Could you become pregnant while on the pill if you had sex on the last day of the pill cycle?

Not very likely: If you are taking your pill as you should, pregnancy is not very likely even though the pills in the last row (week) are fake. This is because the ovulation process doesn't occur until about 9-10 days after your period, in the absence of the pill. Starting your new package of pills on time should prevent ovulation, thus prevent pregnancy. Taking your pills on time is the key.