A TENS machine is a hand-held device which sends small, safe pulses of electrical current via fine leads to pads on your skin which can be effective in relieving lower back pain in the early stages of labour.
There are no known side effects of using a TENS machine although it is advised that you shouldn’t use them until after 37 weeks of pregnancy. If you want to try this, ask your midwife where you can hire or borrow a TENS machine.