My private 20 week scan picked up a "low lying" placenta which I then mentioned at my hospital '20 week scan' at 22 weeks. The sonographer had a good look and eventually mumbled "oh yes, it's low" and flagged it on my notes. I didn't get any grade or any explanation and was sent home. I then had a big bleed at 23 weeks which triggered contractions and early labour (I was pregnant with twins so not the most straightforward of pregnancies) which they blocked with drugs after a late night dash to the labour ward. I then had further bleeds at 25 weeks (with another blocked labour) at 26 weeks and at 28 weeks (another blocked labour). At 30 weeks I had another whopper bleed and a slight water leak (woke up dreaming that my waters had gone) and then was in hospital until I delivered by emergency section at 35 weeks. Both babies born healthy and are now three years old!
I think my bleeding was quite extreme. Further scans revealed that it was partially covering my cervix and I was warned that it was unlikely to move as both placentas were on my posterior wall and the fact that there were two of them meant space was at a premium. I had constant braxton hicks if I walked any faster than a crawl and I do wonder if this irritated the cervix which in turn triggered a bleed. I got no warnings when I was about to bleed - literally nothing to imply that it was going to happen. The big ones happened at night, which was very scary and distressing even though I was prepared for it to happen. Smaller bleeds happened during the day but they were less frequent.
If you haven't bled yet then that's a really, really good sign. I also have a friend who was pregnant with twins the same time as me and had total, Grade 4 praevia i.e. entirely covering her cervix. She was told she had to have a caesarean at 37 weeks and was warned that a bleed was a certainty, but unlike me didn't bleed at all and had a safe and uneventful delivery. I'd say the fact that you haven't bled yet is a really good sign.
As security, before I went to hospital I always had a small 'bleed bag' packed. I had a nightgown, small bag of toiletries, book, pen, cardigan, flip flops. The basics to get me through a couple of night's stay in hospital if needs be. I also always carried my hospital notes with me and had the number of my labour ward punched into my mobile in case I needed to call them.
Praevia is crap but you'll be in really safe hands and the fact that you haven't bled is a really good sign that things are stable.
Kits