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PubMed This is a summary of 22 peer-reviewed journal articles Updated
Obstetrics

The Biology of MCDK: How and Why It Forms

At a Glance

Multicystic dysplastic kidney (MCDK) occurs when normal fetal kidney development is interrupted, causing the organ to form fluid-filled cysts instead of working tissue. While unilateral MCDK affects one kidney and has an excellent outlook, bilateral MCDK affects both and is a severe condition.

To understand Multicystic Dysplastic Kidney (MCDK), it helps to look at how a kidney “grows” in the womb. Kidney development is like a complex conversation between two specific types of tissue. When that conversation is interrupted, the kidney forms into a collection of cysts rather than a working organ [1][2].

The Biological “Missed Connection”

Early in pregnancy, two structures must meet and “talk” to each other to create a kidney:

  1. The Ureteric Bud: This eventually becomes the drainage system (the ureter and collecting ducts).
  2. The Metanephric Mesenchyme (Blastema): This tissue eventually becomes the filtering units of the kidney.

In a healthy pregnancy, the ureteric bud grows into the mesenchyme, and they exchange signals that tell the tissue to start forming nephrons (filters) [3][4]. In MCDK, this interaction fails [1]. Because the tissue doesn’t receive the right signals to become filters, it turns into fluid-filled sacs called cysts and fibrous, non-working tissue [5].

Unilateral vs. Bilateral MCDK

The impact of this developmental “missed connection” depends entirely on whether it happens to one kidney or both.

Unilateral MCDK (One Side)

In the vast majority of cases, MCDK only affects one kidney [6]. Because the other kidney is healthy and functional, it produces enough urine to fill the amniotic sac, allowing the baby’s lungs to develop normally [7]. The prognosis for these children is excellent, and they typically lead normal lives with one fully functioning kidney that grows to handle the extra work [8][9].

Bilateral MCDK (Both Sides)

If both kidneys are affected, the situation is much more serious. Because neither kidney can filter blood or create urine, there is a lack of amniotic fluid (oligohydramnios) [10]. Amniotic fluid is essential for lung development; without it, the lungs remain small and cannot function after birth (pulmonary hypoplasia) [11]. This set of physical findings—including specific facial features and limb changes caused by the lack of fluid—is known as Potter sequence [12].

A diagnosis of bilateral MCDK carries profound emotional weight. If your baby has been diagnosed with bilateral MCDK, specialized neonatal and pediatric palliative care teams will step in to support your family, discuss your baby’s specific outlook, and help you navigate the difficult decisions ahead [13][14].

The Role of Genetics

While most cases of MCDK happen by chance, doctors are increasingly finding that some cases are linked to specific genetic changes.

  • HNF1B Mutations: This is one of the most common genetic links to kidney cysts. This gene helps control the development of the kidneys, pancreas, and liver [15].
  • 17q12 Microdeletion Syndrome: This is a small “missing” piece of genetic material on chromosome 17 that includes the HNF1B gene [12]. Children with this deletion may have kidney issues along with other findings, such as a smaller-than-average pancreas or learning differences [15].

When to Consider Genetic Counseling

Your doctor may recommend a meeting with a genetic counselor to discuss testing if:

  • The MCDK is bilateral (on both sides) [16].
  • There are extrarenal anomalies (findings in other organs like the heart or brain) [17].
  • There is a family history of kidney cysts, single kidneys, or early-onset diabetes [18].
  • The medical team suggests a Chromosomal Microarray (CMA) or Whole-Exome Sequencing (WES) to get a clearer picture of your baby’s health and future needs [19][20].

Genetic testing can help your team provide a more accurate prognosis and tailor your baby’s follow-up care [21][22].

Common questions in this guide

How does a multicystic dysplastic kidney form?
MCDK forms early in pregnancy when the two types of tissue needed to create a kidney fail to communicate properly. Without the right developmental signals, the tissue turns into fluid-filled cysts and non-working scar tissue instead of a healthy filtering organ.
What is the difference between unilateral and bilateral MCDK?
Unilateral MCDK affects only one kidney and generally has an excellent prognosis because the healthy kidney compensates for the affected one. Bilateral MCDK affects both kidneys, leading to a dangerous lack of amniotic fluid and severe developmental complications for the baby.
Is multicystic dysplastic kidney genetic?
While most cases happen by chance, some are linked to specific genetic changes like HNF1B mutations or 17q12 Microdeletion Syndrome. A genetic counselor can help determine if testing is appropriate based on your family history or ultrasound findings.
What is Potter sequence in bilateral MCDK?
Potter sequence is a group of physical findings caused by a severe lack of amniotic fluid when neither kidney functions. Without sufficient amniotic fluid, the baby's lungs cannot develop properly, leading to a life-threatening condition called pulmonary hypoplasia.
When is genetic counseling recommended for MCDK?
Genetic testing is often recommended if the MCDK affects both kidneys, if there are abnormalities in other organs, or if there is a family history of kidney issues. Tests like a Chromosomal Microarray or Whole-Exome Sequencing can help your care team provide a clearer prognosis.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does the ultrasound show the MCDK on one side (unilateral) or both sides (bilateral)?
  2. 2.Was the interaction between the ureteric bud and the tissue normal on the healthy side?
  3. 3.Are there any other structural findings in the heart, pancreas, or limbs that might suggest a genetic syndrome?
  4. 4.Given our specific findings, do you recommend a chromosomal microarray (CMA) or whole-exome sequencing (WES)?
  5. 5.How does the presence or absence of a genetic mutation change our monitoring plan for my baby?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (22)
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    Sequence variants in the renin-angiotensin system genes are associated with isolated multicystic dysplastic kidney in children.

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This page provides educational information about the biology and causes of MCDK. Always consult your obstetrician, genetic counselor, or pediatric specialist for guidance regarding your pregnancy and baby's health.

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