Before they had a baby, Mel Jahnke and her husband, Mike, were “best friends,” Ms. Jahnke says. They laughed a lot, did chores together and coasted through five years of wedded bliss.

Then came a bouncing baby boy – followed by sleepless nights, endless laundry and Mike's retreat into what Ms. Jahnke calls “the man cave.”

Bickering ensued.

“The more I nagged, the less stuff got done,” Ms. Jahnke explains, adding that her husband's diagnosis with multiple sclerosis made matters worse.

What little free time they had, the couple stopped spending together.

“It was really hard for both of us,” says Ms. Jahnke, who lives in a suburb of Milwaukee, Wis.

Eventually, they got help with childcare so they could reconnect emotionally. Now that their son, Gabe, is more than two years old, the couple is happier than ever, she says.

But the Jahnkes have ruled out having a second child, which Ms. Jahnke fears could injure their marriage beyond repair. “I guess I'm not willing to take that risk,” she says.

Today's parents seem prepared to set aside all kinds of pleasures to nurture a child – from their sex lives to careers, not to mention happy-hour beers – but there's one thing even the most kinder-friendly couple is loath to give up: their marriage.

And when one child rocks the marital boat, a growing number of couples are sticking with just one to keep their union strong.

That doesn't make them wimps, though. In fact, research suggests that children are more taxing on marriages than they used to be.

In an eight-year survey of 218 couples, 90 per cent reported a decline in marital satisfaction after the birth of the first child. The recently published study noted a spike in communication problems and a crisis of faith in the marriage, especially during the adjustment period after the birth.

Some of the couples said their relationships were stronger post-birth but the vast majority reported a general deterioration in their marriages over time that was more pronounced than for childless couples.

For most parents, marital satisfaction doesn't rebound until after the last child has left home, according to Daniel Gilbert, a Harvard psychology professor who analyzed studies and made the conclusion in his 2006 book Stumbling on Happiness .

The problem may be partly a modern one, social scientists say. Couples are having children later, are more invested in their careers and don't get as much help from extended family members as they did in the past.

Given the fragile state of the modern marriage, it's no wonder only children are on the rise.

In 1980, just 10 per cent of American kids were “onlies.” Today, more than 20 per cent of children are singletons and the figure is closer to 30 per cent in cities such as New York, based on data from Rutgers University.

In Canada, one in four children is the only child living at home, according to the 2006 Census. But the figure doesn't indicate whether the child has younger siblings on the way or a brother or sister who lives in a different household.

Parents end up with just one child for all kinds of reasons, including finances and fertility problems, says Carolyn White, editor of Only Child, an online magazine launched 12 years ago. Nevertheless, marital tension is high on the list.

“We get thousands of letters from all over the world and this issue of having a child affecting the marriage is pretty common,” she says.

Some couples stop at one child as a preventative measure.

Jen Arbo of New Westminster, B.C., remembers suffering emotionally during her parents' messy divorce when she was a child. As a new parent, Ms. Arbo says, she's very protective of her decade-long relationship with her husband, Ross.

The couple argued more after the birth of their son, Kale, now 10 months old, she says. Their marriage has bounced back since then, she says, and the couple is determined to keep it that way. Having another child would add stress to their lives and “you don't want to mess with a good thing.”

Adult children of divorce may be particularly sensitive to changes in the marital climate, says Robbie Wagner, a family therapist for 30 years and chief executive officer of the Calgary Counselling Centre.

“The risky times for people are generally in the first three years after a baby is born,” Ms. Wagner says.

Most people adjust to their new role as parents, she adds, but the time available for the couple decreases with each child. “Having two is more than double one,” Ms. Wagner says.

Baby-induced marital stress can range from a brief cranky period to what some parents describe as a form of trauma.

“It was really sudden and quite destructive,” says Petra, a mother who declined to give her real name to protect the privacy of her husband and four-year-old daughter.

Both partners were even-tempered before their daughter was born, she says. But Petra began to snap after she developed Type 1 diabetes during pregnancy and then coped with a child who didn't sleep through the night for two years.

Petra's distress made her husband anxious, she says, so “I stopped talking to him about things that would worry him.”

The couple drifted apart before they went into counselling four months ago, she says. Sometimes Petra dreams of having a second child but her husband doesn't want another, she explains, and their marriage is still on shaky ground.

“I have to choose what's good for our daughter, too, and I think that having a good relationship [with my husband] is better for her.”

Other parents say they've made peace with the decision to have a one and only.

Ms. Jahnke is delighted to be a mother without the burden of caring for multiple children or the added strain on her marriage, she says. “It gives me the best of both worlds.”

More kids, less happiness

Having more kids isn't merrier, according to research from the University of Pennsylvania.

In a 2005 study of almost 35,000 adult identical twins in Denmark, sociology professor Hans-Peter Kohler concluded that mothers with one child are about 20 per cent happier than their childless siblings.

But mothers with second or third children are less happy than mothers with only one child, the study found.

A father's happiness increases after the birth of the first baby, however, additional children have little effect on the father's mood.

By studying identical twins, researchers were able to zero in on external causes for happiness and control for genetic predispositions, according to Dr. Kohler.

His advice to married couples who expect children to bring joy? Stop at one kid.