Welcome to the forum! If a woman has a regular cycle (period comes after fixed number of days, 2-3 days this
side or that) then no, a woman cannot become pregnant by having sex one day before the due date of periods.
However, if her cycle is irregular, then she cannot predict for sure when her next period will be and her
expected due date may not be the actual date when she will be getting her periods. In which case, it is
difficult to predict the day before the due date. Pregnancy may occur in such cases because the day the woman
thinks is a day before her next due date may not be so.
Hope this helps. Take care!