Besides smoking and drinking alcohol, parents' health including obesity and poor diet can have "profound implications" for the growth, development and long-term health of their children before their conception, says a series of studies published in the journal Lancet.
The findings showed that smoking, high alcohol and caffeine intake, diet, obesity and malnutrition in either or both parents, potentially increases a child's lifelong risk of heart attacks, stroke, diabetes, immune and neurological diseases.
The research emphasises the need for greater awareness of preconception health and improved guidance, with greater focus on diet and nutrition to improve the health of future generations.
"Research is now showing that our gametes and early embryos are sensitive to a variety of environmental conditions including poor parental diet. These effects can change the process of development, affecting growth, metabolism and health of offspring, so makes the case for both parents to have a healthy lifestyle well before conception and pregnancy." said Tom Fleming, professor at the University of Southampton.
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 poor sperm quality, quantity and motility associated with many of the same conditions.
"The preconception period is a critical time when parental health -- including weight, metabolism and diet -- can influence the risk of future chronic disease in children, and we must now re-examine public health policy to help reduce this risk," said Judith Stephenson, professor from the University College of London.
"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," Stephenson added.
The results were based in part on two new analyses of women of reproductive age - 18 to 42 - in the UK and Australia.
The team also found that women are often not "nutritionally prepared" for pregnancy. Some 96 per cent of the women, for example, had iron and folate intakes below the recommended levels, 14.8 milligrams and 400 micrograms per day, respectively.
Adjusting diet after a pregnancy has begun is often not good enough to fundamentally improve child health, the researchers said.
They propose that behaviour change interventions, supplementation and fortification starting in adolescence, by schools could help young adults prepare for healthy parenthood in the future.