Small for Gestational Age (SGA) describes newborns who are smaller than expected for the time they spent in the womb. Fetal growth assessment compares the baby’s size to the anticipated size for that specific point in time during pregnancy. Identifying a baby as SGA is an important diagnostic step, flagging newborns who may require specialized monitoring and care immediately after birth and into early childhood. Understanding the definition, causes, and potential outcomes of SGA helps ensure the best possible start for these infants.
Defining Small for Gestational Age
A baby is formally classified as Small for Gestational Age if their birth weight falls below the 10th percentile for that specific gestational age and sex. This means 90 out of 100 babies born at the same time would weigh more. The 10th percentile serves as the statistical cutoff to identify infants significantly smaller than the average population.
To apply this definition accurately, gestational age must be precisely determined, typically using the mother’s last menstrual period or an early ultrasound measurement. This age is plotted on specialized growth charts, often customized to account for factors like maternal height, ethnicity, and parity. The infant’s weight is compared against a reference population to determine if it falls within the smallest 10%.
Distinguishing SGA from Intrauterine Growth Restriction
While Small for Gestational Age and Intrauterine Growth Restriction (IUGR) are sometimes used interchangeably, they describe different concepts. SGA is a descriptive term based on a statistical measurement of size at birth, indicating the baby’s weight is below the 10th percentile compared to peers.
IUGR, in contrast, is a diagnosis implying a pathological process where the fetus failed to reach its genetically determined growth potential. A baby can be SGA because they are “constitutionally small,” meaning they are healthy but genetically predisposed to be small. However, a baby with IUGR has experienced restricted growth due to a problem, which often results in an SGA classification at birth.
This distinction is clinically important because not all SGA babies have experienced a growth problem, and some babies who experienced IUGR may still be born above the 10th percentile. A baby whose growth rate slowed significantly late in pregnancy, indicating IUGR, may not be classified as SGA if their starting size was large. Conversely, a healthy SGA baby who is genetically small will not show signs of the malnutrition or wasting often seen in growth-restricted infants.
Primary Causes and Risk Factors
The factors leading to a baby being born SGA fall into three main categories: maternal, placental, and fetal.
Maternal Factors
Maternal factors involve the mother’s health and lifestyle, which can directly or indirectly impact fetal nourishment. Chronic conditions such as hypertension, pre-eclampsia, and advanced diabetes with vascular disease can restrict blood flow and nutrient delivery to the fetus. Lifestyle choices like smoking, substance use, and poor nutrition are well-established contributors to restricted fetal growth. Additionally, maternal age, especially being an adolescent or over 35 years old, and having a low pre-pregnancy weight are recognized risk factors.
Placental Factors
Placental factors are often the most common cause of pathological SGA, which is essentially IUGR. Placental insufficiency means the placenta is not functioning properly, limiting the transfer of oxygen and nutrients to the baby. Other placental issues, like placental abruption or structural abnormalities, can similarly impair the supply line to the fetus.
Fetal Factors
Fetal factors involve conditions within the developing baby that limit growth, regardless of a healthy placenta or mother. These include chromosomal abnormalities, genetic syndromes, or congenital infections like cytomegalovirus (CMV) or rubella. Multiple gestation, such as carrying twins or triplets, is also a factor because shared resources can lead to one or more babies being born SGA.
Health Outcomes and Monitoring
Newborns classified as SGA face an increased risk of specific health issues immediately following birth. A frequent concern is hypoglycemia, or low blood sugar, because these babies may have fewer fat and carbohydrate reserves. They also often struggle with thermoregulation, finding it difficult to maintain a normal body temperature due to their low body fat and a large surface area relative to their weight.
Other potential immediate complications include polycythemia (an abnormally high concentration of red blood cells) and a higher risk of perinatal asphyxia (a decrease in blood flow or oxygen before or during delivery). Specialized monitoring and care, often involving frequent blood sugar checks and thermal support, are routinely provided to SGA infants in the hospital.
For most otherwise healthy SGA infants, the prognosis is good due to a phenomenon known as “catch-up growth” occurring during the first two years of life. About 90% of SGA babies reach a normal point on the growth curve by age two to four. Children who remain small may require follow-up with a pediatric endocrinologist, and growth hormone therapy may be considered. SGA is also associated with an increased risk of adult diseases, such as type 2 diabetes and cardiovascular issues, particularly if rapid weight gain occurs in early infancy.