Hearing that your baby is “measuring small” on ultrasound can be worrying.
You may be told that your baby’s estimated weight is below the 10th percentile and immediately wonder:
Does this mean my baby has IUGR?
Not necessarily.
A baby who is small for gestational age and a baby whose growth is truly restricted are not always the same thing.
Here is the difference between SGA (Small for Gestational Age) and FGR (Fetal Growth Restriction), previously commonly called IUGR.
What Is SGA?
SGA stands for Small for Gestational Age.
It generally describes a fetus whose estimated size or weight is small for the current gestational age, commonly below the 10th percentile.
But being small does not automatically mean that something is wrong.
Some babies are naturally small because of genetics or parental body size.
These babies may remain small throughout pregnancy while continuing to grow steadily. Their amniotic fluid and Doppler blood flow may also remain normal.
What Is FGR or IUGR?
FGR stands for Fetal Growth Restriction.
You may also hear the older term IUGR (Intrauterine Growth Restriction).
FGR means that the fetus may not be reaching its expected growth potential.
One important cause is placental insufficiency, in which the placenta may not provide the fetus with enough oxygen and nutrients to support optimal growth.
This is why doctors look at much more than fetal weight alone.
SGA vs FGR: What Is the Difference?
An easy way to understand the difference is:
SGA = The baby is small.
FGR = The baby’s growth may be restricted.
A fetus can be small and healthy.
On the other hand, fetal growth restriction is concerned not only with size, but also with how the baby is growing over time and whether there are signs of placental dysfunction.
How Does Ultrasound Help?
When a baby is measuring small, ultrasound assessment may include:
- Estimated Fetal Weight (EFW)
- Abdominal Circumference (AC)
- Head Circumference (HC)
- Femur Length (FL)
- Growth compared with previous examinations
- Amniotic fluid volume
- Placental assessment
- Umbilical Artery Doppler
- Middle Cerebral Artery (MCA) Doppler
- Cerebroplacental Ratio (CPR), when indicated
These findings help healthcare providers determine whether the baby is simply small or whether closer monitoring for fetal growth restriction is needed.
Why Is Abdominal Circumference Important?
Abdominal Circumference, or AC, is an important measurement when assessing fetal growth.
When placental function is impaired, fetal abdominal growth may be affected.
In some growth-restricted babies, the abdomen may therefore appear relatively smaller compared with other fetal measurements.
However, AC should never be interpreted in isolation. The overall growth pattern, fetal weight, Doppler findings and clinical situation all need to be considered together.
Does Below the 10th Percentile Always Mean Something Is Wrong?
No.
Being below the 10th percentile does not automatically mean that a baby is unhealthy.
Some babies are constitutionally small and continue to grow normally.
This is why the pattern of growth over time can be just as important as a single estimated fetal weight measurement.
Serial growth ultrasounds can help show whether the baby continues to grow along an expected pattern or whether growth is slowing.
What Happens If My Baby Is Measuring Small?
If your baby is measuring small, your healthcare provider may recommend additional growth ultrasounds and, when appropriate, Doppler studies.
The goal is not simply to determine how many grams the baby weighs.
The more important questions are:
Is the baby continuing to grow?
Is the placenta supporting the baby adequately?
Is the blood flow reassuring?
A small baby is not always a growth-restricted baby.
Regular monitoring helps healthcare providers distinguish between a healthy, naturally small fetus and a fetus that may require closer surveillance.
This article is for general pregnancy and ultrasound education and is not a substitute for individualized medical advice, diagnosis or treatment.
