Elias Thorsteinsson weighed 6 pounds, 11 ounces, when he was born in January 2010. That put him in the 25th percentile of U.S. babies, meaning 25% of babies were smaller than he was at birth. But he didn't gain weight as quickly as other infants and dropped to the 1st percentile on the pediatric growth chart when he was 6 months old. 'We were freaking out,' says his mother, Randi Stebbins, a lawyer in Santa Clara, Calif., who rented an infant scale to weigh him daily at home.

Elias gained weight rapidly once he started eating solid foods. His head grew even faster -- to the 99th percentile on the head circumference chart. His pediatrician wanted to do an MRI to rule out brain abnormalities. But Elias was developing normally, so Mrs. Stebbins refused. 'We'd been through the wringer with these percentiles,' she says. 'I said, 'I have a big head. His father has a big head. He'll grow into it.' '

Now 2 1/2 years old, Elias is 37-inches tall and 32 pounds, which puts him in the 85th percentile in body mass index for his age. 'Now they're giving us BMI and I'm like, 'Really?' ' says his mom.

Parents often worry that their children are too tall, too short, too fat or too thin. These days, however, more kids are measuring 'off the charts' -- either above or below the standard ranges for height and weight that pediatricians use.

The wide variations are due in part to rising obesity rates, an increase in premature infants who survive, and a population that is growing more diverse. Yet the official growth charts from the Centers for Disease Control and Prevention still reflect the size distribution of U.S. children in the 1960s, '70s and '80s. The CDC says it doesn't plan to adjust its charts because it doesn't want the ever-more-obese population to become the new norm.

Growth standards for newborns through age 2 are particularly confusing because two different charts are in use in pediatric offices: a CDC version that reflects mostly formula-fed infants and a newer World Health Organization chart based exclusively on breast-fed babies in ideal conditions in six countries.

Breast-fed babies typically gain weight more slowly than formula-fed babies, starting at about 6 months. By 24 months, a baby in the 50th percentile on the WHO chart weighs 25 pounds -- 1 1/2 pounds less than a baby in the 50th percentile on the CDC chart.

Thus, breast-fed babies may look like they aren't gaining weight adequately on the CDC chart, even though they are following a typical growth curve on the WHO version, according to the CDC. The agency has warned that could prompt some pediatricians and mothers to abandon breast-feeding unnecessarily.

Both the CDC and the American Academy of Pediatrics have urged doctors to adopt the WHO chart since 2010, since they agree that breast-feeding is healthier for babies. But the AAP hasn't issued new guidelines on it yet and says it doesn't know how many pediatricians have made the switch.

'I feel like women who breast-feed are constantly fighting to do what's best for their babies,' says Sarah Weltzer of Bellingham, Wash., whose daughter, Lockhart, dropped from the 50th to the 10th percentile on the CDC chart in May. A pediatrician filling in for Ms. Weltzer's doctor said Lockhart needed to gain weight urgently and that supplementing with formula was the only option. Ms. Weltzer, who notes that Lockhart is still in the 25th percentile on the WHO chart, opted to nurse her more often instead.

Pediatricians say it is important to measure children's growth against standard reference ranges because deviations can signal serious health issues. Unusually rapid growth could be caused by a pituitary tumor or genetic abnormality; unusually slow growth, sometimes called 'failure to thrive,' could be due to malnutrition, infections, cystic fibrosis, liver disease, heart problems or Type 1 diabetes.

When evaluating growth issues, the percentile a child falls in isn't nearly as significant as changes in percentiles over time, experts say. 'My advice to parents is, don't just look at one point -- it'll drive you nuts,' says Joseph Hagan, a pediatrician in South Burlington, Vt., and chairman of the AAP's guidelines committee.