Contractions occur due to an increase in CO2 in your bloodstream, not a lack of oxygen.
Nerve signals are sent to your diaphram causing it to contract and stimulate breathing in order for the oxygen-CO2 levels in your blood to reach the normal equilibrium. However, you can actually 'hold on' for much longer than the urge-to-breath and the contractions seem to suggest!
Oxygen levels in your body tend to lower at a slower rate than CO2 levels rise. Therefore, you feel the urge to breath before your body actually has a significant, and potentially dangerous, lack of oxygen. Don't get me wrong however, too much CO2 is just as dangerous as too little oxygen.
However, your body can handle quite high levels of CO2 before it begins to become dangerous. In any case, unless you are highly trained, there is a point, usually after several contractions, where your body will not be able to handle it anymore and you will unvolontarily breath before reaching the point of black out. Therefore, it is perfectly safe to feel contractions and to let them occur since you cannot hold your breath to death(above water obviously).
To be even safer, and thus more relaxed, lie face up on a bed to do statics so that even if you do lose conciousness, you will not fall and hurt yourself and your airways will be open.
Finally, in my view, you should not feel as though you HAVE TO endure contractions whilst you are not relaxed. There is no point in 'enduring' contractions as you will feel very uncomfortable. Only attempt to overcome them when you feel completely relaxed. In the end there is no point in putting yourself through misery!
