The Anatomy of Abby and Brittany Hensel: A Detailed Medical and Biological Overview

Abby and Brittany Hensel are dicephalic parapagus conjoined twins, a rare anatomical condition where two distinct heads and brains are joined to a single torso with a shared lower body. While each twin maintains independent neurological control over her respective side—Abby controlling the right half and Brittany the left—they share vital organ systems below the waist while maintaining separate hearts, stomachs, and lungs in the upper chest.

What Is Dicephalic Parapagus Conjoining?

To understand the anatomy of Abby and Brittany Hensel, one must first understand the embryological classification of conjoined twins. Conjoined twins develop from a single fertilized egg (monozygotic twins) that fails to fully separate during the early stages of blastocyst development, typically around the second week after fertilization.

Medical science classifies conjoined twins based on the anatomical region of fusion and the direction of the join. The term dicephalic parapagus breaks down into specific medical definitions:

• Dicephalic: Refers to having two separate heads. Each head houses an independent brain, brainstem, central nervous system, and skull structure.
• Parapagus: Indicates that the fusion occurs laterally (side-by-side). Unlike thoracopagus twins (joined front-to-front at the chest) or craniopagus twins (joined at the head), parapagus twins lie adjacent to one another along a single transverse plane.
• Dibrachius Tetrapus / Tribrachius: These secondary terms describe the limb configuration. Abby and Brittany are dibrachius, meaning they have two functional outer arms. At birth, they also possessed a rudimentary central arm between their necks, which was surgically removed during early infancy to allow normal head alignment and posture.

Dicephalic parapagus represents one of the rarest forms of conjoined twinning, accounting for less than 11 percent of all conjoined twin births worldwide. Because the majority of dicephalic parapagus infants suffer from severe structural cardiac anomalies or underdeveloped organ systems, survival into adulthood is exceptionally rare. The survival, growth, and physiological stability of Abby and Brittany offer medical science an unprecedented view into human anatomy and motor neurology.

• Abby & Brittany Hensel (Dicephalic Parapagus)
  • Head: Abby
    • Separate Brain
    • Right-side Control
  • Head: Brittany
    • Separate Brain
    • Left-side Control
  • Shared Torso
    • 2 Hearts (Shared Circulatory Loop)
    • 4 Lungs (Partially Joined Cavity)
    • 2 Stomachs
    • 1 Shared Liver & Gallbladder
    • 3 Kidneys
  • Shared Lower Anatomy
    • 1 Shared Reproductive System
    • 1 Shared Lower Digestive / Urinary System
    • 1 Shared Pelvis
    • 2 Functional Legs (Right: Abby / Left: Brittany)

Detailed Anatomical Breakdown of Internal Organs

The internal anatomical layout of Abby and Brittany Hensel features a combination of duplicate, shared, and modified organ systems. Moving from the cranial region downward, their anatomy transitions from completely duplicated organs to fully integrated single organs.

The Upper Chest and Cardiovascular System

In the upper thoracic cavity, each twin possesses her own heart. Abby’s heart lies on the right side of the broad chest, while Brittany’s lies on the left. However, these two hearts operate within a single, intertwined circulatory system.

The cardiac muscle contraction of each heart occurs independently, with separate sinus rhythms and pulse rates. Despite this independent pumping, the vascular network features cross-connections (anastomoses) that allow blood pumped by Abby’s heart to flow through Brittany’s vascular bed and vice versa.

This shared circulation has major physiological implications:

1. Medication and Hormones: Any drug, hormone, or chemical introduced into Abby's bloodstream (such as an antibiotic, caffeine, or pain reliever) quickly circulates through Brittany's body as well.
2. Blood Pressure Dynamics: Maintaining systemic arterial pressure requires proportional balancing between both hearts to ensure equal perfusion across the shared capillary networks.

The respiratory cavity houses four distinct lungs (two belonging to Abby and two belonging to Brittany), situated within an enlarged thoracic cavity. The medial (inner) lungs share proximity along the midline of the chest, while the lateral (outer) lungs function identically to those of a non-conjoined individual. Diaphragmatic motion is coordinated between them, allowing both twins to breathe in natural synchronization during sleep and exercise, though each retains independent voluntary control of her own breathing rate.

The Gastrointestinal and Abdominal Systems

Below the chest diaphragm, organ duplication becomes selective:

• Stomachs: There are two distinct, fully functioning stomachs. Abby and Brittany feel hunger independently. Food ingested by Abby passes through her esophagus into her right-side stomach, while food consumed by Brittany enters her left-side stomach.
• Liver and Gallbladder: Below the stomachs lies a single, enlarged liver. This shared liver processes metabolic waste, synthesizes proteins, and produces bile for both stomachs. A single gallbladder stores bile and secretes it into the digestive tract.
• Intestinal Tract: Digested food empties from the two separate stomachs into a single, combined intestinal tract. The upper small intestine integrates the chyme from both stomachs, leading into a single large intestine, colon, and rectum. Nutrient absorption and waste elimination are shared processes.

The Excretory and Reproductive Systems

The lower pelvic cavity is entirely single and shared, supported by a single broad pelvis.

• Renal System: The twins possess three kidneys. Two of these are situated laterally in the traditional anatomical locations (one on the far right belonging to Abby's side, one on the far left belonging to Brittany's side). A third, central kidney lies near the midline and functions as a shared or partially fused filtration unit. All three kidneys empty into a single urinary bladder.
• Reproductive Organs: Below the abdominal cavity, the twins have a single set of female reproductive organs, including one uterus, two ovaries (one associated with each side's vascular supply), and a single vaginal canal.

Neurological Structure and Motor Control

The neurological architecture of Abby and Brittany Hensel is one of the most remarkable aspects of human physiology. Each twin possesses a complete, distinct central nervous system, including:

1. A fully developed cerebrum, cerebellum, and brainstem.
2. An independent spinal cord terminating at the sacral region along her respective side of the combined spine.

Unilateral Motor Control

Motor innervation—the neural signals sent from the brain to control voluntary muscles—is split down the vertical midline of their shared body.

• Abby (Right Twin): Her motor cortex sends electrical signals down her spinal cord to operate the right arm, right leg, right neck muscles, and right-side torso posture. She has no direct voluntary motor control over the limbs or muscles on the left side of the body.
• Brittany (Left Twin): Her motor cortex operates the left arm, left leg, left neck muscles, and left-side torso posture. She has no direct motor control over the right side of the body.

This strict division of nervous control means that basic human movements—such as walking, sitting up, holding an object with two hands, or clapping—require continuous real-time coordination between two separate brains.

Neurological Coordination and Motor Learning

In non-conjoined human beings, bilateral motor tasks (such as typing on a keyboard or walking) are coordinated across the corpus callosum connecting the brain's left and right hemispheres. For Abby and Brittany, inter-individual coordination occurs entirely through external neural pathways and sensory feedback loops.

When walking, Abby must initiate the forward swing of the right leg precisely as Brittany plants the left foot and pushes off. This synchronization is not managed by a shared neural connection between their brains; rather, it relies on subconscious proprioceptive feedback (sensing muscle tension, balance, and spatial orientation) and decades of shared muscle memory built since early childhood.

Their brains have adapted to read subtle physical cues from one another. For example, during complex motor tasks:

• Driving a Car: Abby operates the foot pedals (gas and brake) with her right foot and controls the gear shift with her right hand, while Brittany controls the turn signals and lights with her left hand. Both share control of the steering wheel, with Abby holding the right side and Brittany holding the left.

• Typing on a Keyboard: Abby types the keys on the right half of the keyboard using her right hand, while Brittany types the keys on the left half using her left hand. They can type at normal speed without consciously speaking to each other about key selection.

• Sports and Physical Activity: Activities such as swimming, riding a bicycle, and playing volleyball require precise weight distribution and timing. They coordinate through tactile feedback—feeling the balance shift across their central torso—and rhythm.

• Abby's Brain

  • Right Spinal Cord
    • Right Arm
    • Right Leg
    • Right Torso

• Brittany's Brain

  • Left Spinal Cord
    • Left Arm
    • Left Leg
    • Left Torso

• Shared Balance Line

  • Proprioceptive & Tactile Feedback

Somatosensory Division and Threshold Overlap

Somatic sensation—the ability to feel touch, temperature, pain, and pressure—is also divided vertically.

If an external object touches Abby’s right arm or right leg, only Abby feels the sensation. If Brittany is touched on her left arm or left leg, only Brittany perceives it.

However, medical studies of dicephalic parapagus twins indicate that sensory boundaries are not absolute walls. At the central midline of their torso, where nerve endings overlap within the dermis and deeper muscular tissue, there is a narrow region of shared sensation. Both twins can perceive deep pressure or touch applied directly along the spinal midline or central sternum. Below the waist, visceral sensations from shared internal organs (such as bladder fullness or intestinal cramping) are typically felt by both twins, though perception can vary in intensity.

Physiological Functions: Immunity, Metabolism, and Homeostasis

Despite their shared lower body, Abby and Brittany experience distinct physiological states, highlighting the complex interplay between shared bodily fluids and separate neurological signaling.

Separate Illness and Immune Responses

Because they share a common blood supply, white blood cells, antibodies, and circulating pathogens move freely between both twins. However, their immune systems do not always respond identically to infections.

It is medically documented that one twin can contract a viral upper respiratory infection (such as a localized cold or sore throat) while the other remains symptom-free or experiences only mild discomfort. This phenomenon occurs because:

• Infections in the mucosal membranes of the nasal passages, throat, or specific lung tissue are localized to the tissue innervated and protected by that twin’s mucosal defenses.
• The autonomic nervous system, which influences local blood flow, body temperature regulation, and inflammatory responses, operates independently for each head and upper torso.

Consequently, Abby may run a fever while Brittany’s body temperature remains within normal limits, even though both hearts pump blood through the same central circulatory loop.

Metabolic Rate and Vital Signs

Because each twin possesses a separate brainstem, the autonomic functions governing basic survival—such as resting heart rate, blood pressure regulation, and baseline body temperature—can differ slightly between them.

• Heart Rates: While blood flows between their hearts, their cardiac pacemaker cells (S-A nodes) fire independently. One twin’s heart rate may spike due to stress, excitement, or exertion on her side, while the other twin’s heart maintains a lower resting rhythm.
• Metabolism: Digestion takes place in separate stomachs before merging into a common lower gastrointestinal tract. Energy expenditure varies based on each twin’s muscular exertion and baseline metabolic rate.

Surgical Considerations: Why Separation Was Never Performed

When conjoined twins are born, pediatric surgical teams evaluate whether surgical separation is anatomically feasible and ethically justified. For Abby and Brittany Hensel, medical evaluations performed shortly after their birth in March 1990 determined that separation was impossible.

Anatomical Factors Preventing Separation

1. Shared Vital Organs: The twins share a single lower digestive system, a single urinary tract, a single pelvic girdle, and a single set of lower reproductive organs. Separating them would require splitting these single systems, leaving both twins with severe, life-threatening anatomical deficits.
2. Shared Circulatory and Skeletal Systems: Splitting the single pelvis and separating the complex shared vascular networks would carry an extraordinarily high risk of fatal hemorrhage for both infants.
3. Limb Allocation: With only two functional legs, separation would leave each twin with only one leg and one arm, requiring extensive reconstructive surgery and lifelong dependence on prosthetics and assistance, with no guarantee of survival.

Medical Consensus and Quality of Life

The consensus among the surgical specialists, ethics committees, and the twins' parents was that the risk of mortality or severe disability from surgical separation far outweighed any potential benefits. The decision was made to allow the twins to grow up joined.

To support their physical development, minor orthopedic surgeries were performed during childhood and adolescence:

• Removal of the Rudimentary Arm: A third, non-functional central arm located between their necks was surgically removed during early infancy to prevent movement restriction and skin irritation.
• Spinal Fusion for Scoliosis: During adolescence, rapid physical growth created a risk of spinal curvature (scoliosis) due to the lateral weight distribution of two heads on a single spinal axis. Surgeries were performed to stabilize their spinal alignment and expand the thoracic cavity, ensuring adequate room for lung expansion as they grew.

Daily Functionality and Motor Adaptation

The anatomical reality of living as dicephalic parapagus twins requires continuous physical adaptation. Over decades, Abby and Brittany developed movement patterns that maximize efficiency while minimizing physical strain on their joint structures.

Posture and Weight-Bearing

Because Abby is slightly taller than Brittany, and their leg lengths differ subtly, Brittany walks slightly on her toes on the left side to maintain a level pelvic alignment. This compensation prevents unnatural lateral bending of the lumbar spine while walking.

The shared pelvis bears the combined weight of two upper bodies. Bone density and muscular development in their shared legs and hip joints have adapted to support this load, allowing them to perform normal physical activities, including:

• Climbing stairs without external assistance.
• Running, jumping, and swimming using synchronized kick and stroke cycles.
• Driving standard automobiles using custom motor coordination strategies.

Autonomic and Somatic Harmony

The daily living of Abby and Brittany demonstrates the remarkable flexibility of the human central nervous system. Without direct neural pathways connecting their cerebral cortexes, their brains use real-time visual, auditory, and tactile feedback to coordinate complex bodily movements seamlessly.

Frequently Asked Questions

What type of conjoined twins are Abby and Brittany?

Abby and Brittany Hensel are dicephalic parapagus twins. This means they have two separate heads and brains attached side-by-side to a single torso with a shared lower body, two functional arms, and two functional legs.

Do Abby and Brittany share all internal organs?

No. They have separate brains, spinal cords, hearts, stomachs, and lungs. However, they share a liver, single gallbladder, merged lower digestive tract, single urinary bladder, single reproductive system, and a single broad pelvic structure.

How do Abby and Brittany control their legs and arms?

Each twin controls one vertical half of the body. Abby controls the right arm and right leg, while Brittany controls the left arm and left leg. They coordinate movements through sensory feedback and lifelong motor practice.

Can one twin feel pain or touch on the other twin’s side?

No. Somatosensory perception is split along the vertical midline. Abby cannot feel touch or pain on the left side of the body, and Brittany cannot feel touch or pain on the right side. They only share sensation along a narrow band at the center of their torso and in shared internal pelvic organs.

Why was surgical separation not performed after their birth?

Separation was medically ruled out because the twins share critical single organs, including the lower digestive system, pelvic structure, and major blood vessels. Attempting separation carried a near-certain risk of death or severe disability for both twins.

Do Abby and Brittany get sick at the same time?

Not necessarily. While their shared circulatory system distributes circulating antibodies and medications, localized infections (such as throat or lung infections) can affect only one twin. One twin can experience symptoms like a fever or sore throat while the other remains healthy.

Summary and Key Takeaways

The anatomical structure of Abby and Brittany Hensel provides a rare look into human embryology, organ division, and neurological adaptation. Their physical makeup proves that two independent central nervous systems can achieve complete operational harmony across a single shared torso.

• Anatomical Classification: Dicephalic parapagus conjoined twins sharing a single torso and lower body.
• Duplicated Systems: Two heads, two brains, two spinal cords, two hearts, four lungs, and two stomachs.
• Shared Systems: One liver, three kidneys, one intestine, one bladder, one pelvis, and one reproductive system.
• Motor Innervation: Strict lateral division—Abby controls the right side, Brittany controls the left side.
• Neurological Coordination: Accomplished entirely through learned sensory feedback and shared proprioception without direct brain-to-brain neural links.

Understanding their physical layout clarifies how two distinct individuals can seamlessly navigate daily life, exercise, and function within a single shared body.