I suspect that no doctor would give a pregnant woman the OK to take part in showjumping competitions. It was probably more a case of the doctor saying that she was OK to ride, but there's a world of difference between riding and showjumping.
Some doctors will say its ok for riding, some will not recommend it. I horse ride. When I was pregnant my doctor said I could do some horse riding. Flat work only. I love show jumping but I was not allowed to jump whilst pregnant. I don't believe for a second any doctor would recommend show jumping whilst pregnant as aside from the fact it's riskier, it's very different to that of normal riding and your position is different.
Even the women I know that do ride whilst pregnant, even they won't jump when they are expecting.