The human body actually does a pretty good job at protecting our our rather delicate internals, including unborn children, with evolution equiping us to deal with the likes of lions wanting to introduce us to their collection of sharp, pointy things and similar hazards that could result in a variety of traumas.
But - and there's always a but - nothing is a certainty and even in sports or activities that you'd never imagine could cause a serious injury or a miscarriage, such accidents do occur. You can trip while walking & break bones, tear ligaments & muscles, even kill yourself via a broken neck. So while a doctor might say that paintball should be safe, assuming they truely know what the sport entails, you are still taking a calculated risk.
And that's the big question: is the risk, no matter how big or small, worth the life or long-term health of the baby compared with the loss of a few months of what is a luxury, not a neccessity for life.
And it's not just a question of protecting the woman's torso since miscarriages can be triggered by stress or pain. It would not be impossible to have a situation where a pregnant woman gets mugged, getting shot multiple times in the throat or some other sensitive area and the pain causes a miscarriage.
So is playing paintball, a mere hobby - I know, I know! Please don't flame or stone me as a heretic - really worth the risk? A life lost or potentially severely limited for the sake of what is only an hour or two of actual playing time per month for most of us. Personally I'd say not, that it's no different than stopping playing for a few months while recovering from an injury or operation. You can still be a part of the game, helping the team setup between games or just spectating, or - shock & horror! - even do something else. Ok, silly idea, I forgot I'm talking to hardcore ballers here & we have no life outside of the game.
Assuming the woman choses to play, and that her significant other and the site, judges, other players are willing/happy/whatever to accept this, then I would say that she should wear hard protection. It might be inconvenient to have to maybe have to purchase progressively larger protection, but that's the price she should be willing to accept to reduce risks as much as possible so that she can continue playing.
Obviously it would be absurd to suggest that a woman became pregnant just for the advantage of being able to wear soft protection in the hope of a few extra bounces, but the rules do state that such protection is not allowed, with no provisos or loopholes that some of our less scrupulous brethren might try to exploit. Just because the alternative - hard protection - might be somewhat inconvenient is no excuse to allow an exception.
A last thought, one that is perhaps somewhat cynical and cold given the potential tragedy we're discussing, but it should be said: the possible media reaction to a miscarriage due to paintball could cause just as much damage to the image of the sport as kids shooting people in the streets, etc. It might not make much sense, but the media - with the general public and politicians following swiftly behind - often do form irrational opinions and take absurd stances. Such a situation might not cause paintball to be banned - although I just had a thought about the pro-life looneys in the US - but how about a velocity cap or rate of fire cap being enforced, or any other bizarre solution that the politicians and bureaucrats could then come up with in the desire to be seen to be doing something.