I have a 28 day cycle. i took a opk test and it came out positive on cycle day 16. so it appears my luteal phase is about 10 or 11 days. could this be a problem when i am trying to conceive or is that a long enough luteal phase to sustain a pregnancy?

LPD: 20 years ago luteal phase defect (lpd) was part of every fertility evaluation; it is no longer believed important except maybe in recurring pregnancy loss. If you don't have a rpl history, ignore lpd for now. Your arithemtic is correct, 28-16= 12, but OPK is not precise enough for diagnosis. Only endometrial biopsy should be considered for lpd, and only when clinically indicated. Good luck!