Weight gain is one of many indicators of good health in a newborn baby. Slow weight gain can be a sign of an underlying problem, according to Children's Hospital Boston. That said, sometimes a perfectly healthy baby has a natural growth pattern that is slower than normal. To determine if your baby has a normal weight at one month, your baby's doctor will use growth charts to compare your baby's weight to that of other babies born after the same length of pregnancy.
A baby's birth weight includes excess body fluid, which is lost during the first few days. Most babies lose approximately 10 percent of their birth weight during the first five days but gain it back over the next five, putting them back at their original birth weight by day 10. After regaining their birth weight, most babies grow rapidly. They usually experience growth spurts around seven to ten days and again from three to six weeks.
An average newborn gains about 2/3 oz. per day and weighs approximately 10 pounds by one month, according to the American Academy of Pediatrics' HealthyChildren.org website. A newborn grows from 1 1/2 to 2 inches during the first month.
Boys vs. Girls
At age 1 month, boys normally weigh slightly more than girls do. The weight difference is less than 1 lb. Boys are also slightly longer—by about 1/2 inch—than girls at one month.
If a baby's weight is less than normal at 1 month, the doctor will look at other factors to determine if the slow weight gain is the result of a medical problem. The doctor will check to see if an infant maintains a steady growth rate, even though it is slow, has typical increases in head circumference and length, wakes on his own and breastfeeds or takes a bottle eight to 12 times in a 24-hour period and has soiled and wet diaper counts similar to faster-growing babies. If a baby meets these criteria, he is likely healthy and simply has a naturally slower pattern of weight gain.
Failure to meet certain milestones during the first month may indicate your baby's slow weight gain may be problematic. These include not gaining at least 1/2 oz. a day by the fourth or fifth day after birth and not regaining his original birth weight by the time he is 2 to 3 weeks old.