Are chances high of pregnancy before period (week) before?

Low probability. Unlikely but not impossible. During a 28-day (4-week) cycle (from beginning of one to the beginning of next period) the time of greatest fertility ("fertility window") is during the 2nd and 3rd week (days 7-21) after your period. You are much less likely to become pregnant during the 4th week (the week before your period). (image is from wilcox aj et al. Bmj. 2000; 321(7271): 1259–1262).