By Maternal Age
Small for Gestational Age
Babies born small for their gestational age (SGA) are singletons with a birth weight below the 10th percentile for their gestational age and sex. This means that a SGA baby weighs less than 90% of babies of the same sex and gestational age in a reference Canadian cohort of singleton births. SGA babies can also be low birth weight which means they weigh less than 2,500 grams (or 5 lbs 8 oz) when born at full term. SGA babies are more likely to become ill and/or die than normal birth weight newborns. Babies born as SGA can be at an increased risk of type 2 diabetes and coronary heart disease later in life.
In industrialized countries, maternal cigarette smoking during pregnancy accounts for 30% to 40% of SGA births; "genetically related factors", such as history of SGA pregnancies, maternal race, short maternal stature and fetal sex, account for 20% to 30%; nutritional factors including pre-pregnancy weight, low weight gain and low caloric intake accounts for 10% to 15%; and the number of times the mother has given birth and her general state of health for 5% to 10%.
Large for Gestational Age
Babies born large for their gestational age (LGA) are singletons weighing more than 90% of babies of the same sex and gestational age in the same Canadian cohort. When babies are born large for their gestational age they are at higher risk for birth complications, including shoulder dystocia, brachial plexus injury and Erb’s palsy. The mother is also at higher risk for postpartum hemorrhage.
In Canada, the number of babies born large for their gestational age has increased since 1995. Reasons for this are not entirely clear, although ethnicity and the mother’s diet appear to play a role. Maternal diabetes has been established as an important risk factor for LGA. In turn, a baby with a high birth weight may be at increased risk for type 2 diabetes and childhood overweight/obesity later in life.
In the late 1980s SGA babies were more common than LGA babies, but this trend began changing in the early 1990s. Since 1986, the SGA trend has consistently decreased whereas the LGA trend increased. In 2011, the LGA rate is nearly 1.5 times higher than the SGA rate. In 2011, 7.9% of live singleton births were small for gestational age, whereas 11.8% of all singleton babies were considered large for gestational age.
By Maternal Age
When compared to LGA babies, SGA babies are more common among mothers of younger ages 15 to 24 years, whereas LGA babies are more common among older mothers aged 30 to 39 years.