I used it from a few hours before 4cm (it took me 8 hours to get that far) and then for the next 8 hours, when I was 5-6 or 8-9cm, depending which mw I wanted to believe. It was AWESOME for me, until the boost button started to misbehave and only worked intermittently. In retrospect I remember being advised to put fresh batteries in if that happened, but I didn't remember at the time, and I ended up needing an epidural, because the pain was pretty severe when the boost button didn't do its job. I reckon I could have gone right to the end with only the TENS if I'd remembered to change the batteries.