You can not get pregnant if you are wearing underwear and a sanitary napkin, your boyfriend is wearing underwear, and you rub up against each other until he comes in an explosion of unbridled desire.
The presence or absence of a bra has no effect on the probability of a pregnancy… unless it’s a really GOOD bra. As anyone who has worn one has certainly learned, a really good bra can have disinhibiting effects on a woman’s sense of self restraint. Proper support and a nice bit of uplift, combined with some really sexy lace, have been known to make some women feel so fabulous about themselves that they start picking up men on the street corners and having impromptu, unprotected sex. Having a good boob day is a truly dangerous thing… almost as effective for acquiring new sex partners as pink satin bunny ears.
Explanation for why you can’t get pregnant when well padded
Sperm need to get up through your cervix if you’re going to get pregnant. There is no conceivable way that the little swimmers are going to be able to climb through two layers of cloth and a sanitary napkin. Sperm don’t carry shovels… or ladders. They have neither backpacks nor hands.