Can You Be Born With Only One Kidney?

Roughly 1 in every 2,500 to 5,000 babies is born with a single functioning kidney, a condition doctors call unilateral renal agenesis when one kidney simply never forms, or a solitary functioning kidney when one is present but nonfunctional. Most people born this way lead entirely normal lives, and many never learn about it until an imaging scan done for an unrelated reason reveals the surprise. The story gets more interesting, though, when you look at how the body adapts, what risks quietly accumulate, and why the condition sometimes brings company in the form of other developmental differences.

How Common Is It

Birth-defect registries put the prevalence of being born with one kidney somewhere in the range of about 2 to 4 per 10,000 births. A European registry analysis covering over 440,000 births found a prevalence of 4.0 per 10,000, while a massive Chinese surveillance study tracking over 25 million births between 2007 and 2020 reported a rate of about 1.9 per 10,000 for unilateral renal agenesis.¹² The spread between those numbers likely reflects differences in screening practices and reporting standards rather than a true geographic gap. The Chinese data also showed the detection rate climbing over the study period, which makes sense as prenatal ultrasound technology and access improved.

These numbers only capture cases where a kidney is completely absent. There is a related condition, multicystic dysplastic kidney, where a kidney forms but is filled with nonfunctional cysts and eventually shrivels. Functionally, a child with a multicystic dysplastic kidney ends up living with a single working kidney too. A Korean study found that over 90 percent of those dysplastic kidneys had completely or partially disappeared by age two.³ When you add these cases to the agenesis numbers, the total population living on a single kidney from birth is larger than any one registry captures.

Why a Kidney Fails to Form

During early fetal development, the kidneys take shape through a precise back-and-forth signaling process between two structures: a bud that grows out from the embryonic urinary tract and a cluster of precursor cells that will become the filtering tissue. If either side of that signaling conversation breaks down on one side of the body, the kidney on that side simply never develops. Several genes are known to play a role. Mutations in PAX2, for instance, cause a condition called renal-coloboma syndrome that can include partial or complete failure of urinary tract growth on one side.⁴ A genetic sequencing study of children with unilateral renal agenesis identified variants in genes like CHD7 and PROKR2, both of which are involved in broader developmental pathways that affect multiple organ systems.⁵

Genetics is not the whole story, though. The environment inside the womb matters too. Maternal diabetes, particularly diabetes that exists before pregnancy, has been linked to a higher risk of kidney and urinary tract malformations in the child. A large study using a conditional multivariable model found that pregestational diabetes was associated with about a 67 percent increase in the odds of these malformations, while gestational diabetes that developed during pregnancy did not reach the same threshold of risk.⁶ Animal research has helped explain the mechanism: in mice, maternal diabetes led to offspring with lower kidney weight and fewer filtering units, driven in part by increased cell death in the developing kidney tissue.⁷ A review examining how high blood sugar during pregnancy could affect fetal kidney development points to changes in how genes get switched on and off as the likely bridge between the mother’s metabolic state and the baby’s kidney formation.⁸

How It Gets Detected

Most cases are picked up during routine prenatal ultrasound, but the detection rate is far from perfect. Research suggests that up to 38 percent of cases of unilateral renal agenesis are missed on antenatal screening.⁹ That is a surprisingly high miss rate for what seems like a straightforward observation: one kidney instead of two. The difficulty comes from the fact that the remaining kidney often looks perfectly healthy, and a missing structure is inherently harder to notice than an abnormal one. Fetal position, gestational age, and operator experience all affect the odds of catching it.

When a single kidney is found on prenatal ultrasound, one clue that often shows up is that the detected kidney is already larger than expected. Ultrasound studies of normal fetuses between 13 and 22 weeks have shown that in cases with a single kidney, the kidney length was above the 95th percentile.¹⁰ That oversized kidney is actually the body’s compensation process already underway, which is generally a good sign.

For those who slip through prenatal screening, the condition sometimes surfaces during childhood imaging for a urinary tract infection or an unrelated abdominal issue. In other cases, it is found during a sports physical or routine checkup in adolescence. And sometimes it goes entirely unnoticed into adulthood. A case report described a man born prematurely as a twin who was not diagnosed with a solitary kidney and associated kidney abnormality until adulthood.¹¹

How the Remaining Kidney Compensates

The body’s response to having a single kidney is one of the more elegant adaptations in human physiology. The remaining kidney grows larger to take on the workload of two, a process called compensatory hypertrophy. Timing matters here. If the kidney mass reduction occurs before birth, while new filtering units are still being formed, the remaining kidney can actually create additional units through a process called hyperplasia, not just enlarge the existing ones. After birth, when the window for generating new units has closed, the kidney can only grow by making each existing unit larger.¹²

This distinction is clinically meaningful. People born with a single kidney (where the other side never formed at all) have had their entire gestational period for the remaining kidney to adapt, potentially adding extra filtering units. Research bears this out: about a quarter to half of cases detected before birth show signs of hypertrophy, which is associated with a somewhat reduced risk of kidney injury later.⁹ In children born with a multicystic dysplastic kidney on the other side, compensatory enlargement of the working kidney was common in newborns, dipped in early childhood, and then increased again through adolescence, reaching over 80 percent by age nine or ten.³

This compensatory growth is why many people with a congenital solitary kidney never experience symptoms. The single kidney can handle the body’s filtering demands comfortably for decades. But “can handle” and “can handle indefinitely without any strain” are not the same thing, which is where long-term risks come in.

Long-Term Health Risks

The evidence here is more sobering than the reassuring message many families receive at diagnosis. Research has shown that being born with a single functioning kidney leads to signs of kidney injury before adulthood in over half of those affected.⁹ That does not mean half of these children end up on dialysis; “renal injury” in this context includes early markers like protein in the urine or elevated blood pressure, which are warning signs rather than full-blown kidney failure. But they do signal that the single kidney is working under strain.

High blood pressure is the most common concern. A study comparing children born with a solitary functioning kidney to healthy controls found that the prevalence of hypertension was about 33 percent in the solitary kidney group, compared to roughly 13 percent in controls.¹³ Much of the difference came from what is called masked hypertension, where blood pressure reads normal in a clinic but runs high the rest of the day. That pattern is easy to miss with standard office measurements, which is one reason 24-hour ambulatory blood pressure monitoring is increasingly recommended for this group.

The underlying theory is that fewer filtering units means each one has to work harder, filtering more blood per unit. Over years and decades, that increased workload can gradually damage the filtering structures themselves. Children born with one kidney who show early compensatory hypertrophy tend to do better, possibly because their kidney managed to build a larger workforce of filtering units during fetal development. Those without obvious compensatory enlargement are considered at higher risk.¹⁴ Congenital anomalies of the kidney and urinary tract remain the leading cause of end-stage kidney disease in children, and children with a solitary functioning kidney form an important subgroup within that population.¹⁵

Conditions That Tag Along

Being born without one kidney is rarely just about the kidney. The same developmental window that shapes the kidneys also shapes the reproductive tract, so abnormalities in one system often coincide with abnormalities in the other. In females, unilateral renal agenesis is associated with Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome, where the uterus and upper vagina are underdeveloped or absent. In males, it is linked to Zinner syndrome, involving cysts of the ejaculatory duct system. These genital malformations may go unnoticed until puberty, when they become apparent through absent menstruation or reproductive difficulties.¹⁶

Broader developmental conditions can also include the kidney as one feature among many. Kallmann’s syndrome, which affects sense of smell and hormone production, is associated with absent kidneys in some cases due to shared signaling pathways during embryonic development.⁴ The gene CHD7, identified in children with unilateral renal agenesis, is also responsible for CHARGE syndrome, which involves heart defects, eye abnormalities, and other developmental differences.⁵ The practical takeaway is that when a missing kidney is discovered, doctors should look carefully for other structural differences, even if none are immediately apparent.

Monitoring and Follow-Up

Italian pediatric nephrology guidelines, the most detailed consensus recommendations published for this group, recommend lifelong follow-up tailored to a risk-stratification system. A child with a congenital solitary kidney that shows compensatory enlargement and no other urinary tract anomalies is classified as low risk and needs periodic ultrasounds, urine tests, and blood pressure checks. A child whose kidney has not enlarged or who has additional urinary tract anomalies is medium risk and should also have kidney function measured by blood tests. Those who develop decreased kidney function, protein in the urine, or high blood pressure move into a high-risk category with a more individualized monitoring schedule.¹⁴

For adults, management focuses on blood pressure control, monitoring for protein in the urine, and avoiding substances that are hard on the kidneys. When kidney function does eventually decline to advanced stages, replacement therapies such as dialysis become necessary, and cardiovascular health monitoring becomes particularly important because kidney disease and heart disease share a tight, bidirectional relationship.¹¹

Diet and the Single Kidney

Dietary advice for people with an acquired solitary kidney (such as after donation or cancer surgery) is well studied, and many of the same principles apply to those born with one kidney. Expert recommendations suggest avoiding excessively high protein intake, keeping it under about 1 gram per kilogram of body weight per day, and limiting sodium to under 4 grams daily. Adequate fiber from plant-based foods and maintaining a body mass index under 30 are also recommended.¹⁷

The rationale behind the protein recommendation comes from decades of animal and human research showing that high protein intake increases the filtering rate per unit of kidney tissue. In rats with reduced kidney mass, a high-protein diet led to higher filtering rates but also significantly more scarring of the filtering units compared to rats on a lower-protein diet, and the degree of injury tracked directly with how much kidney mass was missing to begin with.¹⁸ You do not need to become a vegetarian or obsessively count protein grams. The concern is about consistently very high intakes, the kind common in bodybuilding diets or heavy meat-centric eating patterns. A balanced diet with moderate protein is generally sufficient without formal restriction unless kidney function tests suggest otherwise.

Sports and Physical Activity

One of the most common worries for parents of children born with a single kidney is whether contact sports are safe. The fear is straightforward: if your only kidney takes a hard blow, the consequences are catastrophic in a way they would not be if you had a spare. The reality is more nuanced. Kidney injuries from sports are uncommon, and the kidney sits in a relatively protected position behind the rib cage and surrounded by fat and muscle. A narrative review noted that there are currently no World Health Organization or European consensus guidelines on sports participation for children and young athletes with a solitary kidney.¹⁹

In practice, many pediatric nephrologists and sports medicine physicians take a case-by-case approach. Some recommend avoiding high-collision sports like football or martial arts in favor of lower-contact activities, while others feel the risk is low enough that blanket restrictions are unwarranted and may do more harm through lost fitness and social development than the minuscule chance of catastrophic kidney trauma. Protective equipment, including padded vests designed for the torso, is sometimes suggested as a compromise. The conversation is worth having with your child’s doctor, but a diagnosis of a solitary kidney does not automatically disqualify anyone from being active.

When the Diagnosis Comes as an Adult Surprise

A substantial number of people born with one kidney discover the fact well into adulthood. Because the remaining kidney compensates so effectively, there are often no symptoms, no abnormal blood work, and no reason for anyone to look. The discovery typically happens during an abdominal scan for something else entirely, like investigating back pain or checking for gallstones. For someone who has been healthy their whole life, learning they have been operating on a single kidney can be disorienting, even unsettling.

The important context here is that having reached adulthood without kidney problems is actually a good prognostic sign. The kidney has already proven it can handle a full lifetime of demand without failing. That said, the same long-term monitoring recommendations still apply: regular blood pressure checks, periodic urine tests to look for protein, and attention to kidney-friendly habits around diet and hydration. People diagnosed late should also mention the finding to any new doctor, because it affects decisions about certain medications and imaging procedures that use contrast dye, both of which require extra caution when there is only one kidney doing all the work.

The discovery also sometimes raises questions about family planning. Because some causes of unilateral renal agenesis have a genetic component, a person found to have the condition may want to discuss it with a geneticist, particularly if they are planning to have children and want to understand whether the trait might recur. Familial patterns have been documented, though most cases appear to be sporadic.