With my last baby I had a midline episiotomy, but still tore badly. What are the chances of a tear happening during the next delivery?
All an episiotomy does, is prevent the superficial tears. Deeper tears are usually the result of a large baby, or if the skin of the perineum isn't given enough time to slowly elasticize, (as with pushing in Stage II of labor). Shortened Stage II's occur with precipitous deliveries or with the use of forceps, generally in emergency situations. The vacuum extractor usually doesn't yield enough outward force to pull a baby over the increased resistance of non-elasticized tissue. (It'll pop off first--a safety feature).
A midline episiotomy is the preferable method (to me, anyway), because the tissue is thinner there. Therefore there's less tissue trauma, less bulk to heal, and less pain. The downside is that if it extends with a large baby, it'll tear right into the rectum. This can be fixed right there, though.
The alternative is a right or left "mediolateral" episiotomy, in which the cut is made from the center of the floor of the vagina down an angle, on either side of the rectum. This might possibly spare the rectum, but in a tear, will shred much thicker tissue than the midline would have, and can cause a mess. A midline is much easier to recover from than a mediolateral. And a midline with an extension tear into the rectum is easier to recover from than a mediolateral with extension tears along irregular paths into all of that thicker lateral tissue.
The chances of the same thing happening are less, since the tissues of the vagina and perineum have already been "elasticized" once. But that's in a perfect world where it's assumed that all other parameters are the same--same doctor, same type of episiotomy, same size and position of baby, same type of labor, etc. No two pregnancies are alike, however, so I'm afraid it's going to be "I don't know." Generally, the more babies one has, the less likely the prudence (notice I didn't say necessity) of an episiotomy.
Do know this: most obstetricians love to get by without an episiotomy at all. I know I do. But I'm not afraid to cut one if I can see that it's the only thing holding the head back without tearing. It's a last-second call.
It would be a good idea to discuss your doctor's policy on episiotomies. Are they automatic? Are they midline or mediolateral, and why? Are they repaired with quickly dissolving suture or delayed absorbing suture?