During a detailed fetal ultrasound, multiple cyst-like spaces may occasionally be seen in one of the baby’s kidneys.
One possible diagnosis is Multicystic Dysplastic Kidney (MCDK).
Hearing that there is an abnormality in a baby’s kidney can understandably be concerning. However, when MCDK affects only one kidney and the other kidney is normal, the overall prognosis is generally favorable.
Let’s look at what MCDK means and what sonographers and doctors evaluate on prenatal ultrasound.
What Is Multicystic Dysplastic Kidney?
Multicystic dysplastic kidney is a congenital condition in which one kidney does not develop normally.
Instead of normal renal tissue, the affected kidney contains multiple cysts of different sizes.
Because the normal renal architecture is replaced by abnormal cystic tissue, the affected kidney usually has little or no useful function.
Fortunately, MCDK most commonly affects only one kidney, which is called unilateral MCDK.
What Does MCDK Look Like on Prenatal Ultrasound?
The typical ultrasound appearance includes:
- Multiple cysts of varying sizes
- Cysts that generally do not communicate with one another
- Loss or distortion of the normal renal architecture
- Reduced or poorly identifiable normal renal parenchyma
The kidney may look like a cluster of several round, fluid-filled spaces rather than a normally shaped fetal kidney.
This characteristic appearance can often raise suspicion for MCDK during a prenatal ultrasound.
MCDK vs. Hydronephrosis
One important differential diagnosis is severe hydronephrosis.
They can sometimes appear similar because both conditions may produce multiple fluid-filled spaces within the renal area.
However, there is an important difference.
In hydronephrosis, the renal pelvis and calyces are dilated and usually communicate with each other.
In MCDK, the cysts tend to be separate, variable in size, and non-communicating, with abnormal or absent normal renal architecture.
In some cases, follow-up prenatal ultrasound and postnatal imaging are needed to establish the final diagnosis.
Why Is the Other Kidney So Important?
When unilateral MCDK is suspected, evaluating the contralateral kidney is extremely important.
The ultrasound examination should assess:
- Size and appearance of the opposite kidney
- Renal echogenicity
- Renal pelvis and collecting system
- Possible urinary tract dilation
- Fetal bladder
- Amniotic fluid volume
- Other associated fetal abnormalities
The unaffected kidney may eventually become larger as it compensates for the poorly functioning kidney. This is known as compensatory hypertrophy or compensatory growth.
Why Do We Check the Fetal Bladder?
Seeing the fetal bladder fill normally is reassuring.
During the second half of pregnancy, fetal urine contributes significantly to amniotic fluid.
Therefore, when the opposite kidney is functioning normally, the fetal bladder usually fills and the amniotic fluid volume generally remains normal.
This is why we don’t evaluate only the abnormal kidney.
The opposite kidney, bladder, and amniotic fluid are equally important parts of the examination.
What Is the Prognosis for Unilateral MCDK?
When the findings are:
Unilateral MCDK + normal opposite kidney + normal bladder + normal amniotic fluid + no major associated abnormalities
the prognosis is generally very good.
A person can live a healthy life with one functioning kidney.
Over time, the healthy kidney may grow larger and take over the necessary renal function.
Does the Abnormal Kidney Keep Growing?
Not necessarily.
The affected kidney may initially appear enlarged because of the multiple cysts.
During later pregnancy or after birth, however, the dysplastic kidney may gradually become smaller.
In some children, it eventually becomes very small or difficult to visualize.
For this reason, follow-up imaging is usually recommended.
What Happens After Birth?
After delivery, the baby will usually have a renal ultrasound to reassess both kidneys and the urinary tract.
Doctors may evaluate:
- The appearance of the affected kidney
- The size and structure of the healthy kidney
- The renal pelvis and urinary tract
- Whether additional testing or follow-up is necessary
Most babies with uncomplicated unilateral MCDK do not automatically require surgery.
Long-term follow-up may be recommended to monitor the functioning kidney, blood pressure, and overall renal health.
What If Both Kidneys Are Affected?
Bilateral MCDK is very different from unilateral MCDK.
If both kidneys are severely affected, fetal urine production can become significantly reduced.
This may lead to severe oligohydramnios, which can interfere with normal fetal lung development.
Therefore, one of the most important questions after detecting MCDK is whether the condition is unilateral or bilateral.
A Key Point for Parents
Finding multiple cysts in a fetal kidney can sound frightening, but the ultrasound findings need to be considered as a whole.
When MCDK affects only one kidney, the most important questions are:
Is the other kidney normal?
Is the bladder filling normally?
Is the amniotic fluid normal?
Are there any other fetal abnormalities?
If the answers are reassuring, isolated unilateral MCDK generally has a favorable prognosis.
This article is intended for general educational purposes and does not replace individualized evaluation or diagnosis by an obstetrician, maternal-fetal medicine specialist, pediatrician, or pediatric nephrology/urology specialist.
