As labour progresses, and increases in intensity, quite often the woman's breathing will follow. The question of "How do I breathe in labour?" is a frequently asked one. In the past, there was a lot of focus placed on preparing for the labour with breathing techniques or 'patterned breathing'. This was usually a series of breathing levels for the different stages of the labour. In recent years, the importance of 'breathing' has become less of a focus in childbirth preparation.
In practice, for most women, once the labour becomes intense the 'breathing technique' goes out the window! Or alternatively the woman can put so much energy into 'breathing correctly' that she does not relax enough for the labour to progress. This can make her prelabour (or the early 1st phase of labour) a long, drawn out affair! Try to keep it simple, and breathe naturally. You can try inhaling through your nose (or mouth), whilst keeping your jaw and throat as relaxed as possible.
Some women will imagine their breath going deep down, into their body to their baby, and then exhale between open, relaxed lips. Sometimes, as the woman moves towards the transition phase, it can become harder for her to keep her jaw, face and throat open and relaxed. Your partner, or support person, might be able to remind you to relax during a contraction, but if you find this impossible, it is important that you try to soften your body, jaw, face and throat between the contractions. Telling the woman to 'flop' or 'loosen up' her shoulders and muscles may get a better response than 'relax'. Some women find the word 'relax' completely opposes what they are capable of in labour!
Another way of thinking about your breathing, is that instead of meeting the contractions with resistance and tension, you try to let your neck and shoulders relax, breathing in and out softly, through an open, soft mouth. Your mouth, eyelids, vagina and anus are all 'sphincteral muscles' that tend to work together.