glasto saw your post in the hut... what a cow your BF's girlfriend is!! I would have balled my eyes out right there and then so everyone would see what a horrible person she is.
anyway, in answer to your question - i've heard a short luteal phase can sometimes be a problem with ttc if your lining starts to shed before the egg has a chance to implant. this normally happens 7-10 days past ovulation (but can happen as early as 5 days or as late as 12 days).
when an egg doesn't implant after 10 or so days, the corpus luteum stops producing progesterone and your womb lining starts to break down, a day or so later you get AF.
if you get an egg implant, then the corpus luteum keeps going with the progesterone until the placenta takes over.
if you have a short luteal phase it could indicate that you have low levels of progesterone - which a doctor can give you a supplement for.
some doctors don't believe that a luteal phase 'defect' (i.e. short one) is a problem, and will just tell you to continue ttc for the full year before investigating it any further.
hth