Although many women have very regular cycles, there are also many women who do not have regular cycles.  Irregular cycles can vary between cycles that are too long in length and cycles that are too short  Also, cycles can have no ovulation at all, in which case bleeding is secondary to a sudden drop in estrogen and overgrowth of the lining of the uterus.

Most often pregnancy occurs during normal cycles at mid-cycle.  You can get pregnant when cycles are not regular as long as ovulation is occurring.  Irregular cycles just make it harder to predict when ovulation is occurring. Changes in cycle length, regularity and ovulation can also occur when taking certain contraceptives or transitioning off of contraceptives.

It is uncommon to get pregnant at the last day of your period as hormone levels are not conducive to ovulation. Another consideration is some women will continue to have bleeding despite being pregnant, so it is possible that the pregnancy occurred with the last ovulation or prior. To determine which day is closest to the day of ovulation you can keep track of your cycles for at least two months and then subtract 14 days from the expected start date of your next period.