The poll, for marketing company Onepoll, also found 25% of men ate more food to make their partner feel better about her weight gain.
One fathers’ group said dads had to be aware their unhealthy behavior could influence their partners.
The average weight gain equated to men putting on around at least two inches around their waist, and a quarter were forced to buy a “paternity” wardrobe.
A fifth of fathers-to-be said they had only realised they had put on weight when their existing clothes stopped fitting.
But 19% said it was their friends who had felt they needed to make it clear they were fatter than before – via jokes about having a “bun in the oven”.
However, only a third joined their partners on a post-pregnancy diet.