Is she a virgin? How long have you been married? Does she get regular gynecological exams? Vaginismus is a disorder that causes involuntary spasms around the vagina that make penetration extremely painful, if not impossible. This can be diagnosed with an exam. There are other causes of painful intercourse, so your wife really needs to see a physician about this problem. She should start with her gynecologist to rule out physical problems. After that, if she is still experiencing problems, she can talk to a psychologist or psychiatrist who treats for sexual dysfunction. Either way, I advise you not to put it off. This can cause a great deal of problems in your marriage if it isn't dealt with, and neither one of you should have to suffer through this when there is help out there.
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Lori