During a pregnancy ultrasound, you may hear:
“The amniotic fluid looks lower than expected.”
The medical term for this is:
Oligohydramnios
It simply means that the amount of amniotic fluid surrounding the baby is lower than expected for the stage of pregnancy.
Naturally, many parents immediately wonder:
Is my baby okay?
Is the placenta not working properly?
Did my water break?
Will I need to deliver early?
The answer depends on much more than one fluid measurement.
The most important questions are:
How low is the fluid?
How many weeks pregnant are you?
Is the baby growing normally?
Could the membranes have ruptured?
Do the kidneys and bladder look normal?
What Is Amniotic Fluid?
Amniotic fluid surrounds the fetus inside the amniotic sac.
It provides space for fetal movement, cushions the baby, and contributes to normal development.
During the second half of pregnancy, fetal urine becomes a major source of amniotic fluid.
The fetus continuously:
swallows fluid
and
produces urine.
So the amount of amniotic fluid reflects several parts of fetal and placental physiology.
How Is Low Amniotic Fluid Measured?
Ultrasound commonly evaluates fluid using:
Deepest Vertical Pocket — DVP
or
Amniotic Fluid Index — AFI
For a DVP measurement, the sonographer identifies an appropriate pocket of fluid without fetal parts or umbilical cord crossing through the measured area.
A:
DVP < 2 cm
is commonly used as a threshold for oligohydramnios.
An:
AFI ≤ 5 cm
has also traditionally been used.
The method used may depend on the clinical setting and type of pregnancy.
Why Doesn’t the Sonographer Just Measure Any Dark Space?
On ultrasound, amniotic fluid appears dark.
But not every dark area is an appropriate fluid pocket.
The sonographer needs to avoid including:
fetal limbs
umbilical cord
or
other structures
inside the measurement.
Color Doppler can sometimes help confirm that an apparent pocket does not contain umbilical cord.
This is why fluid assessment is more than simply finding the biggest black area on the screen.
What Can Cause Oligohydramnios?
There are several possible explanations.
Rupture of membranes
If the amniotic sac leaks or ruptures, fluid may escape.
This can cause the amniotic fluid volume to decrease.
Placental dysfunction
Later in pregnancy, reduced placental function may be associated with decreased fetal urine production and lower fluid volume.
This is particularly important when low fluid occurs together with fetal growth restriction.
Fetal urinary tract abnormalities
Because fetal urine contributes significantly to amniotic fluid later in pregnancy, abnormalities involving:
kidneys
urinary tract
or
bladder outflow
can sometimes result in very low fluid.
Post-term pregnancy
Amniotic fluid may decrease as pregnancy extends beyond the due date.
Medications or maternal conditions
Certain maternal medical conditions and medications can influence amniotic fluid.
And sometimes:
no single clear cause is identified.
Why Does Gestational Age Matter So Much?
Low fluid at 18 weeks is not interpreted the same way as low fluid at 39 weeks.
Early and severe oligohydramnios deserves particularly careful evaluation because amniotic fluid is important for fetal movement and lung development.
Near term, isolated low fluid presents a very different clinical situation.
So whenever you hear:
“The fluid is low,”
the next piece of information should be:
“At what gestational age?”
Why Are the Baby’s Kidneys Checked?
During the second half of pregnancy, the fetal kidneys continuously produce urine.
Therefore, when fluid is markedly reduced, the sonographer may carefully evaluate:
- both kidneys
- renal appearance
- renal pelvises
- urinary bladder
- bladder filling
- urinary tract
- fetal anatomy
Seeing both kidneys and repeated bladder filling can provide useful information.
Why Is the Bladder Important?
A visible fetal bladder indicates that urine is reaching the bladder.
During an ultrasound, the bladder may:
fill
and later
empty.
If severe oligohydramnios is present and the bladder is persistently not visualized, the urinary system deserves particularly careful evaluation.
But a temporarily empty bladder on a single image does not automatically indicate a problem.
Timing matters.
What If My Water Has Broken?
Low fluid can be a clue to rupture of membranes, but ultrasound alone does not diagnose every case.
If you have:
persistent watery vaginal leakage
a sudden gush of fluid
or
concern that your membranes may have ruptured,
contact your maternity team promptly.
Clinical examination and additional testing may be required.
Is Low Fluid a Sign That the Placenta Is Failing?
Not automatically.
But later in pregnancy, oligohydramnios can sometimes occur with placental insufficiency.
If the placenta is not delivering enough oxygen and nutrients, fetal circulation can adapt.
Reduced renal blood flow may lead to lower fetal urine production.
That is why low fluid combined with:
fetal growth restriction
may lead to a more detailed Doppler assessment.
Why Is Fetal Growth Checked?
The sonographer may measure:
- head circumference
- abdominal circumference
- femur length
- estimated fetal weight
If the baby is small and the fluid is also low, clinicians may become more concerned about placental function.
On the other hand:
normal growth + reassuring Doppler + mildly reduced fluid
creates a different clinical picture.
Again, context matters.
Will I Need Doppler Ultrasound?
Possibly.
If fetal growth restriction or placental dysfunction is suspected, umbilical artery Doppler may be used to assess placental resistance.
Other Doppler studies may be added depending on the clinical situation.
However:
Oligohydramnios alone does not automatically mean every Doppler study will be abnormal.
Can Dehydration Cause Low Amniotic Fluid?
Maternal hydration can influence measured amniotic fluid volume to some degree, and increasing oral fluid intake may modestly increase fluid measurements in some situations.
But significant oligohydramnios should not simply be dismissed as dehydration.
If fluid is genuinely low, clinicians still need to consider the underlying cause.
Drinking water is not a substitute for appropriate evaluation.
Can Amniotic Fluid Increase Again?
Yes, sometimes.
Fluid measurements can change.
A mildly reduced measurement may be higher on a subsequent examination.
Changes can reflect:
true fluid variation
maternal hydration
fetal urine production
measurement variation
or changes in the underlying condition.
This is why trends can be more useful than one borderline measurement.
What Is Borderline Low Fluid?
You may sometimes hear the phrase:
“borderline AFI.”
This generally refers to fluid that is toward the lower end of the expected range but does not necessarily meet the diagnostic threshold for oligohydramnios.
Definitions vary.
A borderline measurement should therefore be interpreted according to:
gestational age
fetal growth
Doppler findings
and
the trend over time.
Can Low Fluid Affect the Baby?
The implications depend strongly on:
severity
gestational age
and
cause.
Severe oligohydramnios very early in pregnancy can interfere with normal lung development and fetal movement.
Later in pregnancy, low fluid may be associated with:
umbilical cord compression
and changes in fetal heart rate during labor.
But mild isolated low fluid near term is a very different situation from prolonged severe oligohydramnios early in pregnancy.
They should not be treated as the same diagnosis.
Why Can the Cord Be Compressed?
Amniotic fluid creates space around the fetus and umbilical cord.
When fluid becomes very low, there is less cushioning around the cord.
During fetal movement or contractions, this may increase the chance of intermittent cord compression.
This is one reason fetal monitoring may become important, particularly during labor.
Will I Need NST or BPP Monitoring?
Depending on gestational age and the clinical situation, your provider may recommend fetal surveillance such as:
NST — Nonstress Test
or
BPP — Biophysical Profile
A BPP includes ultrasound assessment of fetal activities and amniotic fluid.
The monitoring schedule depends on the reason for the oligohydramnios and the overall pregnancy.
Does Oligohydramnios Mean I Need a C-Section?
No.
Low amniotic fluid alone does not automatically mean cesarean delivery.
Many patients can still have a vaginal delivery.
However, fetal heart rate monitoring during labor may be particularly important because reduced fluid can increase the likelihood of cord compression.
A C-section may become necessary for the usual obstetric reasons or if fetal monitoring becomes concerning.
Does Low Fluid Mean I Need to Deliver Early?
Sometimes—but not always.
Delivery timing depends on:
- gestational age
- severity of oligohydramnios
- fetal growth
- Doppler findings
- membrane status
- fetal surveillance
- maternal conditions
- other pregnancy complications
Near term, confirmed oligohydramnios may influence delivery timing.
Earlier in pregnancy, clinicians must balance the risks of prematurity against the risks associated with the underlying cause of low fluid.
There is no single delivery week that applies to every case.
Is Oligohydramnios the Opposite of Polyhydramnios?
In terms of fluid volume, yes.
Polyhydramnios
More amniotic fluid than expected
Oligohydramnios
Less amniotic fluid than expected
But both are descriptions, not complete diagnoses.
With either finding, the important task is figuring out:
Why is the fluid abnormal?
Did I Cause It?
Usually, no.
Oligohydramnios is not simply caused by:
working too much
walking
sleeping in the wrong position
or
something you ate.
And although maintaining appropriate hydration is sensible during pregnancy, significant low fluid should not be treated by blaming yourself for not drinking enough water.
The underlying reason needs to be evaluated.
Questions to Ask Your Doctor
If oligohydramnios is identified, useful questions include:
- What is my DVP or AFI?
- How low is the fluid?
- Is this true oligohydramnios or just low-normal fluid?
- Could my membranes have ruptured?
- Do both kidneys look normal?
- Is the fetal bladder filling normally?
- Is my baby growing normally?
- Are Doppler studies reassuring?
- Has the fluid changed since my previous scan?
- Do I need another ultrasound?
- Will I need NST or BPP monitoring?
- Does this affect delivery timing?
And perhaps the most useful question:
“Do we know why the fluid is low?”
Key Takeaway
Oligohydramnios means there is less amniotic fluid than expected.
A deepest vertical pocket below 2 cm is commonly used to define it, while an AFI of 5 cm or less has also traditionally been used.
But the measurement is only the beginning.
The real ultrasound assessment asks:
Is the baby growing normally?
Are the kidneys and bladder normal?
Could the membranes be leaking?
Are there signs of placental dysfunction?
How many weeks pregnant are you?
Is the fluid stable or getting lower?
A mildly low measurement near term and severe oligohydramnios early in pregnancy are not the same clinical situation.
So if you’re told:
“Your fluid is low,”
don’t focus only on the number.
Ask:
“What does the rest of the ultrasound tell us about why?”
💧👶
About the Author
This article was written by a sonographer with over 20 years of hands-on clinical ultrasound experience, including fetal, breast, and thyroid imaging.
When amniotic fluid looks low, I don’t consider the fluid measurement in isolation. I also look at fetal growth, the kidneys and bladder, cord-containing pockets, and—when clinically appropriate—Doppler findings. Gestational age matters enormously: the same fluid measurement can have very different significance depending on when and why it occurs.
This article is for general educational purposes and does not replace individualized prenatal diagnosis, evaluation for rupture of membranes, maternal-fetal medicine assessment, or medical advice from your healthcare provider.
