The size of an infant at birth is determined by the mother's nutrition and environmental conditions, placental function, and the baby's growth potential. Birth weight is primarily determined by gestational age, and standards for various gestations are available—that is, a baby born at 37 weeks will have a different weight expectation than one born at 41 weeks.
So, what does that mean for baby growth percentiles, those charts your doctor uses to compare your child's development to the averages of other children in the country?
- Children who are below the 10th percentile (smaller than 90 percent of other infants) are considered small for gestational age (SGA).
- Children who are above the 90th percentile (larger than 90 percent of other infants) are considered large for gestational age (LGA).
- Infants who are between the 10th and 90th percentiles are considered appropriate for gestational age (AGA).
The Advantages of Being Average
AGA infants have the lowest number of problems in the newborn, period— and the best long-term outcome. With regard to birth weight and size, you want your child to be average. Birth weight does not accurately predict later size for most AGA and many LGA infants, but rather reflects maternal and placental conditions, and not long-term growth potential. In fact, the bigger a baby is, the greater chance that infant (and the mother) has of a difficult birth with possible injury. Therefore, there is no advantage to bigness as long as a child is within the 10th and 90th percentiles for birth weight.