CHICAGO - Under pressure from health and children's advocacy groups, McDonald's Corp. is making changes to its famed Happy Meals.

The fast-food chain will add a serving of fruit or vegetables to all of the meals, which are aimed at children, and shrink the portion of French fries.

The changes, announced Tuesday, will take effect in September in some markets and then roll out to all 14,000 McDonald's restaurants in the United States by April.

McDonald's said it first experimented with cutting fries entirely from the Happy Meals, but children and parents rebelled.

"People come to McDonald's and, first of all, they want the choice and the control to be theirs, but their expectation of a Happy Meal does include a fry," said Jan Fields, president of McDonald's USA. "When we did it without fries, there was a huge disappointment factor."

The new French fry holders in Happy Meals will contain 1.1 ounces of potatoes, down from 2.4. Apple slices will often be included as the healthful side dish, but it could also be carrots, raisins, pineapple slices or mandarin oranges, depending on the time of year and the region in which they're being served, Fields said.

Although subject to variation depending on what's ordered, the new meals will represent, on average, a 20 percent decrease in calories, the chain said. Fields said Happy Meal prices will not go up as a result of the changes.

As the world's largest restaurant chain by sales, McDonald's has been under intense scrutiny for the nutritional quality of its food and its marketing to children. Critics have challenged the chain's practice of selling kids' meals that include a toy, connecting it to the nation's obesity crisis.

Last year, San Francisco and Santa Clara County, Calif., banned toys with meals at fast food restaurants if the meals didn't meet certain nutritional criteria. Similar legislation has been proposed in New York.

Critics complained that McDonald's latest changes didn't go far enough. Kelle Louaillier, executive director of a group called Corporate Accountability International, said McDonald's is "taking steps in the right direction" but also is trying to get ahead of impending regulations that will restrict the marketing of junk food to children and require restaurants to post nutrition information on menus.

Cindy Goody, McDonald's senior director of nutrition, said that the new directives are "absolutely not" related to new regulations, but a response to customers seeking healthier choices.

The Associated Press contributed to this report.