Small for Gestational Age: What You Need to Know

Small for gestational age refers to a situation in which the fetus measures below the 10th percentile of its gestational age. This means that 90 percent or more of fetuses are larger at this point in pregnancy. Doctors determine the size of the fetus with two measurements on ultrasound: the fetal abdominal circumference and the estimated fetal weight. The size of the fetus is then assessed using growth charts. In the process, the fetus is compared to what the size should be based on the mother’s height, weight, ethnic group, and the number of times that she has been pregnant. When the fetus is below the 3rd percentile, this is categorized as severe small for gestational age. A birthweight below 2,500 grams (5.5 lb) is defined as low birthweight.

Causes of being small for gestational age fall into two main categories. A fetus can be what doctors call “constitutionally small”, meaning that the fetus matches up with other family members and is growing appropriately along the growth chart, even if it is small compared with the overall human population. The other category is intrauterine growth restriction, also called fetal growth restriction. This condition refers to a fetus that is small, even accounting for the family. It means that not enough nutrients and/or oxygen are getting to the fetus by way of the placenta and the blood vessels of the umbilical cord. This is due to pathology, meaning that something is wrong and it is associated with complications (morbidity and mortality). Causes of fetal growth restriction themselves fall into two categories: placenta-mediated growth restriction and non-placenta-mediated growth restriction.

Placenta-mediated growth restriction is a condition in which the growth restriction involves a problem with the transmission of nutrients through the placenta. It can be idiopathic, means that the cause is not clear. Alternatively, placenta-mediated growth restriction can be secondary to any of a variety of conditions. These include maternal smoking, preeclampsia, maternal alcohol use, maternal anemia, maternal infection, malnutrition, and other health issues of the mother. In non-placenta-mediated growth restriction, the fetus is small, due to structural abnormality, a genetic condition, infection, or problems with metabolism.

Signs of fetal growth restriction, other than the fetus being small for gestational age include abnormal Doppler studies revealing abnormal circulation in the placenta, reduced volume of amniotic fluid, reduced movement of the fetus, and abnormal continuous fetal heart monitor readings.

Both short-term and long-term complications can result fetal growth restriction. Short-term complications include birth asphyxia (insufficient amount of oxygen reaching the fetal brain during delivery), stillbirth (fetal death), neonatal hypothermia (newborn body temperature is too low), and neonatal hypoglycemia (blood sugar concentration is too low in the newborn). In the long-term, babies with growth restriction have an elevated risk of cardiovascular disease. In particular, they are at risk of developing hypertension (high blood pressure), obesity, and type 2 diabetes. Additionally, they may be at risk of developing mood disorders, such as major depressive disorder, a condition that, in turn, when present in mothers, increases the risk of fetal growth restriction.

Factors increasing the risk that a pregnant woman will have a fetus that is small for gestational age include having a previous fetus that was small for gestational age, smoking, diabetes, high blood pressure existing prior to pregnancy, preeclampsia, high maternal age (generally above 35 years), carrying a multiple pregnancy, low levels of pregnancy-associated plasma protein A, antiphospholipid syndrome, and antepartum hemorrhage (bleeding before giving birth).

Your doctor will perform fetal monitoring, more or less frequently, depending on your risk level and various measurements. In some cases, doctors may perform what’s called Doppler studies of umbilical vessels. In addition to frequent monitoring of the fetal size, doctors will also measure the rate of fetal growth, such as how fast the abdominal size is getting bigger. If doctors detect problems of fetal growth in a woman who is at risk, or they detect problems with umbilical blood flow, generally the frequency of fetal monitoring sessions will be increased. This means that you will have to come in for more appointments.

As for dealing with small for gestational ages fetuses, first they must be identified. If you are at risk of preeclampsia, your doctor may prescribe aspirin. Modifiable risk factors must be adjusted. If you smoke, for instance, your doctor will advise smoking cessation therapy. If fetal growth is slowing noticeably, doctors may advise early, induced delivery or cesarean section. Furthermore, doctors may perform a variety of tests to identify underlying causes of fetal growth restriction.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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