Good evening, thank you for your question.
Absent a custody order, where parents are married, each is presumed to have equal custody of the child and can freely take the child. However, to prevent this, you could go to court and seek a temporary order of physical custody to keep your child in the state. You can get the forms from your local Clerk of Court. There would have to be a hearing on the matter. If your wife leaves the state before the hearing and takes the child, she would have to return with the child, and if the court decides that the child should stay with you, the child would have to be returned to your custody as well.
Your wife's idea to move first and then file for divorce doesn't really work, because in order to do so, she would have to first establish residency, which depending on the state, can take between 6-12 months.
Please let me know if I have not fully addressed your concerns.
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