A couple of people have talked about previa and bleeding, and there are a few things to keep in mind.
First, not all previas will bleed early enough for that to be useful as a warning sign. Most will. Some won't bleed until labor.
Second, pregnancy is not always a blood-free time. In particular, many women have bloody show before labor starts, and many bleed during labor. Bleeding from previa is bright red, not mucus-y, and usually (although not always) painless. Because bleeding can be started by cervical dilation, you may have bleeding associated with contractions. In my experience (read my birth story if you want), a crisis involving placenta previa also involves a *lot* of blood. The most useful thing anyone said to me while I was pregnant was "if there's blood splashing around your ankles, call an ambulance."
Ultrasound does offer a lot of certainty about placental location, and IMO, that's a good reason to have one.
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