Everything I think I know about this comes from having US, so IDK if it's helpful if you aren't having any.
My placenta was completely covering my cervix at 20wks, and I knew it couldn't have been in the front because I felt baby movements early for a FTM, sometime around 14wks, which I attribute to the placenta not being in the way. Funny, because most people equate movement with health. However, I had no idea it was going to be so low.
By around 25 wks I had been visualizing placenta movement to try and correct the previa diagnosis, and I had been picturing it moving up towards the front all along. US showed it to be moving up but towards the back.
Later I noticed that I was particularly sensitive to not being able to lie on my back. Doctors and my masseuse were surprised at how soon I started complaining about feeling sick when lying flat or even reclining. IDK if that's coincidence and more to do with other anatomical factors though. It's just a correlation I noticed.
With regards to baby positioning, I think that's unlikely to be useful. Assuming my belly mapping skills are as good as I think they are (and I think so because I predicted her position correctly at every US I had), my baby spent some time in pretty much every position there is from 28wks when I started trying to feel for it through now, 37wks, so I can't imagine baby position would be a good one.
These days my placenta is anterior and low-lying, but high enough to attempt vaginal delivery. I haven't noticed anything else that might have been useful signs if I wasn't getting US. Maybe if it was still a previa position I would have had bleeding by now, but I've heard of plenty of women who didn't ever bleed.
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25yo FTM to a Wiggle Panda , student teacher , newlywed
With my first pregnancy my MW knew that my placenta was low. I'm not sure exactly how she knew but she used a doppler so that's my best guess. My MW and I felt an ultrasound was indicated so had one to rule out previa.