The provision that people are talking about when they tell you you cannot establish your legal rights because the mother is married to someone else is Mass. General Laws chapter 209C, section 5(a), which says, in pertinent part, "provided, however, that if the mother of the child was or is married and the child’s birth occurs during the marriage or within three hundred days of its termination by death, annulment or divorce, complaints to establish paternity under this chapter may not be filed by a person presumed to be or alleging himself to be the father unless he is or was the mother’s husband at the time of the child’s birth or conception."
Section 5(b) says that you and the mother may file a voluntary acknowledgment of paternity "only if the mother and the person who was the spouse of the mother at the time of the child’s birth or conception sign an affidavit denying that the spouse is the father of the child; provided, however, that where the marriage has been terminated by annulment or by the death of either spouse, paternity of the putative father may only be established by filing a complaint to establish paternity as provided in this chapter."
If they go through their divorce without her husband denying paternity and settle or adjudicate their divorce with findings or acknowledgment that the new baby is a "child of the marriage" you will have additional burdens to overcome. But being named as the paramour in a fault divorce for adultery has significant legal ramifications such that you would be well advised to retain counsel to help you negotiate the process of establishing your rights.
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YOU CAN TRY BY FILING A PATERNITY ACTION.
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It may be possible to intervene in their divorce, It is entirely possible that her Husband has no knowledge of this event. Gaining knowledge may compel him to amend his complaint for divorce and disavow this child.
You will need an experienced attorney to help you navigate this process.
A child born during the marriage is presumed to be a product of that marriage, but it is a rebuttable presumption. Jumping in may assist your rights with regards to the child but will surely give you duties of support and other complexities. You should definitely seek individual counsel.
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