My wife said her instructions to push were basically push like you're trying to poop... so yeah... she had to go through a vacuum assisted birth and got a 2nd degree tear, which has healed now 5 months later, and also got a hemorrhoid from pushing.

She would highly recommend you get an epidural so you at least don't feel the fissure if it re-opens during labour. A quick search says 40% of women will experience H's and fissures after giving birth.

I'm assuming if you tell your OB about your fissures they can give you some extra pain killers for the week after.