Women who cohabit may put on weight as a consequence

Eating man-sized portions and indulging in richer foods and wines can lead to weight gain, a review in The Lancet medical journal says.

When you combine that with going on the contraceptive pill or having babies, a woman's waistline can severely suffer, says author Dr David Haslam.

"Men are very bad for women really," he said.

If you are eating with a partner the evening meal is a social event...you may eat more and maybe more extravagant stuff

Dr Haslam

He said research showed women tend to gain weight once they cohabit and begin to share meals with men who intrinsically have higher energy needs and therefore appetites.

"If you are eating with a partner the evening meal is a social event and its no longer just filling a gap. You may eat more and maybe more extravagant stuff."

The weekly shopping list may change from the basic fruit and veg to include indulgent treats.

Couples may go out to restaurants for meals more often.

Women may do less exercise when they are in a relationship, skipping a trip to the gym to spend quality time with their partner.

Multiple insults

The contraceptive they chose to use can also have an impact.

The Pill, 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.

"Each individual factor for weight gain is serious enough but put them all together and you can get a significant increase," said Dr Haslam.

Societal changes mean women may be less physically active than their great-grandmothers were, doing less housework thanks to time-saving gadgets, getting partners to share the load or paying others to do it for them.

Some studies have shown that married couples gain weight while those who divorce lose weight.

Dr Haslam stressed that it was important not to generalise and that being in a relationship had numerous positive influences and gains too.

He said the important thing was for individuals to look at their own lifestyles and make healthy changes if they needed to.

Obesity has been shown to decrease life expectancy by seven years at the age of 40.