abortions or fetal losses.

Why not? The authors speculate that aborted fetuses won’t be “made up for” with conception at a later date, because many of those pregnancies are unintended. Similarly, fetal losses are more likely with unplanned pregnancies, so one wouldn’t necessarily expect a rebound effect there either.

Overall, the data reflect something surprising: fewer conceptions that result in abortions and fetal losses—but not live births. So the cumulative effect is that conception rates fall with higher temperatures, but this doesn’t necessarily mean fewer babies born. At least, that story is consistent with the data.

Climate change has many negative effects, and reduced human fertility is likely to be one of them. Additionally, higher temperatures may have implications for fetus development, and this deserves more careful study as well. But one unintended consequence of global warming could be a decline in unplanned pregnancies. While this is not all good news by any means, it’s not all bad either. Like most things in life, we often have no choice but to take the good with the bad.


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