But not everyone agrees with the concept of extended breastfeeding. Aside from nutrition, for example, could it harm the child socially? "I would have to suspect that at some point it becomes an unusual attachment rather than a true nutritional or medical benefit," says Andrea McCoy, MD, associate professor of pediatrics and medical director of ambulatory pediatrics at Temple University in Philadelphia.
Dettwyler -- who breastfed two children until ages 4 and 5 1/2 (a third was weaned much earlier) -- wanted to know how prevalent the practice of nursing beyond age 3 was in the U.S. In 1996, she constructed a simple, eight-question survey and began looking for these nursing mothers and children. Her emails and flyers were forwarded through lactation support groups to moms across the country.
What turned up surprised her, she tells WebMD. So far, she has found 1,280 children who were nursed until ages 4, 5, 6, 7 -- well into third grade. "I have five people who nursed until the age of 9," Dettwyler says. "They're all grown up now; they were kids who nursed back in the 1970s and '80s.
In many families, moms were breastfeeding more than one child at a time. "We call it tandem nursing," she says. "There's a lot of it out there.
She asked mothers basic stuff: how long did you nurse, did you wean your child or let the child wean themselves, level of education, socioeconomic status.
"The mothers were overwhelmingly american, extremely highly educated, lots of doctors, lawyers, PhDs, middle and upper class," Dettwyler tells WebMD. "They were not immigrants -- who you might likely expect. Most of the women work outside the home.
Through previous research, focused on the physiology of primate gestation and birth, Dettwyler concluded that "humans as a biological species should be nursing six to seven years," she tells WebMD. "The brain continues to grow until six or seven. We know from research that there are chemicals in human milk that are better for human brains that you don't find in cow's milk. Cow's milk is designed to help the calf grow, to mature very quickly. There are no chemicals for developing brains. Excuse me, but we're not growing a calf here.
Thus far, health outcome studies have focused on children nursing up to two years, says Dettwyler. "But it all shows that the longer you nurse, the better the outcome. It doesn't show that the benefit stops or that it becomes harmful. The 18-24 month group has the best outcome, better than those who nurse shorter periods.
For many of these moms, health benefits are a side issue. Ask the typical woman nursing a 4-year-old if she's doing it for any incremental health benefits, she would say 'no,'" Dettwyler tells WebMD. "She'll more likely says 'It's because my child likes to nurse. It's good nutrition for him, but it also keeps him healthy and happy.
"Research shows that sucking on anything lowers heart rate, respiration and blood pressure, that it's very calming," says Dettwyler. "And human milk has specific chemicals that also help the child calm down and go to sleep. That's why you see them nursing before naps, before bed, when they're scared. It's very comforting emotionally.
The problem is cultural, she tells WebMD. "Our culture has sexualized breasts. A lot of people have trouble separating the breast as sex object from the fact that they're supposed to be for feeding babies. So when they hear about an older child nursing, they jump to the conclusion that they're having sex. Of course they're not. But that's hard for some people to understand, that breasts are not intrinsically sexual, that in other cultures around the world breasts are not considered sexual, they're considered like elbows, for instance.
But what affect does long-term nursing have on the child's social development, according to Dettwyler?
She admits that the bulk of breastfeeding research focuses on health and cognitive development -- not on social development
"I can tell you anecdotally that they tend to be very loving, caring, compassionate children, extremely bright," she tells WebMD. "Other than that, they're just other kids. ... There's certainly no evidence of any sort of negative affect.
Nursing a toddler is quite different from a newborn, she says. "You can have conversations with them, talk about stuff. They're usually nursing a few minutes before they go to bed, and first thing in the morning. When they're sick, they nurse more.
Among the toddlers themselves, it doesn't seem to be an issue, says Dettwyler. "There's been no research on this, but from own children's experience and from talking to other moms, I can tell you that kids don't sit around and talk about this.
When her oldest child was in kindergarten, his teacher told her there was something special about him and his friend Emily, Dettwyler remembers. "I'll tell you what it is," she told the teacher. "They're the only two still nursing. The teacher's mouth fell open.
Speaking for many pediatricians: "I like to see all mothers breastfeed through the first year of life ... and nursing to age two is perfectly acceptable to me, although medically and socially it isn't," says McCoy
It's not that mothers plan to nurse long-term, says Cynthia Garrison, senior lactation consultant at Magee-Women's Hospital in Pittsburgh.
"When mothers find themselves nursing a baby a year or older, they're usually as surprised as anybody," she tells WebMD. "The majority start out they're thinking it's a wonderful thing for the baby. But there's no need to rush it, to wean to the bottle ... If women are nursing over one year, they're usually closet nursers. They're not letting anybody know.
Until the Industrial Revolution, nursing 4-year-olds was a really common occurrence in the U.S., says Garrison. "As more women were drawn into the workforce, it made it less and less common. Now, since you don't encounter it everyday, people tend to think there must be something negative about it. You become a member of the counter-culture.
"We're still fighting the battle of having breastfeeding viewed as normal," Garrison tells WebMD. "Certainly extended breastfeeding has a long way to go in this culture.
So how long should mothers breastfeed?
"We turn it around," says Garrison. "We say, 'you're the expert on your baby.' When it gets to the point that either one, either the baby or mother, feels uncomfortable with it, then we give ideas on gradually helping the toddler learn other ways for comforting. Technically, once eating other foods, weaning has begun. It just may take more years sometimes."