A women only ovulates once a cycle, which is typically during the middle of it -which is when you had sex. So if she hasn't missed a pill at all and is taking it correctly her b/c was around 99% (depends on the pill but usually 98-99). I don't know if it's true that it only drop to 96 after using antibiotics, but that's still a 4/100 times that she would conceive- is that worth the risk? My cousin is part of the minority - she took antibiotics for a week while on birth control and then 3 days after getting off them had sex and conceived. Your girlfriend's chances of getting pregnant will likely be less than "normal", but that doesn't mean you're out of the woods.
As far as 3-5% that sounds a little iffy, I've heard more around the 25% mark. I might be mistaken, I have no idea how long ago or where I heard/read that. But for a normal healthy couple, trying to conceive it really comes down to catching the ovulation time correctly.
Thank you for the quick reply. I know there are many factors playing in this incident but would you say that there is a very little chance that she conceived? I know the pill is 99% effective and from reading online, antibiotics drop that down to around 96% effective. I have also read that when people try to conceive it is usually only 3-5% of the time they are successful. I know there is still a chance that she may have conceived; however, would you say the odds are that it is unlikely?
Yes it's possible that that could be enough to get her pregnant. Antibiotics make birth control ineffective so even though she was on birth control for so long that's no guarantee. Also because sperm can live for almost a week inside the body and there is no guarantee how long you need to be on antibiotics before the b/c is ineffective it's possible that starting them even a couple says after intercourse is enough. So overall -yes, it's possible she conceived. However I don't know how likely it is that she did, so can't give you any numbers.
For now I suggest don't try it again- at least for a while. Birth control needs to be used for about an entire cycle before it's most effective. So I'm assuming that it would be "safe" 3 weeks after she's done taking the antibiotics. Note this is still NOT 100% effective and any time there is sperm involved there is a possibility of conception. Birth control is very effective when taken perfectly, but keep in mind nothing is 100%.
If it's been less than 3 days, 72 hours, perhaps you should talk to her about the morning after pill. Some women will not be willing to take it, but it is a conversation worth having. If it's been more than 3 days all you can do is wait. Just keep in mind that if a women is even slightly stressed her period could be off, she could bloat, feel nauseous, and have appetite changes. So even if something like that happens that's no guarantee you have to panic. After 2-3 weeks, or if she misses her period, have her take a home pregnancy test with her first morning urine.