I totally have experience with this, and yor doctor's advice sounds right to me. DS has been a big bruiser since birth, was 16 lbs at 2 months old, always at 99th percentile of height and, until about nine months, at the 98-99th percentile of weight. Pediatrician suggested I try to make him wait longer between feedings at 2 months, to slow his weight gain, but he was exclusively breastfed and feeding as much as he wanted and needed to. I kept bfing him on demand, exclusively for 6 months then introducing healthy foods. His weight stayed high, proportionate to his height, until 9 months. I wasn't worried. If you're breastfeeding, you're feeding the healthiest food you could possibly feed your baby. If baby is hungry and you don't feed him or her, that's sub-optimal (and, if you're breastfeeding, it can affect your milk supply). Also, I was a butterball of a baby, but a very slender child, and my husband and I are both slim adults. So I knew he'd slim out eventually.
Sure enough, at 9.5 months, he started walking. Scheduling complications meant he didn't get another well-baby visit until he was almos 15 months old. In six months, he had grown more than five inches, and gained less than a pound. Walking makes a huge difference! Your toddler might use up all his baby fat now that he's walking, too.
(Then our pedi worried that he might not be gaining enough weight, as he had plummeted to the 90th percentile, while remaining at the 99th percentile of height. But, as he now sees, our baby hits every milestone early, is healthy and happy and active, and is probably destined to be a tall, slim child. Whatever! We don't live in Lake Wobegon, "where every child is above average.")
I wouldn't worry a lot about percentiles if you're feeding healthy food. I wouldn't worry AT ALL about weight percentiles for your younger one if you're exclusively breastfeeding until six months.
Your 14-month-old is really tall, too, almost as tall as DS was at that age. Is your pedi concerned about his weight? I wouldn't be surprised if he slims out with the increased activity level walking allows. If he gets lots of exercise, eats healthy food and doesn't eat junk, he's probably fine. As our doula pointed out to us, Look, someone has to be in the 99the percentile. It's not necessarily unhealthy to be among the heaviest kids, especially when he's amongst the tallest, as well. Sure, some kids in this weight range are unhealthy, but the top two percentile of weight includes parents who feed their kids soda! That's unhealthy, but it doesn't mean your son is unhealthy just because he's the same weight."
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