There are a ton of comfort measures that can be used during labor. One of my all time favorites is the Transcutaneous Electrical Nerve Stimulation (TENS) Unit. This device will provide mild electrical impulses that will transmit through the skin to stimulate nerve fibers.
I personally used one during my induction, which was a life saver! I believe in them so much I took a safety course and bring one in my doula bag for my clients to use. Whether or not you plan for pain management options, like the Epidural, the TENS unit is an amazing option for getting through the early stage of labor or the time leading up to your epidural.
Benefits of the TENS Unit
Portable - Battery Operated
Non-Pharmaceutical - No prescription needed and alternatives to or before other pain management options. You can even use at home for relief from pregnancy discomforts.
Self Controlled - As you feel the contraction building/ending you increase/decrease the impulses.
Helpful for back labor - Couple with the peanut ball this is a life saver!
Does not interfere with external fetal monitoring.
There is TWO ways the TENS unit works:
1. Gate Control: By selectively exciting A-beta nerve fibers in the skin with TENS, the amount of painful stimulation being transmitted by smaller diameter A-delta and C-fibers can be reduced, causing the perception of pain to be reduced.
In Layman Terms: You shock yourself a little as the contraction is coming on and those feelings get sent up to the brain before your contraction pains! So you focus more on the stimulation of the TENS unit and not your contractions.
2. Increased Endorphins: Stimulating small A-delta fibers reduces the release of excitatory neurotransmitters such as aspartate and glutamate and increases the release of inhibitory neurotransmitters such as GABA (gamma-aminobutyric acid) and serotonin (Sluka and Walsh, 2003), and endogenous opioids such as endorphins and encephalin. (2) This takes place particularly in early labor when the TENS unit is in a lower frequency mode (burst mode), for the span of about 30-60 minutes. Research findings on TENS have shown that laboring women using the device use less pain medication than women using a “sham” TENS device (3). The majority of women surveyed in the UK National Birthday Trust Survey rated it as moderately or very helpful in relieving pain and would use it again in a future labor (4). A study that investigated the use of TENS for back pain in labor found that "TENS has a specific beneficial effect on pain localized in the back."(1).
In Layman Terms: Endorphins are hormones secreted within the brain and nervous system in situations of stress or pain. Endorphins GOOD...Pain BAD.
Placement for the Electrodes Pads:
For optimal effects, I think the best sized pads are the 2"x4" pads. Bigger is better in this case. You'll want 4 of them placed like below. Two right at the panty line or sacrum and two around the bra line. If you're proficient in the spines anatomy S2-S4 and T10-L1.
When you shouldn't use a TENS unit: TENS may be contradicted in patients with demand cycle pace-makers and seizure disorders, but does not have any other known risks or side effects (4). A TENS unit should not be used in water, (shower or bath), with the use of an epidural, or internal fetal monitoring.
What TENS unit should I buy?
You'll hear a lot of different recommendations for this. Personally I prefer ones designed for labor/birth and one that doesn't have a EMS setting.
If your in the market for a TENS Unit here is one of my favorite TENS Units (not an ad I've just tried a bunch):
"EFFECTIVE, PERSONALISED PAIN RELIEF
Our most advanced maternity TENS device to date, the Perfect Mama+ comes complete with a contraction timer and log so you can track your progress throughout labour without having to worry about keeping a manual record.
The device offers 3 clinically-proven pre-set programmes and a boost button to provide additional pain relief during contractions, giving you ultimate control. Each of the electrode pads have been specially designed to be comfortable to lean on and reduce risk of accidental disconnection as you move between contractions.
Following the birth, the unique breast electrode pads can be used to help initiate lactation and facilitate the breastfeeding process in those early days." -Current Body
References (1)Bundsen P, Peterson L-E, Seistam U. Pain relief in labor by transcutaneous electrical nerve stimulation: A prospective matched study. Acta O bstetricia et Gynecologica Scandinavica 19 81; 60:459-468. (2)Sluka KA, Walsh D (2003) Transcutaneous electrical nerve stimulation: basic science mechanisms and clinical effectiveness. Journal of Pain; 4: 3, 109-121. Walsh D (1997) TENS: Clinical Applications and Related Theory. London: Churchill Livingstone. Walsh D et al (2009) Transcutaneous electrical nerve stimulation for acute pain. Cochrane Database of Systematic Reviews; Issue 2, Art No: CD006142. DOI: 10.1002/14651858.CD006142.pub2 (3) Chamberlain G, Wraight A, Steer P. Pain and Its Relief in Childbirth: The Results of a National Survey Conducted by the National Birthday Trust. Churchill Livingstone: Edinburgh, 1993.  Ericksson M, Schuller H, Sjolund B 1978 Hazard from transcutaneous nerve stimulators in patients with pacemakers. Lancet 1: 1319
Additional Reading Davies R. Transcutaneous electrical nerve stimulation. In: Understanding Pain and Its Relief in Labour, S. Moore (ed). Churchill Livingstone: Edinburgh, 1997. Eappen, Sunil; Robbins, Deborah. Nonpharmacological Means of Pain R elief for Labor and Delivery. International Anesthesiology Clinics. 2002; 40(4):103-114. Gentz, Brenda A. Alternative therapies for the management of pain in labor and delivery. Clinical Obstetrics & Gynecology. 2001; 44(4):704-732. Kaplan B, Rabinerson D, Luirie S, Bar J, Krieser U, Neri A. Transcutaneous electrical nerve stimulation (TENS) for adjuvant pain-relief during labor and delivery. Int. Journal of Gynecology & Obstetrics, 19 98; 60:251-255.