In the U.S., approximately 12.7% of reproductive age women seek infertility treatment every year. But that statistic excludes men with infertility issues, which is just one of many reasons actual rates are hard to ascertain.

“There is an idea that rates of infertility are on the rise,” Dr. Erica Johnstone, president of the Society for Reproductive Endocrinology and Infertility and an associate professor at the University of Utah, tells Fortune. But, she adds, “It’s not actually clear if that’s true.”

For starters, infertility is officially defined, for public health data collection, as the failure to achieve pregnancy after 12 months of regular unprotected sexual intercourse—and not everyone trying to have a baby is doing it the old-fashioned way. The Centers for Disease Control and Prevention, meanwhile, tracks infertility by looking at the number of married women between 15 and 49 who report having unprotected sex for at least a year and don’t get pregnant—and, of course, not everyone trying to get pregnant is married.

And many people are simply choosing to have less children, or no children at all.

But to the best of the U.S.’s ability to calculate them, it appears that rates of infertility have plateaued in recent years, according to a 2022 analysis, with fluctuations that have ranged from 5.8% and 8.1% since 2006.

Those numbers have concerned Health and Human Services nominee Robert F. Kennedy, Jr., for one. He has called the infertility rate “alarming” and pledged to look into the role of chemicals and nutritional habits when it comes to both women’s infertility and what he calls “declining” sperm counts (though there is no formal agreement on whether rates are dropping or not).

So what do we know about what’s driving infertility cases—about 15% of which are “unexplained”—for men and women? Below, Johnstone walks us through the science.

Maternal ages are rising

“The No. 1 factor in chances of getting pregnant is female age,” says Johnstone, “and so the fact that many people are starting to try to get pregnant at later ages than they used to is the No. 1 most well-established, definite factor when it comes to having more difficulty, needing more time to get pregnant, or just not getting pregnant.”

While the effect of age is not major when comparing, say, somebody from their late 20s to early 30s—or even in one’s late 30s, when there is a slight decline, it’s trying to conceive at age 40 or older when it becomes much more difficult, the doctor explains.

“And the proportion of women in their 40s who are trying to conceive is going up,” she says—while also pointing out that, despite it being more difficult, “many of the people who start trying to conceive at 35 or 38 or 40 will be successful.”

Obesity plays a big role

Obesity is another clear factor when it comes to infertility—for both men and women, says Johnstone, who notes that it’s associated with decreased sperm counts as well as irregular ovulation.

“But even for women with obesity who are ovulating regularly, they are still probably about 15 or 20% more likely to be infertile,” she says. “And so we think obesity just affects the endocrine system in multiple ways.”

A 2021 committee opinion on obesity and fertility published by the American Society for Reproductive Medicine, in fact, noted, “Obesity has adverse effects on reproduction, including on ovulatory and menstrual function, natural fertility and fecundity rates, infertility treatment success rates, infertility treatment safety, and obstetric outcomes.”

It is also associated with polycystic ovary syndrome, or PCOS, which causes irregular ovulation, the opinion notes; obesity also leads to higher rates of miscarriage.

And obesity rates have been increasing worldwide, including in the U.S., according to the CDC, with the most recent data (from 2023), showing that, in 23 states, more than one in three adults (35%) had obesity—while, before 2013, no state had an adult obesity prevalence at or above 35%.

The legalization of marijuana has led to more users—and possibly less sperm

Particularly for male infertility, says Johnstone, “marijuana is definitely a concern.” That’s because it can decrease sperm counts and also affect the DNA-quality and integrity in sperm.

“That is something that I do think, with increasing legalization of marijuana, is a concern,” she says.

A 2021 study, as well as earlier ones, found that cannabis use decreased the volume of semen and sperm count, and altered the sperm’s shape, for example. But a 2019 study contradicted that, surprising researchers by finding that men who had smoked marijuana at some point in their life actually had significantly higher concentrations of sperm than those who had never used cannabis. So more research is needed.

“There are some concerns in women as well,” Johnstone adds, “but it’s just less well-studied.”

The jury is also still out on environmental toxins

When it comes to our exposure to various chemicals—”glyphosate, BPA, heavy metals, xenoestrogens, endocrine-disrupting chemicals, and so on,” according to RFK Jr.’s list of infertility worries, the science is not always super clear.

“I would say there are concerns about them, but not clear proof that they are causing infertility on a large scale,” says Johnstone.

A 2023 study of 1,032 women did find that exposure to PFAS—synthetic compounds known as “forever chemicals,” found everywhere from nonstick pans to drinking water—may reduce fertility for women by up to 40%.

Further studies have implicated various other culprits for women, including exposure to endocrine disruptors and pesticides, while research has also connected air pollution, chemicals, and pesticides to male infertility through their effect on sperm quality. But more research is needed.

“I think it does mean, for an individual who wants to conceive, that trying to limit their exposure to things like BPA [a forever chemical] is a wise choice,” Johnstone says. “But there isn’t strong evidence that it is causing individual people to become infertile, or that if you’ve been exposed in the past but stop that exposure, that stopping that exposure improves fertility.”