Thirty-four per cent of couples found their relationship went into decline after the birth of their first child. Thirty-five per cent of British women and 32 per cent of men felt the level of conflict in their relationships increased, with just 9 per cent reporting less.
The survey of 4,055 people, of whom 563 were parents with children under five, was carried out to launch a book on the effect of children on relationships, The Transition to Parenthood: How a First Child Changes A Marriage, by Dr Jay Belsky.
Briony Hallam, spokeswoman for the Meet A Mum Association, said: 'Having a baby puts immense pressure on a relationship and they are not prepared for the change. It cuts both ways. Women are prepared for the delivery - but less time is spent preparing them for after the baby is born.
'Men are under pressure too. They have no role models - very often their fathers will have taken no part in child- rearing. They are totally unprepared for the changes in their wives and in the economic situation.
'It's make or break time. If the marriage survives the first child they have a good foundation for the future.'
Women, especially, feel let down after the birth with 28 per cent saying their partners fell short of their expectations for child care and being understanding. However, 38 per cent said their partners had fulfilled their expectations.
Ms Hallam said: 'Communication is the most important thing. They must keep talking. Very often the woman becomes resentful and nit-picks and he keeps away from home just for a rest - the end result is that they stop talking.'
Donna Glean, 32, has a 13- month-old son, Thomas. Her unplanned pregnancy put a great strain on her relationship with her partner, Richard Gordon, 28. 'It's a very big step because you're thinking of mortgages, bills and things like that. I don't think that we have much time together now. We don't make love as much as we'd like either. We're not as physically close - but mentally we are just as close. But we still have time for a kiss and a cuddle from time to time]'
Leading article, page 13