I didn't have one, so I may not be the best one to answer you, but I didn't want to read and run.
If the head stretches you a bit too much, there is obviously the risk of a tear (ouch, to be honest I can't even bear the thought) of the perineum to get the baby out. Tears are generally messier, ie you can't control how you tear, it could go any direction, and generally is just uncontrollable and once you have it you have to deal with it. If you opt for an episiotomy, they will give you a local anaethetic, cut you and then get the baby out.
Either way you would have a cut which needs to be stitched back up, but if they do an episiotomy, the cut is likely to be a lot tidier and the doctors have done them before etc etc. The tear on the other hand is unpredictable and could be bigger, need more stitches, and of course you had no local anaethetic there when it happened.
I was lucky in not needing either. I don't know why, I just was, and my Ryan was 8lb 7oz, so not a small baby.
If you google Perineum Massage, that's what people do to *supposedly* help the perineum be stronger and more flexible and less likely to tear. I think you use some sort of oils, but the ladies in 2nd/3rd tri's may know more, or just use google. I was far too lazy to bother to be honest!!!
Also Arnica.. I believe.. is to ease bruising. I think you can take it when you are in labour and if you have cuts or grazes down there it can help to heal them better? I didn't use that either.
But there's a couple of ideas for you and hopefully someone with some experience may be able to help you better!
Good luck with your pregnancy!
And you'll see on this thread -
http://www.pregnancyforum.co.uk/forum/v ... hp?t=38603 - that I was bricking it about the exact same thing, so you're not alone! x