Weight is a key consideration in determining whether an infant is thriving. Doctors and parents use growth charts to compare a baby's weight to other infants of the same gender and age. No one number, however, can capture how a baby is doing. Because each child is different, doctors focus most heavily on the growth curve, meaning the baby's weight gain over time. By age 4 months, babies tend to double their birth weight, and by one year they generally triple it. This holds true no matter whether the child's weight is high, low or average compared with peers.
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A typical healthy, full-term newborn weighs between 6 and 9 lbs., according to the American Pregnancy Association. A 6-month-old boy weighing 17.5 lbs. would fall into the 50th percentile, according to National Center for Health Statistics growth charts. If he weighed 14.5 lbs., he would be in the 10th percentile, and if he weighed 20 lbs. he would be in the 90th percentile. A weight of 16 lbs. puts a 6-month-old girl in the 50th percentile. A weight of just under 14 lbs. would land her in the 10th percentile, and 18.3 lbs. would put her in the 90th percentile.
A baby who is born early is likely to weigh less than one who was born at full term, meaning 38 to 40 weeks gestation. Other factors that can come into play include the mother’s health and nutrition during pregnancy, the parents’ build and whether the birth is a singleton or multiples, according to the American Pregnancy Association. On average, breastfed babies grow more quickly than formula-fed infants during the first two to three months of life and more slowly from three months to 1 year, according to Kelly Bonyata, a certified lactation consultant who runs KellyMom, a parenting and breastfeeding website.
Though slow weight gain can result from a baby's natural growth pattern, it can also signal a problem, according to Children's Hospital Boston. A doctor should investigate if your baby fails to put on at least 1/2 oz. (15 grams) per day by the fifth day after birth or to regain birth weight by three weeks following birth. Additionally, according to the hospital, failure to gain at least 1 lb. (454 grams) per month for the first four months is cause for concern, as is a pronounced falloff in the growth rate from the previous curve. Potential causes include infection, chronic disease, genetics, poor emotional health and poor nutrition, according to Medline Plus. Delays in development often accompany delayed physical growth.
Excessive weight gain is a problem for some infants. Obesity occurs when weight gain is far out of proportion to growth in height, according to Penn State Children's Hospital. A baby with such a problem will, according to the hospital, "look fat, not just chubby." Possible causes include overfeeding, providing only high-calorie drinks, using food as a reward or to relieve stress, and not offering sufficient opportunities to exercise. Typically, a healthier diet is prescribed under a physician's supervision to slow the rate of weight gain.
Doctors often put too much emphasis on what the scale says when evaluating the health of babies, writes Dr. Jay Gordon, a pediatrician and author who maintains a website about breastfeeding and other health topics. As a result, he says, mothers too often turn to formula over breast milk. He advises doctors and parents to consider, before turning to supplementation, whether the child is alert, is urinating and defecating well, is moving arms and legs vigorously and is meeting developmental milestones.