I have a 4 yr old and a 2 yr old who both have swcah. I just found out that I am pregnant again and started dex again yesterday. With my son, I started it early and then did a CVS at 12 weeks. Because of the way my placenta was placed, they were unable to retrieve a piece of my placenta to test. This test really freaked us (my husband and I) out and we decided once we found out we were having a boy not to continue with any further testing (amnio) while pregnant and I went off the dex. When my son was born, we tested him with the newborn screening test but that took 5-7 days to get the results back. He ended up in the ICU by the 5th day knocking on heavens door. The doctors were very surprised that he was still alive by the time we got him to the emergency room because his potassium level was over 2 x's what it should have been. They said that he should have died by then. By the grace of God, I felt something was not right all day because he had stopped eating. I took him to my pediatrician and she said that he seemed normal and that he was probably just "adjusting". We discussed waiting on the results of the newborn screening test and she said that she'd call when they came in. That night, his breathing was very labored and so we headed to the ER at just the right time. The results came back the next day while we were sitting in the ICU that he did indeed have swcah. Of course, by then we had already figured that out the hard way.
This pregnancy, I have a new OB and a new pediatrician who are both very aware of things and I feel are going to work very close together. I will also see a high risk perinatologist (I did as well with my son) and will probably preceed with having either a CVS done or an amnio. I feel like my son was closer to death that night in the ER than any risk of miscarriage if we would have done testing in utero. I am curious if there is any other testing once the baby is born in which the results are faster than the 5-7 days for the newborn screening test. My son didn't have 5-7 days of not being treated.
As far as side effects, I didn't experience really any last pregnancy and this one is too early to tell. I gained a lot of weight with my son (50 pounds) but I gained the same amount with my daughter and I wasn't on any dex. I guess I just liked to eat while I'm pregnant (I start out weighing 115 so 50 pounds is a lot on my frame!).
Good luck with your decision and I hope that my experiences help!