If you’ve been told that your baby is measuring small, you may come across two terms:
FGR and IUGR.
They are often used to describe a similar concern—a baby that may not be reaching its expected growth potential—but the terminology has changed over time.
Today, FGR (Fetal Growth Restriction) is generally the preferred term in modern obstetric practice, while IUGR (Intrauterine Growth Restriction) is an older term that is still commonly used by patients, healthcare professionals, and online resources.
So what’s the difference?
What Does FGR Mean?
FGR stands for Fetal Growth Restriction.
It describes a fetus that may not be achieving its expected growth potential.
FGR is more than simply having a small baby.
Doctors evaluate fetal size together with other information that may include:
- estimated fetal weight (EFW)
- abdominal circumference (AC)
- growth trajectory
- umbilical artery Doppler
- other Doppler studies when indicated
- amniotic fluid
- placental findings
- maternal and pregnancy factors
The exact criteria used to diagnose and manage FGR can vary depending on clinical guidelines and the individual pregnancy.
What Does IUGR Mean?
IUGR stands for Intrauterine Growth Restriction.
For many years, IUGR was widely used to describe impaired fetal growth during pregnancy.
You may still hear the term from:
- patients
- healthcare professionals
- older medical records
- websites
- pregnancy forums
In many contexts, IUGR and FGR are referring to essentially the same clinical concept.
However, FGR is now generally preferred terminology.
Why Did the Terminology Change?
The term FGR focuses specifically on fetal growth rather than simply describing growth occurring “intrauterine.”
It also helps emphasize an important concept:
A fetus can be small without necessarily being growth restricted.
And a fetus may show concerning changes in growth even when a single measurement does not tell the entire story.
This distinction is important when interpreting ultrasound results.
FGR Is Not the Same as SGA
Another term you may encounter is:
SGA — Small for Gestational Age
SGA generally describes a baby or fetus whose size falls below a defined percentile for gestational age.
FGR, on the other hand, refers to concern that the fetus may not be reaching its biological growth potential.
These concepts overlap, but they are not identical.
A baby can be small and healthy.
Some babies are naturally small because of genetics and normal biological variation.
These babies may remain on a lower percentile while continuing to grow steadily.
A baby can be growth restricted.
In other pregnancies, a fetus may become smaller than expected because growth is being affected.
This is why doctors do not look at fetal weight alone.
Is Every Baby Below the 10th Percentile FGR?
Not necessarily.
The 10th percentile is an important threshold in fetal growth assessment, but a percentile by itself does not describe the entire pregnancy.
For example:
Baby A
28 weeks → 9th percentile
32 weeks → 10th percentile
36 weeks → 9th percentile
Doppler findings are reassuring and growth remains relatively consistent.
This baby may simply be constitutionally small.
Baby B
28 weeks → 45th percentile
32 weeks → 22nd percentile
36 weeks → 8th percentile
This baby shows a significant downward growth trajectory.
These two situations may need different clinical interpretation even though both eventually measure below the 10th percentile.
What Causes FGR?
There are many possible causes.
One important cause is placental dysfunction, where the placenta may not support fetal growth as effectively as expected.
Other factors can include:
- maternal medical conditions
- pregnancy complications
- fetal conditions
- chromosomal or genetic conditions
- some infections
- placental abnormalities
Sometimes no single clear cause is identified.
How Is FGR Evaluated on Ultrasound?
Ultrasound plays an important role in evaluating fetal growth.
Measurements commonly include:
BPD — Biparietal Diameter
Width of the baby’s head.
HC — Head Circumference
Circumference of the baby’s head.
AC — Abdominal Circumference
Circumference of the baby’s abdomen.
FL — Femur Length
Length of the thigh bone.
These measurements help calculate the estimated fetal weight (EFW).
Doctors then interpret fetal size together with the overall clinical picture.
Why Is Abdominal Circumference Important?
The abdominal circumference (AC) is particularly useful when evaluating fetal growth.
Sometimes the baby’s overall estimated weight may remain above a certain percentile while the abdominal circumference measures relatively small.
This finding may prompt closer evaluation or follow-up.
A small AC does not automatically mean FGR, but it can be an important part of the assessment.
What Does Doppler Tell Us?
Doppler ultrasound evaluates blood-flow patterns in fetal and placental vessels.
One commonly assessed vessel in suspected FGR is the umbilical artery.
Doppler findings can provide additional information about placental circulation and help doctors assess and monitor pregnancies affected by FGR.
Depending on the clinical situation, other Doppler measurements may also be used.
Can You Have FGR With Normal Doppler?
Yes.
A baby with suspected or diagnosed FGR can still have reassuring Doppler findings.
This is one reason a normal Doppler does not necessarily mean that follow-up is no longer needed.
Doctors may continue monitoring:
- fetal growth
- growth trajectory
- Doppler findings
- amniotic fluid
- fetal well-being
The monitoring plan depends on the individual pregnancy.
Early-Onset vs Late-Onset FGR
FGR can also be described according to when it develops.
Early-Onset FGR
This develops earlier in pregnancy and may be associated with more significant placental dysfunction or other underlying conditions.
Late-Onset FGR
This develops later in pregnancy and can sometimes be more subtle.
A baby may not appear dramatically small at first, which is why changes in growth trajectory and other findings can matter.
Can FGR Improve?
The answer depends on what is causing the growth restriction and how the pregnancy progresses.
Doctors cannot simply make a fetus grow faster through ultrasound monitoring.
Instead, monitoring helps determine whether the baby continues to grow and whether the intrauterine environment remains reassuring.
The goal is to balance continued pregnancy with the baby’s well-being and the risks of delivery at a particular gestational age.
FGR vs IUGR: The Simple Answer
For parents trying to understand an ultrasound report, the simplest explanation is:
IUGR is the older term.
FGR is the term more commonly preferred today.
Both are generally used to discuss concern that a fetus is not growing as expected.
But remember:
FGR does not simply mean “small baby.”
Doctors consider fetal size, growth over time, Doppler findings, amniotic fluid, and the overall clinical picture.
Key Takeaway
If you see IUGR in an older report or online article and FGR in a newer one, they are usually discussing the same broad clinical problem.
The more important distinction is often:
Is this baby naturally small, or is the baby not reaching its expected growth potential?
That question cannot be answered by one percentile alone.
Serial growth measurements and the rest of the pregnancy assessment provide the bigger picture.
This article is for general educational purposes and does not replace individualized medical advice from your obstetric healthcare provider.
