The only thing I've ever heard that is under your control that *might* reduce your risk of placenta previa is don't smoke crack. I would recommend that course of action anyway. I don't think, however, that this results in practical risk reduction for your case.
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I had placenta previa (full at the 18 week scan, partial when I started bleeding like crazy at 32 weeks), and a surgical delivery. They wanted two units of blood in the OR with me, in case the damage to the placental blood vessels resulted in dangerous amounts of bleeding. Placenta previa is a b!tch of a complication.
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While KaraJMay is right that placenta previa can't get worse, please keep in mind that not all obstetrical imaging equipment is created equal. I had a scan at L&D at 25 weeks that showed partial previa, and another scan at a different L&D at 28 weeks that showed complete previa. It wasn't that my placenta moved "backwards" it's that the ultrasounds were insufficiently clear (and with the baby's head right on top of the placenta, unless you do a transvaginal scan, it's really hard to be sure what the placenta covers). If you are diagnosed with this condition, you should insist on having a high-level ultrasound (including transvaginal scanning) to determine the location of the placenta before making decisions about non-emergency care. However, please keep in mind that emergencies are fairly common in previa cases, and you may not have the choices you would want to have.
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All that said: Previa persisting to delivery occurs only about every 1 in 200 pregnancies. I wouldn't worry about it unless you're diagnosed.