icky-female-stuff-warning
I don't know this from experience, but I've seen a few friends go through it. Pregnancy symptoms are easy to misinterpret, even if you want to be pregnant. sometimes its just unexpected, and people think the dizziness and nausea is just "something I ate." female athletes have notoriously irregular "cycles" anyway, so it could take even longer for an athlete who isn't always regular to know for sure...took a dancer friend of mine almost three months to figure it out.
there's a reason radiologists (x-ray techs) almost always put a lead vest over a woman's middle, even if she tells the radiologist she's absolutely certain she's not pregnant. A woman, especially if its the first child, may not be completely familiar with the feelings and symptoms of pregnancy. might be ignorance, might be wishful thinking, or whatever, but even if someone is trying to get pregnant, they don't know whether or not they're successful for a few weeks... they may be trying to have a child, test for their belt thinking they're not pregnant, and then find out they are because they miscarry because they got kicked...
all I gotta say is...why risk it? if you gotta kick, kick the butt, sweep the leg, or do something else.