In a national survey of 1,334 mothers, conducted by the Centers for Disease Control and Prevention (CDC), 40% said they gave their baby solid food before they were 4 months old, with 9% starting as early as 4 weeks. Doctors now recommend waiting until a baby is at least 6 months old.

For at least 20 years, the American Academy of Pediatrics had advised against feeding babies solid food before they turned at least 4 months old. Last year, encouraged by growing evidence of the health benefits of breast milk, the group raised that age, saying babies should be fed nothing but breast milk for six months. When breast milk in not an option, formula is an acceptable alternative, the group says.

However, the survey suggests that mothers are not aware of the recommendations or find them difficult to follow. Popular reasons for giving solid food to babies before 4 months included “my baby is old enough,” “my baby seemed hungry,” “I wanted my baby to sleep longer at night” and -- most alarming to researchers -- “a doctor or health care professional said my baby should begin eating solid food.”

“Clearly we need better dissemination of the recommendations on solid food introduction,” said Kelley Scanlon, an epidemiologist with the CDC and an author of the study. “Health care providers need to provide clear and accurate guidance, and then provide support to help parents carry out those recommended practices.”

The study suggested that economics were a factor in the decision to introduce solid food, with poorer women who saw formula as too expensive more likely to feed solids too soon. Women who were feeding their children exclusively formula or a mix of formula and breast milk were not only more likely to introduce solid food early, but to say their doctors gave them the go-ahead.

“It makes me want to know more about the other advice that those parents were getting on infant feeding,” Scanlon said.

Further, the women in the survey who turned to solid food early were more likely to be young, less educated and unmarried. They also had lower levels of income or education and were more likely to participate in the Special Supplemental Nutrition Program for Women, Infants and Children.

While many pediatricians are sympathetic to the difficulties parents face feeding their child nothing but breast milk or formula for six months, they say little good can come from feeding solid food to a child before he or she is physically ready.

“When a baby is ready to start eating food, he will put his hands in his mouth, and you will see him actually making chewing motions,” said Dr. T J Gold, a pediatrician with Tribeca Pediatrics in Brooklyn. “At 2, 3 months, they can’t even hold their heads up well, and they can’t sit,” making it difficult, if not dangerous, to put solid food in their mouths.

They also have yet to develop the proper gut bacteria that allow them to process solid food safely, potentially leading to gastroenteritis and diarrhea, Gold said. The early introduction of solid foods has also been linked to increased risk of obesity, diabetes, eczema and celiac disease.

One reason parents turn to solid food early is the persistence of myths about solid food helping babies sleep through the night or put on weight.

“That big fat bottle at the end of the night isn’t why your baby is sleeping -- it’s a skill you acquire,” Gold said. “And if you think giving your child more calories is going to help him gain weight, but it gives him more diarrhea, then he’s not actually absorbing as much.”

However, even parents who are aware of the guidelines can have trouble following them, particularly if they are struggling to buy enough formula to feed a rapidly growing child. “The formula gets really expensive, especially in the 4-to-6-month window,” Gold said. “And if you have more than one child and you’re already preparing food for the whole family, it’s much easier to just start sweeping things off your plate.”

Pediatricians can help parents delay solid food by helping them better understand their baby’s signals, Scanlon said. “When the baby is fussy, they need to help them understand that doesn’t necessarily mean they’re hungry and need solid foods,” she said.

Parents should also know the signs that their child is ready for solid food, like sitting up, being able to take food off a fork and not closing the mouth when food is offered, Scanlon said.