"I don't understand how screaming at the top of my lungs during and between contractions at 2cm is more energy conserving than breathing gas and air. That doesn't make sense to me at all. Sorry, but it doesn't.
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No but perhaps having pethidine and relaxing might be better.
I don't think anyone could do gas and air for 18 hours which is the average labour for a first time mum.
So what's better - pethidine at first, get to 6-7cm maybe on that and then have gas for the last few hours.
Or use gas till you're so tired you feel your eyes are bleeding, beg for pethidine or more likely an epidural at that point. End up having pethidine nearer the birth and get b/f off to not the best start. Or the epidural increasing the chance of an instrumental.
I'm not saying what's better - not my place to. But its something that should be discussed with the woman so she can make an informed choice.
However where I work we can only give gas and air to someone in establised labour - 4cm. I wouldn't be allowed to give you gas and air before then unless for a short period, like during an examination. Its hospital policy. Pethidine/diamorphine can be given at 2cm if prescribed by a Dr.