What are some warning signs to look out for in the 2nd trimester?

Please ask your OB . You should be asking any preg related ?S to your ob, you should not be having any uterus pains, or bleeding or abnormal discharge, you should have already had a least 1 ultrasound and possibly 2 and u should be getting another one around 19-20 weeks to check the baby- fetal anantomy scan and the cx to make sure it is the pr0per length, if u are worried about something specific then call your ob.
Depends on Risks. Depends if any known risks exist. Otherwise watch for abnormal discharge, preterm labor or contractions, any abnormal bleeding. Notifify your OB about any of these and any signs of a bladder infection, or high fever.