Whoever observed that the price of democracy is eternal vigilance must have been a marriage counsellor. Maintaining a genuinely mutual relationship is a lot like pole vaulting, or brain surgery. When other people do it, it looks so easy. But try doing the same trick at home - especially after you have had a baby. Just when you thought the dangers of gender-typing were dead and buried, they rise up like unquiet spirits from the postpartum wreckage. Suddenly, she is "Mum" and he is "Dad", and - struggle and resist though they might - there isn't a hell of a lot they will be able to do about it.

Bringing substantive industrial reform to that most hidebound of all workplace settings - the marital home - is never going to be easy. Research consistently shows that the vast majority of men and women are committed to equality in marriage.It also shows that most are astonishingly ineffectual at achieving it.

Yet more and more young couples start out married life committed to equality in everything from decision-making clout to the domestic division of labour. That is hardly surprising. Research shows that today's couples increasingly mirror one another's educational attainments and earning power. They share a sense of entitlement to the good things in life - from sex and fitness to travel and shopping. They also share the expectation that the 50/50 partnership they take for granted will last until death - or more likely divorce - do them part.

And then they have a baby.

Having a first child, Nora Ephron once remarked, is like throwing a hand grenade into a marriage. If that image seems harsh and exaggerated, you are obviously not a parent. Or a sociologist. Like it or not, the evidence supporting Ephron's observation is as overwhelming as a second-stage contraction, and about as pleasant.

In a nutshell, the research tells us that parenthood exaggerates and hardens gender differences within marriage, pushing husbands to become more "husbandly" and wives more "wifely" - and then leaving them to get on with it.

The birth of a first child, however wanted or loved, sets in train a predictable cascade of marital events. Almost all of them are negative. First, household division of labour becomes "more traditional" (ie women go from doing most of it to doing almost all of it). Second, conflict increases - not just among some couples but in a mind-blowing 97% of marriages, according to a recent large-scale study. Third, marital satisfaction and frequency of sex decline. Precipitously. Fourth, so do "positive interchange" between partners and even "feelings of love for the spouse".

The good news (if that's what you want to call it) is that despite all this, marital stability actually increases. One study found that the divorce rate among families with a pre-school child is half that of childless couples. When partners become parents, it seems, they like each other less and less, but stay together more and more.

The resulting fallout - in the form of tension, frustration and barely restrained rage - is compounded by confusion. For an increasing number of men and women, the reversion to traditional gender roles after the birth of a first child is not only unwelcome, it is downright mysterious. One day a woman is conducting herself like an emancipated, autonomous adult, and the next she has metamorphosed into "Mummy": a sub-human pod person who cares about the whiteness of her laundry and gets off on discussing the ins and outs of breast pumping. Is it any wonder new mothers find their friends without children are, in the words of one recent study, "especially likely to disappear" from their lives?

"It's not just that couples are startled by how the division of labour falls along gender lines," observe US researchers Carolyn and Philip Cowan. "But they describe the change as if it were a mysterious virus they picked up when they were in the hospital having the baby."

The shock of encountering the ghosts of gender roles past is particularly acute for couples who were sure it would never happen to them. Hardest hit of all are middle-class couples in their thirties and forties in which both partners are professionally employed. The upside for such couples during the transition to parenthood is that they have a better chance of maintaining personal wellbeing. The penalty is that their marital satisfaction will take the worst bludgeoning of all. "The birth of a child entirely revamps the internal landscape of marriage," writes leading family psychologist Judith Wallerstein.

But there's a secret even darker and more depressing than this one. The birth of a child will affect a woman's internal landscape like an earthquake, followed by a flood, followed by a volcanic eruption. For a man, it will be more along the lines of a heavy shower.

A 20-year study of stable marriages in a culturally diverse sample found that three-quarters of fathers reported being satisfied with their marriages during the years of childrearing, compared with only half of mothers. It is not hard to see why.

Motherhood will introduce momentous, and often unanticipated, changes to a woman's core identity. It also brings a shitload of extra work. New mothers, who will typically have performed two to six times as much housework as their male partners even before the birth, will find their unpaid workload increasing by anywhere from 50 to 90%.

A large proportion of this increase will be time devoted to primary childcare, both physical and "managerial". Mothers are far more likely to function as what psychologist Diane Ehrensaft calls "the psychological task-manager", their intellectual hard drives all but consumed by details regarding the baby's feeds, sleeps, toileting, clothing and minute-to-minute wellbeing. In the US, according to current research, women still do about 80% of the child care - as much as in the sixties.

Notwithstanding all the rhetoric, much of it heartfelt, about the New Dad, the time spent by fathers in primary childcare will be negligible. One large-scale study found the arrival of a first child increased the average wife's domestic load by 91%. Fathers' contributions did not increase by a single minute. Men do spend more time with their young families than they used to. Yet the lion's share of the increase is concentrated on the "fun bits" - what researchers call "skimming off the cream" of parenthood. In Australia, for example, men perform on average 16% of the labour involved in childcare - yet they claim 40% of time spent playing with their kids.

One US study found that 70% of a random sample of fathers were not responsible for any childcare tasks, and an additional 22% were responsible for only one such task. One reason fathers may pick up less than their share at home is that their hours of paid work actually increase after the birth of a first child. Researchers are unclear as to whether this reflects the breadwinner dynamic in action or simply a desire to avoid the unaccustomed stresses of life with a baby.

The fact is, the dream of shared parenting remains just that, and the reimagination of fatherhood is just another in a series of "stalled revolutions" on the exit ramp of 20th-century life. Co-parenting is not impossible, but it is surprisingly labour intensive, in part because it means making almost everything up as you go along, at a time when both partners' creative energies are at a low ebb. For wives, caving in to tradition is an obvious case of taking the path of least resistance. For both partners, it may reflect an implicit acknowledgment that having some rules, even bad ones, may be better at times of stress than no rules at all.

The collateral damage extends from a couple's social life to their sex life. US researchers Carolyn and Philip Cowan found both husbands and wives reported that the frequency of lovemaking bottomed out in the early months of parenthood, after having declined for about half in the last stages of pregnancy. Yet a woman's libido is no more likely to bounce back to its pre-pregnancy level than her belly button is.

One reason is simply sleep deprivation: a condition of new motherhood (and often new fatherhood) that can last not weeks, or months, but years. It is no exaggeration to say that the struggle for sleep consumes the lives of mothers. For fathers, the battleground often becomes the struggle for sex with those mothers.

There is something absurd about the notion that children are bad for marriages. Yet, absurd or not, it is an undeniable part of the social reality in which we are mired. What we expect of ourselves as partners, and what we require of ourselves as parents, tallies poorly, if at all. The ideology of equality notwithstanding, maternity continues to be the great leveller among women - and the inexorable tipping point for gender balance in marriage. The result is a deal in which everyone feels short-changed without ever quite understanding how they got that way.