In couples with recurrent pregnancy loss, the men also had reduced sperm motility and morphology, compared to their peers. As the semen quality goes down, the risk of miscarriages goes up because of bad sperm, which, as you’ve read, are increasingly common.

To put this in perspective, consider this: between the ages of twenty-five and thirty-five, women have a 25 to 30 percent chance of getting pregnant with well-timed, unprotected sex in any given month, a 10 percent risk of having a miscarriage, and a 1-in-900 chance of having a baby with Down syndrome; by contrast, women who are forty have a 10

In particular, the ubiquity of insidiously harmful chemicals in the modern world is threatening the reproductive development and functionality of both humans and other species.

The truth is, human reproduction is highly inefficient, especially compared to that of the majority of mammalian species. During a given menstrual cycle, people have—at best, depending on their age—an approximately 30 percent probability of conception with well-timed, unprotected sex.V

On average, women answer correctly 50 percent of questions about the causes and prevalence of infertility; medical students fare only slightly better, typically earning a D instead of an F.

the decline continues at the same rate, by 2050 many couples will need to turn to technology—such as assisted reproduction, frozen embryos, even eggs and sperm that are created from other cells in the laboratory (yes, this is actually being done)—to reproduce.

In a healthy, fertile man, the testicles produce 200 million to 300 million sperm cells per day, only about 50 percent of which become viable sperm.

In Western countries, sperm counts and men’s testosterone levels have declined dramatically over the last four decades, as my own research and that of others has found.

(Though the average cycle is twenty-eight days long, anything between twenty-one and forty-five days is considered normal, and normal periods last two to eight days.)