I managed to be "okay" with my "valve" not working properly during the pregnancy, however I was advised (by 4 different doctors) to continue taking my nexiam during pregnancy. I think this helped a lot with the extra heartburn and gastric reflux. I had really bad morning sickness as well, and I think it must be terrible not to be able to puke while feeling so nauseaus.
I could only eat very small meals, no rich fatty foods, don't drink fluids with meals, and have supper no later than 18:00.
It was still tough during the pregnancy, but I could manage.
My sister on the other hand, who also has the "mantelvlies breuk" was very very nauseaus during both her 2 pregnancies. She was hospatalised twice because she couldn't eat and had to be put on an IVU. So, I think it is difficult to say how much your pregnancy will affect your condition.
Interesting, don't know if you know, but the reason why your condition gets worse while being pregnant is because your body produces a hormone called "relaxin" (I think) that basically "relaxes" all the muscles in your body to be able to stretch, but it also "relaxes" that valve to your stomach more. So, you can imagine while pregnant your valve loossens even more than normally.