The answer is maybe. Keep good records. The law clearly states that both parents are entitled to records held by third parties, unless there is an order to the contrary.
Keep good records regarding your ex's attempts to prevent you from getting the child's medical records. This may not carry the day in a hearing, but if you have other evidence in addition, it will be helpful in future proceedings like a change of custody or a termination of joint custody.
Nothing in this post creates an attorney-client relationship. This post is not meant to serve as legal advice.
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