Hi lwb
I am the world expert on low-lying placentas !
20 weeks is far too early to worry about what your placenta is doing IMHO. When I had my DDs, my hospital actually refused to comment on the position of the placenta at the 20 week scan, as they said it was irrelevant. Most placentas, even those which are low-lying at 20 weeks, will 'move' out of the way by the time you approach your EDD.
(The placenta doesn't actually 'move' btw, it just appears to do so. It was explained to me as being like a piece of sellotape stuck to a balloon. When the balloon is deflated, the sellotape might be quite near the bottom, but as you blow the balloon up - or as your uterus expands - the sellotape appears to move higher up the side of the balloon. This is why most placentas will be well out of the way by the time your baby is big enough to be born.)
Placenta praevia (when a placenta is low-lying enough to prevent a natural delivery) is 'graded' on a scale of 1 to 4. 1 means that the placenta is just touching the edge of the os (mouth of the cervix), 4 means that the placenta is completely covering the os. So, if you are really worried, when you go for your next scan, you could ask whether the position of your placenta would be classed as placenta praevia at the moment, and if so what grade. This might reassure you, and would at least allow you to picture what is going on in there!
My own experience is that in both my pregnancies my placenta was low-lying at 20 weeks. Of course, because the hospital didn't mention it I wasn't aware of this the first time round. However, at 27 weeks I began to bleed quite heavily. I was admitted to hospital and they discovered that I had grade 4 placenta praevia. (They told me that my placenta was sunk in the os like a plug in a sink.) Because I kept bleeding on and off, I was not allowed home and eventually had my DD by c-section at 36 weeks. Please DON'T PANIC! Apparently it is very rare for this to happen (my hospital saw a couple of cases a year). However, if you know that your placenta is low-lying and you do notice any bleeding, however slight, it's important to get to hospital asap to get it checked out. It is VERY RARE for the placenta to come away completely, but IF it does it is an emergency situation and the baby would have to be delivered immediately.
With my second pregnancy I was inevitably worried about the position of the placenta, but they still didn't want to tell me anything about it at my 20 week scan. In the end I insisted and basically said that I was gonna lie there till they told me where it was (I explained my history) and they did admit that it was low-lying again, though not actually covering the os. I had another scan at 34 weeks to confirm where it was (I wouldn't even have been offered this, btw, if I hadn't insisted on being told where it was at the first scan - they are so convinced that it won't be a problem) and it had 'moved', and I had a very successful VBAC with my 2nd DD.
It is odd that your midwife says that you won't be able to feel the baby moving with your placenta where it is, though. That suggests that the placenta is towards the front (which can sort of 'muffle' the kicks, iykwim). Was your midwife looking at a scan when she said this? Or was it just 'a feeling'? If the latter, you may well be worrying about nothing.
Good luck with it all anyway.
P.S. Please don't let what I've said about emergency situations worry you. Everyone knows that all kinds of emergencies CAN occur in pregnancy, but mostly they don't! I was very very unlucky... and even so at the end of the day I had a healthy and beautiful DD who was defiitely worth every minute of the 10 weeks I spent in hospital waiting for her to arrive!
P.P.S. I am not a doctor or a midwife or anything. I just spent 10 weeks in a maternity ward and asked a lot of questions (out of sheer boredom mostly!).