Body Asymmetry: Why Is One Hand or Breast Bigger?

The little idiosyncrasies make us human. Whether your left hand span measures one centimeter more, or your right earlobe seems to hang a little lower in the mirror, small asymmetric variations in double body parts are completely normal ... nothing to worry about.

Sometimes, however, exaggerated physical discrepancies, even if benign, can be important. Some women with uneven breasts receive plastic surgery to make things more symmetric. Moreover, discrepancies in the size of other body parts, like the hands or limbs (arms and legs), can be so substantial that they signify more serious illness.

Intrauterine Growth Retardation and Developmental Instability

Intrauterine growth restriction (IUGR) affects about five percent of the obstetric population and—following prematurity—is the second leading cause of death and disease in unborn babies. The term IUGR is the pathological term for small for gestational age (SGA) and can either be symmetric or asymmetric. Please note that many SGA babies are normal but constitutionally small; IUGR refers to SGA that represents illness or disease.

Asymmetric IUGR is usually due to placental insufficiency, where the baby isn’t receiving adequate oxygen and nutrition via the placenta. This impaired placental blood flow can be due to many things, including smoking, alcohol, drugs, medications, high blood pressure, and genetic disorders.

A fetus with asymmetric IUGR does the best with the blood that is supplied. Blood flow is directed to vital organs, and head circumference is maintained. However, the abdominal circumference is decreased due to a smaller liver size, limbs are scrawny, and the skin is thin because there’s less fat. At birth, scrawny limbs with decreased muscle mass can appear asymmetric.

Asymmetric IUGR doesn’t necessarily mean that mirrored body parts like limbs are discrepant in size, although this can be a consequence. Rather, it means that the growth pattern is asymmetric, with most energy being directed to vital organs like the brain and the heart.

When placental blood flow becomes so impaired that the fetus can no longer compensate, symmetric IUGR can result in more uniform growth restriction, with stunted development occurring even in vital organs like the heart and brain. This development is evidenced by decreased head circumference—a serious complication.

It's possible that IUGR may represent the clinical endpoint of developmental instability. It’s been hypothesized that developmental instability, or disturbance in the intrauterine environment caused by mutation, radiation, or physiological stress, can alter a fetus’s genetics or appearance in more subtle ways, too, thus resulting in minor, yet noticeable, variations in the size of double organs like the ears, hands, and breasts. These little variations may not result in any disability but may be noticeable or concerning for those people born with them.

In other words, if the uterine environment is off by even a little, it’s possible that this disturbance, or developmental instability, may result in slight discrepancies in hand, foot, and breast size.

Why Is One Breast Bigger Than the Other?

Breast asymmetry occurs in many women. It’s common for individual breasts to be of different volumes or different shapes. Furthermore, one breast may develop an excess amount of breast tissue (supernumerary breast).

Although asymmetric breasts are most commonly a normal finding—especially among young women where malignancy is rare—there are aesthetic treatment options. Although there is some research regarding the use of breast surgery to correct for breast asymmetry in adolescent girls , it’s best for a woman to finish with her development and emerge from adolescence to adulthood before receiving elective breast surgery. After all, adolescence is a time of change, and breast asymmetries can go away with further development. In other words, there’s no reason to undergo the risk of surgery if the problem is resolved on its own.

Surgical options to correct for breast asymmetry include breast lift, breast reduction, and breast augmentation. Additionally, in recent years, lipofilling has returned to favor as an aesthetic and reconstructive procedure. With lipofilling, fat taken from a woman’s own body, or autologous fat, is harvested, processed, and grafted to the breast.

The history of lipofilling is curious. In 1987, the American Society of Plastic and Reconstructive (now known as the American Society of Plastic Surgeons) released a statement recommending against this practice. Concerns were raised that this procedure could result in scarring and interfere with breast screening.

Over time, experts realized that such concerns about scarring secondary to lipofilling were unfounded and that this procedure results in no more (and likely much less) scarring than other surgeries like breast reduction. Furthermore, any lesions that do result from this procedure don’t interfere with breast screening. In 2009, the American Society of Plastic Surgeons reversed its position on lipofilling. 

Although uneven breasts are a common and normal finding, there has been some retrospective research done that links discrepant breast volumes and breast cancer. However, such associations need to be supported by further study.

Hemihypertrophy: Why Is One Hand Bigger Than the Other?

Some people have different-sized hands, with one hand being larger than the other. Individually, each hand could appear normally proportioned. Although rare, possible pathological causes of this phenomenon include either hemihypertrophy (hemihyperplasia) or local gigantism. Hemihypertrophy can affect not only the hand but also an entire limb (arm or leg). Local gigantism usually occurs due to a variety of genetic or acquired etiologies, one of which may an arteriovenous fistula of the upper limb.

Hemihypertrophy also presents as a constellation of signs in rare genetic syndromes, or groups of conditions, such as Beckwith-Wiedemman syndrome and Proteus syndrome. Hemihypertrophy can also occur with neurofibromatosis type 1. These syndromes are complicated and managed by pediatricians, geneticists, and orthopedic surgeons. Importantly, the presence of associated tumors must be ruled out.

A difference in size of as little as five percent can be useful in diagnosing hemihypertrophy. Usually, however, parents seek medical attention when differences are larger.

On a related note, muscle imbalances can also result in asymmetry of limbs. For example, if you tend to work one side of your body more than the other, this may result in muscle imbalances evident in your arm or leg muscle groups. Muscle imbalances aren't pathological per se but rather a physiological response to the environment.

Bottom Line

Lots of people have different-sized hands, feet, arms, and breasts. The causes of minimal asymmetries are usually totally benign and more novel than concerning. More rarely, larger discrepancies of certain body parts may point to actual identifiable conditions that warrant diagnosis and treatment by a physician.