Hi. I'm ttc and I would like to know how likely I can concieve 12 days before my period is due?

Depends. Most women ovulate between cycle 12-14. In the average 28 day cycle the egg is produced around day 14. Sperm can live happily 48 hours in the vagina or the uterus so the most fertile time is day 12 to day 16. If your cycles are irregular then the day of ovulation can vary.