A TENS Unit is a small, hand-held device that emits an electronic pulse. It is connected to four stimulating pads that are placed strategically on your back. The stimulation from a TENS unit works to reduce the pain of labor in two ways:
First, the light electrical stimulation encourages endorphin production - especially when the stimulation is pulsed, not continuous.
Gate Control Theory
Second, it overrides some of the discomfort of contractions by "getting to the brain faster." The Gate Control theory of pain says, basically: which ever stimulus gets there first wins. Pain travels slowly (along unmyelinated neurons). Pleasant sensations, like massage, aromatherapy, loving words, and the electronic stimulation from a TENS unit, travel more quickly (along myelinated neurons) and get to your brain faster than the sensations of contractions.
The stimulation from a TENS unit is perceived before the discomfort of labor pain, so your brain registers the discomfort less than it otherwise would.
TENS units are a standard of care in maternity wards throughout Europe, Canada, and Australia. A small group of women, including doulas, midwives, and childbirth educators, are working to promote their use in the United States.