If you're gonna have pethidine have it early rather than late in the labour, and don't expect it to help, anyway.
I have a friend whose baby almost wouldn't breathe at birth because she had pethidine too close to the birth (at midwife's encouragement). Worst part is, the whole experience was nearly repeated with her 2nd baby (midwife saying, "You'll be ages to deliver, have some pain relief", and then 20 minutes later baby arrives, but thankfully friend had declined pethidine). And it nearly happend to another friend ("Yes, we agree you aren't coping with the pain well and you're hours from delivery; oops, is that the baby's head we see crowning?".)
Anyway, I had pethidine when I thought (had been told) I was hours from fully dilated, and no way I could envision another 4-7 hours without some strong pain relief. Then sure enough I was fully dilated 45 minutes later and pushing. I guess I can be very glad it did take hours to push him out...
Plus the pethidine meant constant internal monitoring so I couldn't move around or adopt any sort of birthing position where gravity would help. There were other interventions that only happened to me as a knock-on effect of having the pethidine, too.
And on top of all that, I can honestly say I don't think the pethidine helped relieve my pain relief.
So I guess you can tell I'm not a fan! Too dangerous, and doesn't work, anyway... In My Experience.