According to a UK obesity expert when women move in with their man they very often put on weight as a result.
Men says Haslam are really very bad for women.
He says eating with a partner becomes a social event, where women often eat larger portions and indulge in richer foods and wines and more extravagant food.
When those factors are combined with taking the contraceptive pill or having babies, a woman's waistline can severely suffer, says Haslam.
Dr Haslam says research shows women tend to gain weight once they cohabit and begin to share meals with men who intrinsically have higher energy needs and therefore appetites.
The weekly shopping list may change from the basic fruit and veg to include indulgent treats, and couples may go out to restaurants for meals more often.
Women also may do less exercise when they are in a relationship, skipping a trip to the gym to spend quality time with their partner.
Apparently the contraceptive they chose to use can also have an impact.
The contraceptive pill, which is taken by 3.5 million British women, has been associated with a slight weight gain among some users, on average 4lb in the first three months.
Repeated pregnancies can also lead to weight gain.
Changes in society's attitude also means women may be less physically active than their great-grandmothers were, doing less housework thanks to technology, and getting partners to share the load or paying others to do it for them.
In the past studies have shown that married couples gain weight while those who divorce lose weight.
Dr Haslam does fortunately stress that it is important not to generalise and that being in a relationship has numerous positive influences and gains too.
He says however it is important for individuals to look at their own lifestyles and make healthy changes if they need to, as obesity has been shown to decrease life expectancy by seven years at the age of 40.
The study is published in The Lancet medical journal.