Placenta previa?

So I'm 17 weeks and I had been having severe cramping and felt pressure for about 5 days. Everyone told me to put it off and that it was nothing. I called my midwife and she said go to the er. So I went, they did an ultrasound and they said my placenta has implanted very low and partially over my cervix. They said it's possible throughout the pregnancy it can move upwards meaning I won't have any complications. Also since I'm not 20 weeks they said they couldn't diagnose me with it. But I'm asking for future reference if anyone has had it, what was birth like? Did it subside? I'm pretty scared honestly. This is my first I don't know what to expect. Thanks.