I'm going to cut and drop in here something I've said before.
Every woman having this problem (not believing the lab results) has a different reason they are anxious.
It could be guilt or shame over her behavior. It could be catastrophizing, such as they are thinking if someone found out they cheated, their whole life as they know it will end and the world will explode. It could be worry they won't be a good parent, or wishing the other guy was the dad and being ashamed of it, or resentment of the father, or fear of childbirth, or feeling God will punish them, or not really wanting to be a mom, or fear of lack of money, or it could be something else entirely.
Those worries are not easy to control, especially if the problem is shame or guilt, since having cheated can't be taken back and the person just has to live with the fact that she did it. (This latter kind of lingering angst often is a specialty of women who have never had to face up to having done something 'wrong' that can't be taken back, glossed, or fixed before.)
When such a feeling drives anxiety, the brain hates that it doesn't feel solvable. A person's mind in this kind of existential stress often lets the anxiety settle on something more cut-and-dried that to the brain feels more controllable. (Like obsessing over what if the the test is wrong.)
No amount of rational explanations will solve worries over the tests if the worries really don't come from disbelieving the test results. If your worries come from something else, like guilt, talk to a counselor about the real fears (not about the tests) and your fears will begin to leave.
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