(CNN) Many moms-to-be know that their health even before they become pregnant -- known as pre-conception health -- can affect the health of their babies.
Now, research is continuing to show that the pre-conception health of fathers also can influence a pregnancy and the baby.
Three papers published Monday in the journal The Lancet detail how the health of both women and men, before they even conceive a child, can have profound impacts on the health of their offspring -- such as birth weight and brain development.
"This is a really important series, and it is important because it helps further re-establish the importance of pre-conception care as a legitimate direction for improving birth outcomes and improving health in children, both at the time of birth but also over their life course," said Milton Kotelchuck, professor of pediatrics at Harvard Medical School and a senior scientist at Massachusetts General Hospital, who was not involved in the new papers.
"Really, almost all of the important epigenetics, all the important embryologic development, takes place in the first few weeks of the pregnancy. That's when lots of the really key things are happening before people even know they're pregnant," he said. "Your brain development, your entire spine, all the nerves are developed in the first couple of weeks."
The first of the three papers turns a spotlight on when the pre-conception period begins and ends and how it's a time when many parents might not even realize that their health can influence their baby's.
"Certainly, the last three to six months before you attempt to conceive really need to be focused on improving or making sure you maintain quality health," said Dr. Haywood Brown, professor of obstetrics and gynecology at the Duke University School of Medicine and president of the American College of Obstetricians and Gynecologists, who was not involved in the new papers.
The first paper calls for the pre-conception period to be redefined according to three perspectives: biological, individual and public health.
Based on the biological perspective, the pre-conception period would be defined as the days to weeks before an embryo develops, but from the individual perspective, the pre-conception period would begin as soon as a couple has a conscious intention to conceive, typically weeks to months before pregnancy occurs, according to the paper.
Based on the public perspective, the pre-conception period would encompass the months or years it takes to address pre-conception risk factors related to diet, lifestyle and chronic diseases such as obesity or diabetes, according to the paper.
For example, "men who are obese have a higher chance of having decreased sperm count," Brown said.
In women, "obesity is a risk factor for congenital birth defects. It's associated with congenital heart defects. ... Obesity also increases risk for things like developing preeclampsia and developing diabetes, and so in that sense, they affect the health of the baby," he said. "We also know that people who are extremely underweight also have a higher risk of babies being small, and so there's an extreme on both ends."
The paper, which reviews separate previous studies on nutrition and obesity among women of reproductive age, notes that substantial risks for maternal and child health are associated with mothers who are underweight or overweight.
A separate previous analysis of the body mass indexes among more than 19 million adults across 200 countries found that the age-standardized global proportion of underweight women decreased from 15% to 10% between 1975 and 2014. South Asia had the highest proportion of underweight women, with an estimated 24% in 2014.
Although the proportion of women who are underweight decreased, the proportion of obese women globally climbed from 6% to 15% between 1975 and 2014. In the new paper, the researchers also noted that in many low-, middle- and high-income countries, up to 50% of women are overweight or obese when they become pregnant.
On the other hand, the second paper in the new series pointed out that a father's diet and weight also can impact his offspring, but more research is needed to better understand exactly how that happens.
Kotelchuck called this the "new science" of pre-conception care.
"There's more to pre-conception health for men -- the impact of men's health on subsequent pregnancies -- than the quality of their sperm," Kotelchuck said.
"Does he help the woman or harm her in getting prenatal care? Does he give support or not support? ... Does he have sexually transmitted diseases?" he said. "The father's genes also are very important in the development of the placenta and whether the placenta's nurtured well enough."
At least, that's what has been found in animal studies.
A study of mules, which have horse mothers and donkey fathers, and hinnies, with horse fathers and donkey mothers, found that genes from the animals' fathers were predominant in the placenta, the temporary organ that joins the mother and fetus during pregnancy. That research was published in the Proceedings of the National Academy of Sciences in 2013.
So during the pre-conception period and pregnancy, the health of both parents can influence an offspring, and the second paper in the new series spotlights how those influences can involve a baby's long-term risks of developing cardiovascular, metabolic, immune and neurological problems.
For the paper, researchers analyzed the pre-conception period and disease risk across four areas: relating to a mother's overweight and obesity, a mother's underweight, paternal factors and the use of assisted reproductive treatment. They reviewed studies in both human and animal models.
The researchers wrote in the paper that there was sufficient evidence that the pre-conception period "is a key window during which poor maternal and paternal physiology, body composition, metabolism, and diet can induce increased risk of chronic disease in offspring -- a lifetime legacy and major driver of health burden in the 21st century. The evidence that similar consequences can result from assisted reproductive treatment practices sharpens the focus on this window."
For instance, the researchers noted that children born as a result of assisted reproductive treatment, compared with their peers, have an increased risk of rare genetic imprinting disorders, such as Angelman syndrome, which primarily affects the nervous system.
The third paper in the series pinpoints strategies to improve approaches to pre-conception care in order to improve the pre-conception health of parents-to-be.
When thinking about pre-conception care, visiting an indoor skydiving school came to mind for Dr. Aimee Eyvazzadeh, a San Francisco-based reproductive endocrinologist who was not involved in the new papers.
"Before it was my turn in the fly chamber, everyone had to take a class in the jump school. A lot of people weren't sure if they wanted to fly. Some people only took the class to see if it was for them and what they should do if they ended up taking a turn in the chamber. If you think about it, having a baby isn't that different," she said. "But where is our version of jump school?"
Although pregnancy has been separated into three trimesters and some people refer to the period after having a baby as the fourth trimester, Eyvazzadeh described the pre-conception period as trimester zero.
"Shouldn't we be talking about trimester zero? What's the fertility's equivalent to the class I took before indoor skydiving? This is the incredibly important time where a doctor can make a difference and help educate a hopeful pregnant person before they jump feet first into something they may know little or nothing about," Eyvazzadeh said, adding that the new papers demonstrate that decisions made during trimester zero can impact a baby.
"Rather than try to get pregnant with zero guidance, all people should get a pre-conception checkup and be taught about what they can do to promote lifelong health in their future offspring, and it certainly starts with the parents," she said.
The third paper pinpointed strategies, such as supplementing the diets of undernourished women or providing micronutrient-rich snacks to women before conception and during pregnancy; offering cash incentives to improve pre-conception health; and addressing unhealthy behaviors, such as reducing alcohol consumption and smoking.
Yet Kotelchuck said there should be an emphasis on developing interventions to help improve the pre-conception health of everyone, including fathers.
While conducting research that is underway and yet to be published, Kotelchuck said, he and his colleagues found that 40% of men at a major hospital in Boston reported that they were not asked questions while accompanying their partners to prenatal care visits.
One study published last year in the journal JAMA Psychiatry found that fathers-to-be can be at risk of depression symptoms if they feel stressed or if they are in poor health while a baby is on the way.
"There's very little talking to men. There's almost no brochures that speak to men and men's interests," he said. "If you do pre-conception care for women because you're interested in the health of not only the baby but also the woman over her life course ... you have to talk to dads in the same way."
Overall, the new papers brought together the viewpoints of those in the medical community who have been arguing for years that pre-conception care is needed before prenatal care, Kotelchuck said.