Well, let me just say that birth control is NOT 100% effective. My daughter is living proof of that.
I had been taking the pill for 3 years when I got pregnant.
Antibiotics also make the pill ineffective.
You CAN get pregnant, even on the days that she is taking the real birth control, not the sugar pill.
The pill is only low-90% effective. The best way to ensure no pregnancy, is to ALWAYS use a condom. Or spermicidal lubricant. Something WITH the pill.
If you're still worried, get a pregnancy test.
$20 for 2. I'm sure the other one will get used.
Usually when a woman becomes pregnant, she KNOWS she is pregnant. IT's the maternal instinct kicking in. She gets that feeling and BOOM, the test is positive.