Researchers at the Coombe Women and Infants University Hospital have urged dads to take better care of themselves, with improved diets, smoking habits and exercise the main targets.
Yet the appeal wasn't just a personal worry for dads, given obese men are two to four times more likely to have obese children.
In a study on 167 men in Ireland, half of the fathers-to-be were overweight, and one in six obese. This is bad news for families, as obese fathers are "more likely to die young, leaving children fatherless, and mothers to support the family single handedly," said Professor Michael Turner, one of the authors of the study.
The study, published in the Australian and New Zealand Journal of Obstetrics and Gynaecology, highlighted the risks of the 'beer belly', which is full of the kind of fat that puts people at greatest risk of diabetes and heart disease.
"Fathers have a responsibility to their children as well as themselves to promote a healthy lifestyle for all the family," said Professor Turner.
Smoking is also a huge problem, given 40 per cent of the dads surveyed are smokers. This increases the risk of cancer and heart disease, and exposes their children to the risks of passive smoking, the researchers warn.
"Men need motivation to start looking after their health. While women are more likely to adopt a healthy life style if they are trying for a baby, the same is not the case for men," said Professor Turner.
He argues that health policy needs to encourage dads, as well as mums, to take care of themselves. "If there is going to be a successful public health intervention against obesity in Ireland it must embrace the whole family, including the parental unit".