The average age when children are successfully toilet trained is going up, and new research has identified certain factors related to this trend.

Children who start toilet training at an older age, or who are constipated, are more likely to be late toilet trainers, according to a study of nearly 400 youngsters. The study also found that children who refuse to use the potty for bowel movements even though they use it to urinate are more likely to be late toilet trainers.

Pediatricians have noticed that the average age at which toilet training is completed has been on the rise recently, the authors point out in their article in the Journal of Pediatrics.

In the 1950s, children completed toilet training at an average age of almost 29 months, and more than 97 percent were toilet trained by age 3. In recent studies, however, just 40 to 60 percent of children have completed toilet training by age 3.

Besides stressing out parents and putting diaper-changing demands on day-care workers, late toilet training may increase the spread of infectious illnesses, such as diarrhea and hepatitis A, at day care centers, the researchers say.

A team led by Dr. Nathan J. Blum of Children’s Hospital of Philadelphia interviewed the parents of 378 children every 2 to 3 months to see how long it took for their children to be toilet trained.

The average age of completed toilet training was about 3 years. But 16 percent of children were not completely toilet trained until after 42 months.

Blum’s team identified three factors that seemed to account for most of the cases of later toilet training.

Late potty trainers tended to start toilet training later than other children. They were also more likely to have frequent constipation. And children who completed toilet training late were more likely to use the toilet to urinate but not to have a bowel movement - so-called stool toileting refusal.

Why parents often start toilet training later these days is uncertain, according to the report. One possibility, according to the researchers, is that parents may have heard that starting the toileting process early does not necessarily lead to earlier toilet training success.

In fact, the results of the study do not mean that toilet training should be started at an earlier age, according to Dr. Nader Shaikh of the Children’s Hospital in Pittsburgh. Shaikh points out in a related editorial “that the younger the age at which toilet training was initiated the longer it took for the process to be completed.”

One of the limitations of the study, the authors point out, is that the children were mostly from white, suburban, upper middle-class families, so the results may not apply to other groups.

SOURCE: Journal of Pediatrics, July 2004.