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.
What It Means to Be Small
Being small for gestational age has several names. Each of these names has a slightly different implication, but generally means the same thing: the infant is smaller than was expected.
Some of these other names are:
- Intrauterine growth retardation
- Small for dates
- Light for dates
There are many causes of being SGA, and your baby’s doctor should make at least a cursory search for an explanation. This is important because some problems cause reduced growth in childhood.
A partial list of factors contributing to being SGA includes:
Maternal environment and condition:
- High blood pressure
- Cigarette smoking
- Maternal drug use
- Maternal malnutrition
- Low maternal weight gain (less than 20 pounds)
- Mother was also SGA
- Maternal chronic disease (advanced diabetes, anemia, etc.)
- Living at high elevations (for example, Denver, Colorado)
- Frequent, heavy physical work during pregnancy
- Maternal age (less than 20 years)
- Racial and ethnic background
- Multiple fetuses
- Rubella, cytomegalovirus, and other infections
- Placental abruption (separation of the placenta from the uterine wall)
Fetus’ growth potential:
- Chromosome disorders
- Metabolic problems
- Fetal infections
SGA Babies’ Outcome
These infants have more problems in the newborn period such as low blood sugar and too many red blood cells. SGA babies also have a higher risk than AGA infants for being small throughout life and for having delayed development.
Catch-up growth usually occurs in the first six months of age. The later in pregnancy that the fetus’ growth slowed down, the more likely that catch-up growth will be complete.
What It Means to Be Large
Although there are many causes for decreased growth, there are relatively few causes of increased growth resulting in LGA infants.
- Maternal diabetes (even gestational diabetes), the most common cause
- Maternal obesity
- Fetal abnormalities (certain syndromes)
LGA Babies’ Outcome
Infants who are LGA have a higher risk of birth injury, low blood sugar, elevated blood count, and immature lungs. It was once thought that baby weight and size normalized later in life and that there were no long-term consequences, though we now know this is probably not true. A recent study suggests that infants born to mothers with diabetes are more likely to develop obesity in childhood. If one parent has diabetes, children have about a one to five percent risk of developing diabetes later in life.
Baby Size Considerations
Size at birth predicts future size mostly at the extremes of birth weight and size. Future size is not accurately predicted by birth weight for babies who are born at full-term and within the usual range (2,500 grams to 4,500 grams or about five and one-half pounds to nine pounds, 15 ounces). Newborns that are born lighter than the norms may have catch-up growth, but many remain somewhat small for their age throughout childhood. Those that are born larger than the norms may have a higher risk of obesity later in life.