You're not selfish at all - in fact, research in mice showed that mice who had mummies who exercised in pregnancy had more brain-power than mice with lazy mummies. OK, so the mice didn't play soccer, but that's not the point.

At 7-8w I was doing hi-impact aerobics and stomach crunches. At 12w I was doing lo-impact aerobics and yoga. I was still running at about 20w, but that's had to stop now - first the sciatica made it hard, then I found I was getting too breathless. Although I'm cutting back, I'm doing it slowly - I'm still dancing around to the radio and doing all my stretching, only more mildly than before.

The main danger is if someone hits you in the stomach, or the ball does, but as the baby is still tucked inside your hips then it should be safe until the second trimester, when the uterus starts to grow out.