What a mother and father eat, and whether they are seriously overweight, in other words, can have "profound implications for the growth, development and long-term health of their children before conception," they warned in a trio of studies.
The findings, reported in The Lancet, a leading medical journal, should heighten awareness of "preconception risk factors," the researchers said.
"Evidence for preconceptional effect on lifetime health is now so compelling that it calls for new guidance on parental preparation for pregnancy, beginning before conception," they concluded.
The studies -- combining a review of earlier literature and new research -- showed that the lifestyle habits of fathers, not just mothers, can have a direct impact on the wellbeing of offspring.
"The preconception period is a critical time when parental health -- including weight, metabolism and diet -- can influence the risk of future chronic diseases in children," said Judith Stephenson, a professor and University College London and lead author of the series.
"While the current focus on risk factors such as smoking and excess alcohol intake is important, we also need new drives to prepare nutritionally for pregnancy in both parents."
Obesity in either or both parents, for example, increases the chances of heart attacks, stroke, immune disease and diabetes in offspring.
Maternal obesity is thought to enhance levels of inflammation and hormones, which can directly alter the development of the egg and embryo.
This, in turn, boosts the odds of chronic disease later in life.
In men, being obese leads to deficiencies in sperm associated with many of the same conditions.