Fetal growth is continuously monitored throughout a healthy pregnancy. If an unborn baby’s estimated weight is lower than expected for the number of weeks of pregnancy, the finding is termed Small for Gestational Age (SGA). This designation is a statistical measure that identifies pregnancies requiring closer attention. SGA is not a diagnosis of an illness, but a descriptive label for a baby whose size falls at the lower end of the normal range for their gestational age. Medical teams then determine the reason for the smaller size and ensure the baby is thriving.
Defining Small for Gestational Age (SGA)
Small for Gestational Age (SGA) is a statistical definition applied to a fetus or newborn whose estimated fetal weight (EFW) or birth weight is below the 10th percentile for that specific gestational age. This means the baby is smaller than 90% of other babies at the same stage of pregnancy. Approximately one in ten babies will be classified as SGA.
The most important distinction is the difference between SGA and Fetal Growth Restriction (FGR), sometimes called Intrauterine Growth Restriction (IUGR). SGA is strictly a measurement of size. FGR, however, is a clinical diagnosis implying a pathological process has prevented the fetus from reaching its full growth potential.
Many babies classified as SGA are “constitutionally small,” meaning they are healthy due to natural genetic factors, such as having smaller parents. In these cases, the fetus is growing appropriately but is positioned on a lower growth curve. Conversely, a fetus with FGR is pathologically small because of an underlying issue, often related to the placenta, which compromises the delivery of oxygen and nutrients.
It is the presence of FGR that raises the risk for complications, not the SGA label alone. The clinical focus shifts to determining if the small size is due to healthy, constitutional smallness or a more concerning growth problem. A fetus with FGR is almost always SGA, but an SGA fetus is not always growth-restricted.
Primary Factors Contributing to SGA
The factors that lead to restricted growth are broadly categorized into issues originating from the mother, the placenta, or the fetus itself. Placental factors are the most common cause of true Fetal Growth Restriction (FGR). Poor placental function, or placental insufficiency, reduces the blood flow that delivers oxygen and nutrients to the developing baby, directly limiting growth.
Maternal factors can also significantly influence fetal growth, especially those that affect the mother’s ability to support the pregnancy. These include pre-existing medical conditions like chronic hypertension, chronic kidney disease, or advanced diabetes with vascular complications. Lifestyle choices such as cigarette smoking, substance use, and significant malnutrition are also associated with restricted fetal growth.
Less common are factors intrinsic to the fetus, such as genetic syndromes or chromosomal abnormalities. Certain congenital infections, like cytomegalovirus or toxoplasmosis, can also directly impede a fetus’s growth and development. These varied factors result in SGA being seen across a spectrum, from healthy small babies to pathological growth restriction.
Diagnosis and Monitoring During Pregnancy
The initial screening for potential SGA often begins with routine measurements of the mother’s fundal height, which is the distance from the pubic bone to the top of the uterus. After approximately 20 weeks of gestation, this measurement in centimeters should roughly correspond to the number of weeks of pregnancy. If the fundal height is consistently low, it prompts further investigation.
The definitive confirmation of SGA is made using ultrasound biometry, which provides an Estimated Fetal Weight (EFW). The technician measures various fetal parts, including the head circumference, abdominal circumference (AC), and femur length, to calculate the EFW. A key indicator of growth restriction is an abdominal circumference measurement or EFW that plots below the 10th percentile.
To distinguish between a healthy SGA fetus and one with FGR, specialized monitoring techniques are employed, most notably Doppler velocimetry. This test measures blood flow within the umbilical artery and sometimes in fetal brain vessels, like the middle cerebral artery. Abnormal Doppler results, such as reduced blood flow, indicate a redistribution of blood flow—often called the “brain-sparing” effect—which is a sign of placental dysfunction and FGR.
Medical Management and Neonatal Outcomes
Once an SGA or FGR diagnosis is confirmed, management shifts to intensive surveillance, as there is no current medical intervention that can cure FGR. This increased monitoring typically involves more frequent ultrasounds, often every two weeks, to track growth velocity and assess amniotic fluid volume. Fetal wellbeing is also assessed using non-stress tests and biophysical profiles.
The primary challenge in managing FGR is balancing the risks of early delivery against the dangers of remaining in a compromised uterine environment. If monitoring indicates a decline in fetal health, such as increasingly abnormal Doppler studies, the medical team may recommend delivery. Delivery may be offered around 37 weeks gestation for SGA fetuses with normal Doppler studies, but earlier if the fetus shows signs of distress.
Newborns who are SGA, particularly those who experienced FGR, face specific immediate health considerations. They are at an increased risk for hypoglycemia, or low blood sugar, because they have limited fat and glycogen stores. They may also have difficulty regulating their body temperature due to a large surface area-to-weight ratio and low subcutaneous fat. These infants often require observation in a neonatal intensive care unit (NICU) to manage these issues, including ensuring stable temperature and blood glucose levels.