thanks for any thoughts
I would say listen to your inner voice, and ask your midwife about a good doctor she recommends that you can see. I think it will give you some reassurance (not that you will necessarily get the answer you want, but at least you will be able to make a more informed decision).
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Your midwife will be able to hear the sound of your placenta as your pregnancy progresses, and confirm that it is moving. Many placentas develop low in the uterus, appearing to be previas, and "move" upward as the uterus grows. Since you had an ultrasound, it is quite likely that your midwife will want you to have another later in the pregnancy to confirm the placenta has moved up out of the way.
Previas sound really scary, but the placental sounds are quite distinct, so you caregiver will be able to track your progress. Even if you had not had an ultrasound, there would be other symptoms later in the pregnancy which would surface warning of previa- spotting or bleeding which would signal to your midwife that you might have a placenta previa and need and ultrasound to confirm.
I hope that helps some...
The Lord bless you!