Vocalisation is a primitive response to pain. If you hurt yourself you cry out. Animals vocalise their pain too. Think dogs yelping.

There are a couple of ways vocalising helps you deal with labour pain. The first being, it's a focus toohil - or something to distract you from the overwhelming sensations of a contraction.

Secondly, a low moan relaxes the vocal chords, throat and at the same time it helps to relax the cervix.

Thirdly, and I love this little titbit of information, a recent study showed that uttering, moaning, or simply repeating an expletive (Yes you can say F**K)releases endorphins which aid in pain relief! (Actually the study said that any repeated word will work, but expletives were particularly effective as they were emotive)

Fourthly, your baby hears everything - certain tones are calming (think low, steady, ocean sounds) others are fearful - (high pitched, scared, panicky sounds) so by moaning low to the baby, you help keep him and yourself calm.

So the trick is to use a tone that is nice and low, feels loose in your throat, and is emotive enough to produce emotions. Given that you do not want to be "thinking" during labour, provided you keep it low - use what comes to you.

many women will have a moo, growl or roar :)