Low lying placenta around 19weeks is a common finding.as the pregnancy advances and a proper lower uterine segment is formed in all probability it will migrate.clitoral stimulation should nt cause any problems to it.avoid insertion of fingers inside specially with u groomed nails as that can cause bleeding from cervix which will mimick like placental spotting and give u some unnecessary stress.

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Consequent orgasm  can initiate labour and and you up in profuse bleeding

Better to avoid.