Many of my clients use the TENS machine. See here for an explanation of how the TENS works. The use of the TENS is hugely under-rated for early labour, during active labour and I discovered recently that it is also awesome for post birth contractions! Many insurance companies will cover the cost of the TENS machine for labour and birth, so check with your insurance provider before purchasing one. Midwife practices also have them often to rent.
As far as I have observed in my practice the TENS never has a negative affect. Some women don’t like the sensation and will realise that soon after putting it on. The earlier in the labour process you start with the TENS the more effective it will be.
It will at some point just become a part of the sensations you are feeling.
The TENS may, usually during the transition phase, feel like it has less of an affect. If you take it off at that point you may miss it and want it back on again. This would suggest it is still having some affect even if it is just because the sensation has become familiar.
The research on the tens machine for labour is not very extensive. Here is a link to Evidence Based Birth for a summary of the research that there is.
The TENS machine is also fantastic for induction. It is best to start using it simultaneously when the synthetic oxytocin is administered and it will trigger the bodies own hormonal system to produce endorphins, your natural pain relief. These are the hormones that are released to help you cope with labour. They are bypassed when synthetic oxytocin is administered into the blood via an IV. This is the reason many women experience induction as more painful than natural labour. The natural hormones are not helping with the process. The TENS can help with that or help tide a woman over until she can get medical pain relief.
The TENS does not interfere with other natural pain relief options except for water of course. Getting your partner or doula to give counter pressure or massage your back does not reduce the TENS working. It also does not interfere with the CTG monitoring of the baby’s heart beat. It is only incompatible with an epidural.