According to the psychologist Gill Cappuccini even the most "right-on" couples are likely to suspend their ideals when they become parents.
Often the man's job will assume more importance than the woman's while the wife does more of the housework and child care.
Ms Cappuccini and her colleague Ray Cochrane, from Birmingham University, studied 100 married couples who were expecting their first babies.
About two-thirds were living together in an equal partnership, both having jobs and sharing domestic chores like housework, shopping, cooking meals and washing-up.
But in almost all cases this arrangement did not last as the wife stayed at home looking after the new baby while her husband went to work.
Ms Cappuccini, who presented her findings at a British Psychological Society conference at Strathclyde University yesterday, said: "In nearly all cases the man's career becomes more important after the birth.
"Sometimes promotion is more sought after because of the extra financial responsibilities, or the woman finds herself stepping back from her career a little bit. Soon this begins to affect everything else and life is never quite the same again.
"Some of these people said what an unexpected shock it was when the baby arrived in the house. They try to make sense of a very new situation and find themselves adopting the old traditional roles."
In the majority of cases people were quite happy with the new arrangement and marital satisfaction remained high.
Some couples who had strict demarcation lines in the home - the husband doing all the cooking, for example - managed to carry on in much the same way after the birth, said Ms Cappuccini. But this was not the case for couples who generally shared domestic tasks.Reuse content