I've just had a good old Google regarding this very question (really, really didn't want to cut down on the booze!).
Apparently, even moderate alcohol consumption has an effect on fertility for women. Massive bummer
. The study I found said a drop in fertility was noticed in women who had over 5 drinks a week (it was a Danish study- maybe they don't work in units?). But, as far as I could work out, it was something like 65% conceived when not drinking, 55% when drinking moderately.
Drinking over your weekly recommended units has an even greater effect. Annoyingly, men can still drink up to their recommended units before any effect is noticed on fertility.
(NB: this is only what I gathered after an evening's Googling, and it's all pretty confusing out there- I'm not a fertility expert or nowt!)
The getting drunk before you know you're pregnant is a separate issue- I've heard that for the first few weeks the embryo is living off the sac of the egg, and not taking its nourishment from you, so should be OK?
However, I also read that alcohol can increase the chances of miscarriage.
The conclusions? That I'm spending far, far too much time googling 
(sigh) Nice cup of herbal tea, anyone?