Hundreds of men are offering up intimate details of their lives — from sex to food to their underwear preferences — as part of a major study on fertility led by researchers at Boston University.

It's called PRESTO, for Pregnancy Study Online, and it's the largest preconception study in North America. When it comes to male participation, it's the largest study of its kind in the world: Nearly 3,500 women and 945 men from the U.S. and Canada have enrolled in the web-based study, and all of the participants are just starting to try to conceive. As part of the study, they're followed for a year, or until they get pregnant.

Researchers are exploring what types of lifestyle and environmental factors might be contributing to a decline in fertility rates nationwide ("fertility" is defined as total births per 1,000 women aged 15–44 in a year) for the past few decades — and what factors might boost couples' chances to conceive.

What's driving the fertility rate is a complicated question: it involves behavioral, economic and biological factors; for instance, the drop in teen pregnancies is one contributor to the decline, but not the whole story. The PRESTO study is trying to tease out the factors around fertility that people can actually modify. It asks hundreds of questions about couples' lives, including their diet (any carrot juice in the past year?), exercise routines, prescription drug use, sex (how many times a week? do you use lubricants?) and work habits (do you use your laptop — on your lap?).

Delaying Childbirth, And More

Infertility, defined as the inability to conceive after 12 months of unprotected intercourse, affects about 10 to 15 percent of U.S. couples.

Massachusetts has the third-lowest fertility rate in the country, and the Bay State also has one of the highest rates of assisted reproductive technology use in the U.S.

Delaying childbirth is clearly a major factor, as women — many highly educated and pursuing careers — wait until they are older to try to get pregnant, lowering the chances of conceiving naturally.

Identifying factors that can improve couples' chances of conceiving naturally is an important public health goal, says Lauren Wise, a professor of epidemiology at the Boston University School of Public Health, and PRESTO's lead researcher.

Also, Wise says, most fertility studies focus on women, but it's important to study both members of a couple. After all, she says, "50 percent of infertility is attributable to the male partner."

To recruit young couples just starting to try to have a family, Wise and her colleagues are reaching out on social media, notably Facebook, and on health-related websites as well as sending email blasts through lifestyle sites, such as The Bump. Wise said that traditional recruitment methods, such as flyers in clinics and physicians' offices, cost more and don't work as well for these types of "hidden populations."

"On Facebook, we are able to target by age, gender, geographic region and relationship status," Wise said.

Men And Caffeinated Drinks

One recent PRESTO-related study, published in April found that caffeine consumption — including energy drinks, sodas and coffee — may impact male fertility.

Among the 662 men in the study, published in the journal Reproductive Toxicology, those who consumed one or more energy drink per day (16 men total) had reduced fertility compared with men who skipped energy drinks altogether. Specifically, researchers report: The one-or-more-a-day energy drink consumers had a 54 percent lower chance of conception in any given cycle compared with non-consumers, after controlling for lifestyle factors.

In addition, men who drank two or more caffeinated sodas per day (39 men) had what researchers describe as a "modest" reduction in fertility compared with non-consumers. (They had a 28 percent lower average per-cycle probability of conception compared to the non-soda drinkers.) The paper acknowledges that the actual number of high-caffeine consumers was low, and urges caution interpreting the data.

That Laptop On Your Lap?

"We’ve generated pilot data showing that short sleep duration, high levels of stress and depression, and factors that increase heat exposure to the scrotal area, such as laptop use in one’s lap, might be harmful to male fertility," Wise said. Keep in mind that this is preliminary data only, based on a sub-study analyzing semen samples from 37 men, so larger numbers will be needed to confirm the findings.

The researchers are also investigating a broad range of factors that may impact fertility in women.

It's already been established that obesity affects the ability to conceive, and women with extremes of BMI — both underweight and overweight — have reduced fertility.

A PRESTO paper published earlier this year confirmed the association between obesity and lower fertility, but also found that where the fat is stored matters: women who were more apple-shaped, that is, with a higher waist-to-hip ratio, had lower fertility than pear-shaped women. That study, which included data from 2,062 women, also found vigorous physical exercise was associated with improved fertility among overweight and obese women only -- not lean women. Moderate physical activity was associated with higher fertility among women of all body sizes.

Severe Depression Associated With Sub-Fertility

Another paper, published in April in the American Journal of Obstetrics and Gynecology, found that severe depressive symptoms were associated with lower fertility. In addition, the study of 2,146 women found no association between the use of antidepressant medication and fertility.

Wise and her colleagues are currently analyzing how trans-fat intake, dairy consumption and lubricant use might be linked to fertility in women. And they're also investigating male cell phone use, and whether it matters where you carry your cell phone, for instance, in your front or back pocket.

Novel Design

Enrique Schisterman, a senior investigator and chief of the epidemiology branch of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, said the PRESTO study is quite novel in design: It's the first internet-based preconception study that recruits both men and women in North America.

"The problem with preconception studies," he said, "is it's very hard to recruit couples who have just recently discontinued contraception use and are actively trying to get pregnant. You have to catch them when they are trying, but before they're already pregnant — that's very difficult."

As for results, Schisterman said the study is just beginning to generate important data, especially in men, but ultimately, there's potential for findings that could change people's lives. "I think that the PRESTO study is still in its preliminary stage," he said, with much of the published data showing "things that we had already suspected but without strong evidence."

Still, he added: "It's important to have empirical support in a study such as PRESTO with large numbers, and it's also important to verify that the internet-based recruitment approach is successful. It is evident from [the numbers of people signed up] that it is in fact successful."

The hope, he said, is that the study "will be able to identify behaviors that are modifiable, that will have a population effect [and] that will help couples to get pregnant naturally, or identify couples who may need to get treatment."

'Improve Our Chances'

Some PRESTO study participants are already modifying their behavior.

Nicholas, a 37-year-old math professor in Framingham, had a bit of an "Aha" moment after answering some of the study questions.

One, for instance, asked how often he worked on a laptop — on his lap. That made him realize that sometimes his laptop gets really hot, which probably isn't great, so, he says, he bought a laptop cooling table for protection.

Another question was about caffeine intake. Again, the question alone was enough to prompt a change.

"I took it as a hint that I should cut down. I drank sodas — about three or four a day — now I'm down to one can," he said.

Nicholas and his 35-year-old wife have been trying to get pregnant since 2014. They've undergone various tests but no clear cause has emerged regarding why they haven't been able to conceive. In fact, they already have a 6-year-old child.

But Nicholas said he and his wife are thinking more about their lifestyle and habits, due to participation in the research. "It just sort of prompted us to think about what we could do to improve our chances."