The Upper Extremities
The upper extremities—that is, each arm and hand considered as a unit—are known to start forming during the fourth through seventh weeks after a child's conception.  At this point in the pregnancy, the fetus is still under an inch long.

Congenital Anomalies of the Upper Extremities
Congenital anomalies are unusual.  They occur when the figers, hand, or extremity do not form "normally" in utero.  Congential deformities may occur from the mother's having some disease (a virus, for example) during pregnancy or taking certain drugs during the time the hands are developing.  Such anomalies may be inherited from the mother, the father, or both.  Genetic counseling is usually recommended to assess the likelihood of subsequent children's being affected.
In the hand, such variations can include webbing between the fingers; fewer or more fingers than five; fingers that cannot bend; and fingers that are short or asymmetrical.  In the arm, congenital anomalies might include incomplete formation of part or all of the arm, the arm's being bowed, or unusually or incompletely formed bones and joints.  Some of these variations are unique to the individual and isolated; some tend to be associated with other known congenital anomalies.
Congenital hand problems may in fact be associated with organ problems involving the heart, kidneys, spine, gut, etc.  An examination by the pediatrician is of course routine.

Origins of Congenital Anomalies
Heredity and Genetics
In the same way that certain vocal qualities or physical characteristics may be inherited, some congenital anomalies are also genetically determined.  In fact, such traits may be passed on for many generations without actually showing up in anyone physically.
Chemical Influences
It is well documented that alcohol, tobacco, certain medications, and street drugs, as well as chemotherapy agents, can bring about birth defects.
Pregnancy
During pregnancy, a baby's fingers or hands can become restricted, wound in threads that have somehow separated from the amniotic membrane.  When this happens, these threads keep the fingers or hand bound so that they cannot grow as they usually would.  Although the mechanics are known, what gives rise to this situation is not.

Emotional Responses
Many parents experience the spectrum of emotions—everything from guilt to sadness, outrage, and fear—upon finding that their child has been born with a congenital anomaly.  Yet, without his or her parents' standards as a yardstick, the baby knows only the normal human needs of an infant.  Neither does he or she yet have the world's standards as a point of comparison.
Once the child's parents can view the situation clearly, with the child's point of view and best interests in mind, they can begin to realistically address the issues that lie ahead.  Professionals such as a therapist, pediatrician, or hand surgeon, as well as other parents who have dealt with similar circumstances, can assist them.  Children with congenital hand differences tend to achieve excellent function even with severe problems.

Treatment
The aim in working with any child who has an upper-extremity anomaly is to assist the child to operate in the world without difficulty.  Many avenues are available, including physical therapy and the use of physical aids such as prostheses.
Surgery can frequently enhance appearance and function.  For example, crooked or stiff fingers can be connected, joined fingers can be separated, and an additional thumb or additional fingers can be removed or reconstructed.
Sometimes more-sophisticated techniques are needed, such as fabricating a thumb out of an index finger (pollicization), or transferring tendons or realigning bones.