All of the anti-thyroid drugs have a disclaimer saying it can harm a fetus. The trick is to take as small a dose of it as you can, monitor your blood often, keep the dose low, and try not to stress out.
Not taking the drug may harm the fetus and may end badly depending on how hyper you are.
The standard initial dose is 300mg per day. Perhaps you can get by on less, which is why I suggest you get your blood checked often.
I have absolutely no personal experience with this drug since I've been treated for cancer and hypothyroidism. However I did find some references.
Before you read them (and proceed to freak yourself out) remember that most ALL medications are not indicated (or carry warnings) - including ASPIRIN - for pregnancy. You need to get your OB/GYN and endo talking ASAP.
http://www.medicinenet.com/propylthiouracil/article.htm
http://www.drugs.com/pro/propylthiouracil.html
http://www.webmd.com/drugs/drug-8883-Propylthiouracil+Oral.aspx?drugid=8883&drugname=Propylthiouracil+Oral
Hopefully some of our meds experts (AR-10) will be along soon to help.
You're due around the same time as my daughter (who is also a thyroid patient). Congrats!