Treat it! The treatment of asthma in pregnancy is the same as treating asthma if one is not pregnant. Out of control asthma is clearly bad for mom but is probably worse for baby.
Take meds when preg. The risk to the baby of not taking your asthma medications while pregnant is generally considered to be higher than any risks to the baby caused by the medications themselves. Mothers who took their medications had higher birth-weight babies, presumably because they oxygenated the growing baby better. Check with your asthma doctor and your OB about the best medications to use when pregnant.
Rule of 1/3. 1/3 of patients have improved symptoms of asthma when pregnant, 1/3 have no change in asthma symptoms and 1/3 of asthma worsens. Things that happen during pregnancy can aggrave so it is important to treat: reflux, vomitting, expanding abdomen.