Understanding Renal Agenesis: Biology & Diagnosis

Finding out during a prenatal ultrasound that your baby may be missing one or both kidneys can be an overwhelming and frightening experience. It is normal to feel a range of emotions, from shock and confusion to deep concern for your child’s future. Renal agenesis is a condition where one or both kidneys fail to develop before birth. While the news is difficult, understanding the biology and the diagnostic process can help you feel more prepared for the conversations ahead with your medical team.

The Biology of Kidney Development

Kidney development is a complex “handshake” between two specific types of embryonic tissue. Around the fifth week of pregnancy, a structure called the ureteric bud (which eventually becomes the drainage system) must signal the metanephric mesenchyme (the tissue that becomes the kidney’s filtering units) ^[1].

If this signaling is interrupted—due to genetic factors or chemical pathways like GDNF/RET signaling—the kidney fails to form ^[2][3]. This is a breakdown in a highly intricate biological process and is not caused by the mother’s diet, stress, or behavior during pregnancy ^[4].

Unilateral vs. Bilateral Renal Agenesis

There are two distinct forms of this condition, and the outlook for each is very different:

• Unilateral Renal Agenesis (URA): This occurs when only one kidney fails to form. It is more common than the bilateral form ^[5]. Most children with URA lead healthy lives because the remaining kidney often grows larger to handle the work of two—a process called compensatory hypertrophy ^[6].
• Bilateral Renal Agenesis (BRA): This is a much more serious condition where neither kidney develops. Because kidneys produce fetal urine, which makes up most of the amniotic fluid (the protective liquid around the baby), BRA leads to a severe lack of fluid called oligohydramnios ^[7][8]. This lack of fluid can affect how the baby’s lungs and limbs develop, a pattern of features known as Potter sequence ^[9].

How Doctors See What’s Missing

Diagnosing renal agenesis prenatally requires specialized imaging. Doctors look for more than just an empty space; they look for secondary signs that confirm a kidney never formed:

The “Lying Down” Adrenal Gland

Normally, the adrenal gland (a small hormone-producing gland that sits on top of the kidney) is kept in an upright or curved shape by the kidney itself. If the kidney is missing, the adrenal gland loses its “shelf” and flattens out, appearing to “lie down” horizontally in the empty space on the ultrasound ^[10]. Seeing this “lying down adrenal gland” sign is a key clue for radiologists ^[11].

Color Doppler Imaging

Doctors often use color Doppler, a type of ultrasound that shows blood flow. By looking at the abdominal aorta (the main artery in the belly), they check for the presence of renal arteries. If no artery is branching out toward where the kidney should be, it confirms that the kidney never started developing ^[11][12].

Fetal MRI

In cases of BRA where amniotic fluid is very low, the ultrasound image can be blurry. A fetal MRI may be used to get a clearer, more detailed view of the baby’s internal structures to confirm the diagnosis ^[11].

Genetic Factors

While many cases occur randomly, research has identified specific genetic variants (such as in the GREB1L or WNT9B genes) that play a role in how the kidneys form ^[13][4]. Sometimes, renal agenesis is part of a broader syndrome that might affect other organs like the heart, spine, or reproductive system ^[14][15]. Genetic counseling is often recommended to help families understand the risk of recurrence in future pregnancies ^[16].