KAYLON BRUNER-TRAN WAS STUDYING FEMALE MICE whose mothers were exposed to toxins. She was interested in endometriosis, which meant she had no need for the male offspring. The professor of obstetrics and gynecology at Vanderbilt University Medical Center often wished there was more to learn about pregnancy from the males.

Bruner-Tran cracked open that research door. “It turns out that dads matter a great deal to the health of a pregnancy,” says Bruner-Tran, who found that the adult males exposed to toxins in utero had misshapen sperm swimming in circles. With 10% of babies in the United States born preterm, “improving the father’s health prior to conception may be the neglected piece of the puzzle to prevent pregnancy disorders,” she says.

A man’s role in pregnancy and birth outcomes is now coming into much sharper focus, in part because of digital research tools. “Understanding and quantifying the father’s contributions requires large amounts of data and computers to analyze them,” says Michael Eisenberg, director of Male Reproductive Medicine and Surgery at Stanford University School of Medicine. “The Big Data revolution has allowed us to tease out potential factors that influence birth outcomes.”

Eisenberg recently published a study that found that a father in poor health before conception correlated with higher odds of pregnancy loss, preterm birth and having a baby admitted to a neonatal intensive care unit (NICU). Another Eisenberg study showed that babies fathered by men age 45 and older were more likely to be preterm, have seizures and need intensive medical care. The mothers in those pregnancies had a higher-than-normal risk of developing gestational diabetes.

The DNA in sperm remains relatively stable throughout a man’s life, but its epigenome—which governs how those genes are expressed—alters through the years. Smoking, diet and lifestyle choices as well as stress or exposure to pollution and environmental chemicals can cause changes. “These epigenetic alterations to sperm can affect fertilization of the woman’s egg, implantation of the embryo, fetal growth and even how a child develops cognitively and neurologically,” says Carmen Messerlian, assistant professor of environmental, reproductive, perinatal and pediatric epidemiology at the Harvard T.H. Chan School of Public Health.

It turns out, for instance, that fathers contribute significantly to the development of the placenta. “A placenta that isn’t healthy will change how the pregnancy progresses, creating a higher risk of pregnancy loss and other complications,” says Eisenberg.

The few months before a couple conceives is when environmental toxins and unhealthy behaviors can do the greatest harm, says Messerlian. She is an investigator with the Environment and Reproductive Health (EARTH) study, which has followed patients at the Massachusetts General Hospital Fertility Center since it launched in 2004. Work with these patients has shown that external factors—lifestyle and exposure to chemicals—affecting both mothers and fathers contributed in roughly equal amounts to preterm births. But risk factors for the father played a bigger role in the baby’s weight at birth.

Understanding the father’s role means that the few months before conception are a time to improve the chances of a healthy pregnancy. Avoiding pesticide-laced fruits and vegetables and cutting out processed meat and trans fatty acids can improve the quality of a prospective father’s sperm. Both men and women can benefit by staying away from processed foods and avoiding environmental toxins from scented personal care products, plastics and microwaveable containers.

“We need to look at pregnancy and birth as couples-based outcomes, especially during the pre-conception window,” says Messerlian. “That is a prime opportunity to modify negative exposures.” Although the original EARTH study ended in 2019, Messerlian is leading new directions of the study, including a pilot study to evaluate whether couples who make pre-conception changes will have better pregnancy outcomes. Another project that follows the children born in the original EARTH study looks at how parents’ exposures affect their child’s neurocognitive development.

Women whose partners have diabetes, obesity or other chronic health conditions had a 19% higher chance of having a preterm baby, a 23% greater chance of having a low-birth-weight baby and a 28% higher chance of a baby being admitted to the NICU, according to Eisenberg’s study. So if those fathers, too, took steps to improve their health, it could benefit their offspring. In the study, fathers in the worst health correlated with an increased risk of complications for their partners’ pregnancies. And from China, a new study found that fathers’ alcohol consumption prior to conception increased the risk of birth defects in their babies.

Although a mother’s health has a larger impact on the pregnancy than the health of a father, dads have an important role in creating healthy pregnancies and babies, says Eisenberg. “We advise women to take prenatal vitamins and to watch their diet before getting pregnant,” he says. “We need to counsel men to optimize their health as well.”