in early pregnancy
sometimes placenta may be low lying and with progression of time the placenta will move upwards called as migration of placenta,
if placenta is completely covering the os then the possibility of migration is less, sometimes when placenta is invading the uterine layers blood can collect inside, injuries over abdomen or hypertension
can also lead to bleeding in placenta, small collections will absorb but large collections can affect the function of the placenta.
in case of low lying placenta sometimes painless bleeding can occur and that can be severe at times, so you and your family members have to be aware of it and if any symptoms develop have to contact the doctor immediately, possibly in next scan some improvement can be expected.