Diphallia, penile duplication (PD), diphallic terata, or diphallasparatus, is a rare medical condition in which a male infant is born with two penises. The first reported case was by Johannes Jacob Wecker in 1609. Its occurrence is 1 in 5,500,000 men in the United States.
When diphallia is present, it is usually accompanied by other congenital anomalies such as renal, vertebral, hindgut or anorectal duplication. There is also a higher risk of spina bifida. Infants born with PD and its related conditions have a higher death rate from various infections associated with their more complex renal or colorectal systems.
It is thought diphallia occurs in the fetus between the 23rd and 25th days of gestation when an injury, chemical stress, or malfunctioning homeobox genes hamper proper function of the caudal cell mass of the fetal mesoderm as the urogenital sinus separates from the genital tubercle and rectum to form the penis.