There are many reasons for this, including voluntary choices as women’s career options and earning potential have improved. But not every factor behind declining birth rates has to do with everyday decisions.
Deep and long-lasting environmental changes mean child-seeking people in the U.S.—as well as the rest of the world—might have odds stacked against them these days.
Successful reproduction relies on hormones, crucial biological regulators that orchestrate everything from puberty to sperm production, fertilization, and pregnancy. This is true for humans the same way it is for most animals, including other mammals, fish, and birds.
Substances such as particle-sized plastics and harmful forever chemicals—a class of substances used in everything from food packaging to some nonstick cookware that do not break down naturally—can mimic or obstruct hormonal activity that develops sexual health or allow successful reproduction, the review found. Even in small amounts, these contaminants are enough to hobble normal processes.
“This can occur at effective concentrations so low they are analogous to a whisper that is powerful enough to redirect a hurricane,” the authors wrote.
The review described several toxins and chemicals that have become ubiquitous in everyday life, although relatively few have been deeply studied for their potential harm to humans.
The authors note how of the 140,000 synthetic chemicals that are currently registered, only 1,000 are known to affect the biological processes that govern hormones, known as the endocrine system. But this is likely a “gross underestimate,” according to the review, given just 1% of these chemicals have been sufficiently researched and the fact that around 2,000 new chemicals are produced and released every year.
These substances are invasive disruptors to animals’ endocrine and reproductive systems. For example, the review included multiple studies that documented the effect of microplastics—plastic particles smaller than a fifth of an inch in diameter, in semen production, collectively finding microplastic exposure led to falling sperm counts and motility in multiple species, including humans.
Pollutants aren’t the only environmental factor hurting fertility. The review also analyzed the role climate change plays in declining birth rates, finding higher temperatures constitute a heavy toll on the reproductive prospects of most animal life.
To be sure, environmental factors aren’t the only reason birth rates in the U.S. and in the rest of the world are declining. Reducing gender discrimination in education has been a crucial driver, as a growing number of women in developed economies opt to pursue schooling and careers over creating a family.
The nosedive in U.S. fertility also has some positive explanations. A large factor in last year’s record low was a sharp decline in teen pregnancy rates, according to the CDC, which found the fertility rate for teenagers ages 15 to 19 had fallen 7% in 2025, the latest in a decades-long series of progress. Since 1991, in fact, the teen birth rate in the U.S. has plummeted 81%.
But with pollution and warming taking their own toll on fertility, the choice to avoid children might be taken out of peoples’ hands.
“We must recognize that chemicals, once released, don’t simply disappear,” the authors wrote. “Instead, they contribute to the larger issue of driving humanity towards the exceedance of planetary boundaries when considered in combination with climate change and other planetary-level impacts.”
