Kudos to you for recognizing it is not safe to take pravastatin (or any cholesterol-lowering medication) while pregnant!
Obviously, you should discuss your pravastatin use and your desire to conceive with your health care provider. In general, pravastatin has a short half-life of about two hours, which means about two hours after you take it, only half the dose is still circulating in your blood. If you completely discontinue pravastatin, it will gradually leave your bloodstream altogether.
There's no way to tell when the drug has completely left your system, and every individual's body eliminates drugs at a different rate, so your best bet is to have a discussion with your health care provider to see how long you should wait before trying to conceive.
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