The article in the latest edition of Nature argues that tackling health problems including obesity, mental health, poor nutrition and substance abuse in young people before they become parents is essential for the best possible start to life for their future children.

Researchers from the Murdoch Children's Research Institute (MCRI) and the University of Melbourne said that taking action once a woman knows she is pregnant is often starting too late.

Young women and men often carry lifestyle and health risks from adolescence into pregnancy, they added, even if this happens in their 20s or 30s.

Lead author Professor George Patton said: "The first 1000 days of a child's life are crucially important, but that is too late to be taking action. Current policies to promote the best possible start to life in Australia along with most other countries are starting too late.

"Health and lifestyle in the months immediately before pregnancy matters for both young mothers and fathers-to-be," Professor Patton said.

"The health system now only kicks into action with a woman's first antenatal visit, most often eight to 14 weeks into a pregnancy. We need the health service system to be engaged before pregnancy -- and it should go beyond its current focus on contraception to tackle broader health risks and emotional well-being in both young women and men.

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"Today's adolescents will be the largest generation to become parents in human history. We need to invest in their physical, social and emotional development to guarantee not only their own future health but that of their children."

The paper brought together data from around 200 countries and from more than 140 recent research papers.

It considered mechanisms other than genes for how health and growth was transmitted between generations, including changes in a father's sperm or a mother's ovum, maternal influences around the time of conception and in later pregnancy, and parenting in the first two years after birth.

In high and middle income countries, the paper highlighted three main areas for action in adolescence: mental health, obesity and substance abuse.

Professor Patton said: "Maternal depression during pregnancy may affect a baby's development before birth and the mother-child bond after birth. Both depression in pregnancy and after birth are generally a continuation of pre-pregnancy mental health problems that date back to adolescence."

There is a rapid increase in obesity across adolescence and young adulthood, according to the authors. Maternal obesity during pregnancy predicts later childhood obesity, poorer cognitive skills and greater childhood behavioural problems.

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Smoking, alcohol and drug use rise steeply in adolescence, the researchers said. They found consistent and clear evidence that persisting maternal tobacco, alcohol, cannabis and other illicit drug use in pregnancy adversely affects offspring growth and development. Stopping use when a woman recognises she is pregnant may be too late to address the early effects on a baby.

"Some risks for children like parental obesity and depression need a long-term approach. At a time when obesity, mental health problems and heavy substance use have become common in young adults, prevention beginning in adolescence will be essential," Professor Patton said.

For many lower income countries, the paper recommended major actions around ending child marriage, delaying first pregnancy through contraception and girls staying in school, and tackling under-nutrition.

"We need health services to go beyond a traditional focus on reproductive health, to a more comprehensive and integrated engagement with adolescent and young adult health; and we need to create health-promoting environments in the families, schools, workplaces and communities where adolescents are growing up," Professor Patton said.

The authors also questioned the age range of adolescence. Current research suggests that physical and neurological growth continues into the 20s. The paper said this, combined with social changes such as the later adoption of adult roles, meant adolescence was better considered to range between 10 and 24.

University of Melbourne and MCRI researcher and paper author Professor Susan Sawyer said: "From this perspective, adolescence occupies a greater proportion of the life-course with greater relevance for human development than ever before. An extended adolescence creates an opportunity for this generation to acquire greater assets and capabilities and that will make a huge difference not only for themselves but for their children."