Introduction: Who Should Undergo Diagnostics

Diagnostic evaluation for small for gestational age babies typically begins during pregnancy and continues after birth. If you are pregnant, your healthcare provider will start monitoring your baby’s growth regularly, especially if certain risk factors are present. Mothers who have high blood pressure, diabetes, kidney disease, or who smoke or use substances during pregnancy may receive closer monitoring because these conditions can affect how well a baby grows inside the womb.^[1]

Women who are teenagers or over 35 years of age, those who have previously had a small for gestational age baby, or those carrying twins or triplets should also expect more frequent checks on their baby’s size. Additionally, if you have certain chronic conditions like heart disease, lung disease, severe anemia, or lupus, your doctor will likely recommend additional testing to track your baby’s growth throughout the pregnancy.^[3]

After birth, all newborns are assessed to determine if their weight and length are appropriate for the number of weeks they spent in the womb. This assessment happens routinely in the hospital, so parents don’t need to request it specifically. However, if your baby is found to be below the 10th percentile for their gestational age, additional diagnostic tests may be recommended to understand why they are small and whether any underlying issues need attention.^[2]

Diagnostic Methods During Pregnancy

During your pregnancy, your midwife or doctor will use several methods to check whether your baby is growing as expected. The simplest technique is called palpation, where your healthcare provider gently presses on your abdomen to feel the baby’s size and position. This manual examination doesn’t hurt your baby because they are protected by the amniotic fluid surrounding them in your womb.^[24]

Starting around 26 to 28 weeks of pregnancy, your midwife will begin measuring your belly using a tape measure. This measurement is called fundal height, and it tracks the distance from your pubic bone to the top of your uterus. The fundus is the name for the top portion of your womb. Typically, your fundal height in centimeters matches the number of weeks you are pregnant, give or take about 2 centimeters. For instance, if you are 32 weeks pregnant, your fundal height should measure somewhere between 30 and 34 centimeters.^[24]

Some healthcare providers use a customized growth chart that takes into account your own height, weight, and ethnic background, as well as how many babies you have had before. These personalized charts help determine what is normal for you and your baby specifically, rather than comparing your baby to all other babies. If your fundal height measurements suggest that your baby may be smaller than expected, your doctor will likely recommend an ultrasound scan for a more detailed assessment.^[24]

Ultrasound scans are the most accurate way to estimate your baby’s size before birth. During an ultrasound, sound waves create pictures of your baby inside the womb, allowing the doctor to measure various parts of your baby’s body. These measurements are used to estimate your baby’s weight. If the estimated weight falls below the 10th percentile for babies at the same stage of pregnancy, your baby may be classified as small for gestational age.^[6]

When ultrasound shows that your baby might be small, your healthcare team may order additional ultrasounds more frequently to monitor growth patterns over time. This repeated monitoring helps doctors see whether your baby is growing steadily (even if slowly) or if growth has slowed down or stopped. The pattern of growth provides important clues about whether there is a problem that needs intervention.^[12]

Besides measuring size, ultrasound can also assess blood flow through the placenta and umbilical cord using a technique called Doppler ultrasound. The placenta is the organ that supplies nutrients and oxygen to your baby. If blood flow is reduced, your baby may not be receiving everything needed for proper growth. Doppler studies help doctors understand whether inadequate blood supply is contributing to your baby’s small size.^[12]

Your doctor may also perform tests to check for infections that can affect fetal growth. Certain infections during pregnancy, such as cytomegalovirus (CMV), rubella (German measles), toxoplasmosis, or Zika virus, can cause babies to be smaller than expected. Blood tests can detect whether you have been exposed to these infections. If an infection is found, it helps explain why your baby is small and guides decisions about care and delivery timing.^[3]

In some cases, doctors may recommend testing to check for genetic or chromosomal problems if your baby is very small or if the ultrasound shows other concerning features. Procedures like amniocentesis or chorionic villus sampling can analyze your baby’s chromosomes to identify conditions such as Down syndrome or other genetic disorders that might be associated with poor growth. These tests carry a small risk, so they are usually only offered when there is a specific reason to suspect a genetic problem.^[2]

Diagnostic Methods After Birth

Once your baby is born, healthcare providers perform a thorough assessment to confirm whether your baby is small for gestational age. The first step is determining your baby’s gestational age, which refers to how many weeks of pregnancy had passed when your baby was born. Gestational age is calculated by counting from the first day of your last menstrual period to the day of delivery, though doctors often adjust this timeframe based on early ultrasound scans done during pregnancy.^[3]

Your baby’s birth weight and length are measured immediately after delivery and compared to standard growth charts for babies born at the same gestational age. If your baby’s weight or length falls below the 10th percentile on these charts, they are considered small for gestational age. Some medical definitions use the 3rd percentile as the cutoff, meaning that only the smallest 3 out of every 100 babies would be classified this way.^[7]

Doctors also examine your baby’s appearance and physical characteristics. Some small for gestational age babies look proportionately small all over, meaning their head size, body length, and weight are all equally reduced. This is called symmetric growth restriction and usually indicates that something affected your baby early in pregnancy. Other babies may have a normal-sized head but a smaller body or lower weight, which is called asymmetric growth restriction. This pattern typically suggests that growth problems started later in pregnancy, often due to placenta issues.^[3]

The medical team will look for other physical signs that might provide clues about why your baby is small. Some small for gestational age babies appear thin with less muscle and fat tissue, and may have sunken facial features. Others may look mature and healthy despite their size. The doctor will also check for any birth defects or unusual features that might suggest an underlying genetic condition or infection.^[3]

If your baby is small for gestational age, the healthcare team may recommend additional tests to look for possible causes. Blood tests can check your baby’s blood sugar levels, as small babies are at higher risk for low blood sugar after birth. Your baby may also be tested for infections that could have been passed from mother to baby during pregnancy. These infection tests might include blood tests or other laboratory examinations depending on what the doctor suspects.^[2]

In some cases, doctors may order imaging tests such as ultrasound of your baby’s brain, heart, or other organs if they are concerned about how the restricted growth may have affected your baby’s development. These tests help identify whether any organs were particularly affected by receiving inadequate nutrients during pregnancy. Not all small for gestational age babies need these additional tests—the decision depends on how small your baby is, how they appear, and whether there are other concerning signs.^[2]

Distinguishing Small for Gestational Age from Related Conditions

It’s important to understand that being small for gestational age is not exactly the same as other terms you might hear, though they are related. Low birth weight refers to any baby born weighing less than 2,500 grams (about 5 pounds, 8 ounces), regardless of how many weeks of pregnancy had passed. A baby can have low birth weight either because they were born early or because they are small for their gestational age—or both.^[2]

Intrauterine growth restriction, often abbreviated as IUGR, is a diagnosis made while the baby is still inside the womb. It describes a situation where the fetus is not growing as expected and is not reaching its full growth potential. Not all babies with IUGR end up being small for gestational age at birth, and not all babies who are small for gestational age had IUGR during pregnancy. The terms describe similar situations but at different time points—IUGR during pregnancy, and small for gestational age after birth.^[12]

Premature or preterm babies are those born before 37 weeks of pregnancy. Small for gestational age babies can be born prematurely, at full term (between 37 and 41 weeks), or even post-term (after 42 weeks). Being premature and being small for gestational age are separate issues, though some babies experience both conditions simultaneously.^[2]

Diagnostics for Clinical Trial Qualification

When researchers conduct clinical trials to test new treatments for children who were born small for gestational age and have not experienced adequate catch-up growth, they use specific diagnostic criteria to determine which children can participate. These standardized criteria ensure that the children enrolled in the study truly have the condition being studied and that results can be compared across different research centers.^[6]

The most common criterion for enrolling children in clinical trials is confirmation that they were born with a birth weight and/or length below a specific percentile for their gestational age. Many studies use the 10th percentile as the cutoff, while others may use the 3rd percentile or define small for gestational age as being 2 standard deviation scores below the mean for gestational age.^[7]

Clinical trials typically require documentation of the child’s birth measurements and gestational age from hospital records. Accurate gestational age is crucial because it determines whether a child’s size at birth was truly smaller than expected. Researchers will review prenatal ultrasound reports and delivery records to verify these measurements.^[6]

For trials studying treatments for short children who were born small for gestational age, researchers usually require evidence that the child has failed to achieve catch-up growth by a certain age—commonly by 2 to 4 years old. This is assessed by measuring the child’s current height and comparing it to standard growth charts. Children must demonstrate that their height remains significantly below the normal range despite having had time to potentially catch up after birth.^[16]

Clinical trials may also require tests to rule out other causes of short stature before enrolling a child. These tests might include blood tests to check thyroid function, growth hormone levels, and other hormone measurements. X-rays of the hand and wrist may be done to assess bone age, which provides information about skeletal maturity. These tests help ensure that the child’s short stature is truly related to being born small for gestational age rather than another underlying condition.^[13]

Some research studies require genetic testing or chromosome analysis to exclude children with specific genetic syndromes or chromosomal abnormalities. While these conditions can cause babies to be born small, researchers may want to study only “idiopathic” small for gestational age children—those whose small size has no identifiable cause. This helps isolate the specific population most likely to benefit from the treatment being tested.^[13]

Trials may also collect detailed information about the mother’s pregnancy, including any complications, maternal health conditions, and substance use. This background information helps researchers understand factors that may have contributed to the child being born small and whether these factors might influence how the child responds to treatment.^[6]