Keep this in mind about 'the pill':
Basically, all it does is raises the acidity levels in the uterus to kill sperm. This acid kills your swimmers.
(There are a few other ideas behind the pill, but this is the main function.)
If you have seen a package of birth control pills, you notice different colors, that is the different strengths.
When a woman is ready to 'ovulate' (best time to get pregnant) the strongest pills are being taken. Should a woman ovulate early or late, she is taking the weaker pills, this can allow your swimmers to make it thru.
Should she take an over-the-counter acid reducing medication. (zantac. rolaids, tums, etc.) This can also lower the acid the pill creates, allowing your swimmers to make it thru.
Any "reducer" over-the-counter meds, (fever reducers, swelling reducers. etc..) can also reduce the acid, allowing you swimmers to make it thru.
A biggy is antibiotics!!! I have a friend who has two wonderful little girls as a result of antibiotics, it voids the acid the pill creates, allowing your swimmers to get thru.
A wonderfully working pill is only 95% effective, now add an outside synthetic, an overly eager ovulator, or human error you can make a baby.
I am not saying this to scare anyone, it is just not a perfect science, I have known two woman, penkitten making the third, to become pregnant while on the pill, it is rare, but not as rare as you may think.
But MOST IMPORTANT guys, please remember the acid the pill creates does not stop sexually transmitted diseases, be careful.