PORTLAND &

Most weekdays, Oona Baker picks up her daughter, Ramona, from preschool and heads home to the Woodstock neighborhood. The two eat lunch, then decide what's next. The library? Or baking cookies? With no siblings to factor in, mother and daughter map their own schedule.

"We're pretty minimal people," Baker, 34, says. "We have a small house. We have a small car. We can walk lots of places. Our life is just easier with one child."

Not just easier, but greener.

In a city where people harvest rainwater, and bicycling to work is a badge of green pride, some Portland families say stopping at one child is global activism at its most personal.

Nationwide, the percentage of women giving birth to just one child has almost doubled, to 17.4 percent, during the past three decades, according to U.S. Census figures from 2004, the most recent data available. Some experts attribute the trend to women giving birth later in life or a generational shift in the expectations of family size.

"From my standpoint, having smaller families helps the planet," Baker says. "There's already so many people living on the Earth, that's one small thing that we can do. It was a personal and bigger-scale choice at the same time."

The environment isn't a common reason people limit their families, but that might change as people recognize the strain a bulging planet places on resources, says Carolyn White, founder of online magazine Only Child.

"Look at what's happening in the food supply," she says. "When I go to the market, I'm appalled at how much things cost. It's not just gasoline, it's everything. . . .

"One American child," she contends, "uses up more natural resources than an entire village in Africa (over) his lifetime."

Like any parenting decision, choosing how many kids to have is fraught with factors. Parents of "onlies" worry they're depriving their children of a built-in support network should illness or crises strike. They face judgmental relatives and others who assume their child will suffer socially.

Of course, having siblings doesn't guarantee close or useful relationships. And years of research shows that only children aren't harmed academically or socially by their status.

But parents who say they're having only one child because of the environment risk clashing with other families in a world where practically every parenting decision can turn competitive.

Southeast Portland mom Heather Kmetz, who has a daughter, worries that green parents might alienate others by touting their decision. In fact, she questions whether anyone really chooses to stop having kids because of the environment.

"That's very far-fetched," she says. She's not sure having one child makes a significant difference on the environment. She and her husband decided to stop with 2-year-old Emily because they both work full time and want to focus on one child.

Her family of three are vegetarians, mostly for environmental reasons. They own a Toyota Prius but bicycle where they think they can with a toddler. They grow their own herbs, pay extra for wind energy and use nontoxic cleaners.

"It's just a difficult analysis," Kmetz, a 37-year-old tax lawyer, says. "It is how you live, not just a quantity. I've seen some large families where they are environmentally responsible, more so than small families."

But for Brenna Bell and John Brush, who live on Tryon Life Community Farm, their ecological footprint amid a booming world population is a key factor.

Living in the intentional community means sharing a kitchen, a washing machine, meals and more with four children and 14 other adults.

"We are really focused on living as lightly on the land as possible," Bell says. "At the same time, we know, even lightly lived, our lives have a disproportionate impact on our resources.

"Bringing more children in is a pretty significant question."

Brush, especially, feels strongly about not having more kids because of the rising global population.

A Sellwood mom and naturopath, Adriana Azcarate-Ferbel, worried about procreating at all. She and her husband, Pedro Ferbel-Azcarate, a Portland State University professor, recently built an addition to their house out of straw bales.

"When I see people with three or four kids, I do get concerned," says Azcarate-Ferbel, whose family in Mexico pressures her to have more children. "What's going to happen to those kids? Are we going to have enough water? Are we going to have enough food for our kids? Are we going to have droughts like we've had in Africa?"

If they want 2-year-old Santiago to have a sister, they could always adopt, she says.

Although parents of only children say they still feel stigmatized, Bell says things are changing. She knows some couples who chose not to have kids at all.

"It's not an assumed thing anymore," she says.

And for her, at least, the door to having another child remains cracked.

"We basically break it down to hormones," the 33-year-old lawyer says. "I have a very strong environmental consciousness, but I also have reproductive hormones flooding my body. I think that's why (my husband) can be more abstractly, philosophically committed to the idea, whereas my biological clock is asking me, 'What next?'"