Sperm counts and teen pregnancy rates.

The state of the universe.
May 3 2006 10:20 AM

What's Really Behind the Plunge in Teen Pregnancy?

It's time to look at boys' contributions.

May 3—in case you didn't know it—was "National Day To Prevent Teen Pregnancy." In the past decade, possibly no social program has been as dramatically effective as the effort to reduce teen pregnancy, and no results so uniformly celebrated. Between 1990 and 2000 the U.S. teen pregnancy rate plummeted by 28 percent, dropping from 117 to 84 pregnancies per 1,000 women aged 15-19. Births to teenagers are also down, as are teen abortion rates. It's an achievement so profound and so heartening that left and right are eager to take credit for it, and both can probably do so. Child-health advocates generally acknowledge that liberal sex education and conservative abstinence initiatives are both to thank for the fact that fewer teenagers are ending up in school bathroom stalls sobbing over the results of a home pregnancy test.

What, though, if the drop in teen pregnancy isn't a good thing, or not entirely? What if there's a third explanation, one that has nothing to do with just-say-no campaigns or safe-sex educational posters? What if teenagers are less fertile than they used to be?

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Not the girls—the boys?

It's a conversation that's taking place among a different and somewhat less vocal interest group: scientists who study human and animal reproduction. Like many scientific inquiries, this one is hotly contested and not likely to be resolved anytime soon. Still, the fact that it's going on provides a useful reminder that not every social trend is the sole result of partisan policy initiatives and think-tank-generated outreach efforts. It reminds us that a drop in something as profound as fertility, in human creatures of any age, might also have something to do with health, perhaps even the future of the species.

The great sperm-count debate began in 1992, when a group of Danish scientist published a study suggesting that sperm counts declined globally by about 1 percent a year between 1938 and 1990. This study postulated that "environmental influences," particularly widely used chemical compounds with an impact like that of the female hormone estrogen, might be contributing to a drop in fertility among males. If true, this was obviously an alarming development, particularly given that human sperm counts are already strikingly low compared to almost any other species. "Humans have the worst sperm except for gorillas and ganders of any animal on the planet," points out Sherman Silber, a high-profile urologist who attributes this in part to short-term female monogamy. Since one man's sperm rarely has to race that of another man to the finish, things like speed and volume are less important in human sperm than in other animals, permitting a certain amount of atrophy among humans.

The Danish study set an argument in motion. Other studies were published showing that sperm counts were staying the same; still others showed them going up. In the late 1990s, however, an American reproductive epidemiologist named Shanna Swan published work confirming the Danish findings. In a well-respected study published in Environmental Health Perspectives, Swan, now at the University of Rochester Medical Center, found that sperm counts are dropping by about 1.5 percent a year in the United States and 3 percent in Europe and Australia, though they do not appear to be falling in the less-developed world. This may not sound like a lot, but cumulatively—like compound interest—a drop of 1 percent has a big effect. Swan showed, further, that in the United States there appears to be a regional variation in sperm counts: They tend to be lower in rural sectors and higher in cities, suggesting the possible impact of chemicals (such as pesticides) particular to one locality.

Swan is part of a group of scientists whose work suggests that environmental changes are indeed having a reproductive impact. Under the auspices of a women's health group at Stanford University and an alliance called the Collaborative on Health and the Environment, some of these scientists met in February 2005 at a retreat in Menlo Park, Calif., to discuss their findings. Among the evidence presented are several trends that seem to point to a subtle feminization of male babies: a worldwide rise in hypospadias, a birth defect in which the urethral opening is located on the shaft of the penis rather than at the tip; a rise in cryptorchidism, or undescended testicles; and experiments Swan has done showing that in male babies with high exposure to compounds called phthalates, something called the anogenital distance is decreasing. If you measure the distance from a baby's anus to the genitals, the distance in these males is shorter, more like that of … girls.

Wildlife biologists also talked about the fact that alligators living in one contaminated Florida lake were found to have small phalli and low testosterone levels, while females in the same lake had problems associated with abnormally high levels of estrogen. In 1980 the alligators' mothers had been exposed to a major pesticide dump, which, some believe, was working like an estrogen on their young, disrupting their natural hormones. A report later published by this group pointed out that similar disruptions have been found in a "wide range of species from seagulls to polar bears, seals to salmon, mollusks to frogs." As evidence that a parent's exposure to toxicants can powerfully affect the development of offspring, the example of DES, or diethylstilbestrol, was also, of course, offered. Widely given to pregnant women beginning in the late 1930s under the mistaken assumption that it would prevent miscarriage, DES left the women unaffected but profoundly affected their female fetuses, some of whom would die of cancer, others of whom would find their reproductive capacity compromised. The consensus was that the so-called chemical revolution may well be disrupting the development of reproductive organs in young males, among others. This research is controversial, certainly, but accepted enough, as a hypothesis, that it appears in developmental-biology textbooks.

Tellingly, the U.S. government is also taking this conversation seriously. Together, the National Institutes of Health and the U.S. Centers for Disease Control are sponsoring a longitudinal effort to study the effect of environment on fertility. This study will track couples living in Texas and Michigan, following their efforts to become pregnant. The aim is to determine whether toxicants are affecting the reproductive potential of female and male alike.

It will be welcome information. In the United States, good statistics about infertility are strikingly hard to come by. There is no government-sponsored effort to track male fertility rates, even though male-factor problems account for half of all infertility. Even among women, who are regularly interrogated about reproductive details, it's difficult to get a good handle on developments. For years, government researchers included only married women in the category of "infertility," creating a real problem for demographers and epidemiologists looking for trends. The National Center for Health Statistics created a second category called "impaired fecundity," which includes any woman, of any marital category, who is trying to get pregnant and not having luck.

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