The Probate and Family Court does, as you allude, have broad equity powers, including powers with respect to child custody and visitation. Case law tends to suggest that in a situation like yours, if you are arguing de facto parent status, you would be wise to also file a Complaint in Equity while the divorce is pending. The Complaint in Equity will most likely track with the same judge hearing the divorce, and hearings could be consolidated. If you do not file a Complaint in Equity, you could always try to argue that the issues relative to custody and visitation should still be heard through the divorce based upon the Probate and Family Court's broad equitable powers, but your argument would be much stronger if you file the Complaint in Equity.
You should speak with a knowledgeable Family Law attorney to discuss your situation in greater detail. Since the divorce is already pending, you need to act quickly. Most attorneys, including my firm, offer free initial consultations. Good luck!
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Unfortunately, there is not enough information here to answer this question definitively. This is an area of law that turns on the specific and exact language of the agreement. You should bring the agreement and documents to an attorney to find out a definite answer for your question.
Generally, you can enforce a contract regarding custody so long as it was signed off by a judge. However, it sounds like the judge never agreed to the custody agreement so there may not be an enforceable claim. . .
Sorry to not be of more help!
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I think you are right to be concerned. The divorce statute gives the Probate and Family Court authority to determine custody, etc. regarding a "child of the marriage". Even an equity judgment that you are a de facto parent of the child does not necessarily make the child a child of the marriage. Without your adopting the child, unless the child was the result of artificial insemination through a medical facility, the male genetic material provider still has parental rights and would have to be joined in any custody action.
If you had gotten married before the child was born, Mass. General Laws chapter 209C, section 6, MAY have given you legal status of parent status.
I say "may" because although the Mass. Equal Protection Act and the Goodridge case generally work to give same gender partners equal rights and eliminate distinctions of gender in statutory language, I have not seen a case since In re Tammy allowed same sex parents to adopt the biological child of one of them that applies 209C:6 to a female de facto parent of another female's child.
If neither biological parent ever challenges you and your soon to be ex-spouse falsely claiming the child as a child of the marriage, the divorce decree may be enough. But, if there is a chance that either bio parent might try to undo it later and assuming that adoption is no longer an option as far as either of the bio parents are concerned, you would be better off with a formalization of your de facto parent status that is applicable as against both bio parents in order to make your custody agreement an order of the court.
You need the advice of an attorney familiar with and friendly to same gender parental relationships to help you through the legal maze of effectuating your and your spouse's current desires in regard to the custody issue. You might want to use the Find a Lawyer function here on avvo.com to search for an attorney experienced in same gender parenting matters or submit your question to gladanswers.org or use the GLAD Lawyer Referral Service: http://www.glad.org/rights/infoline/lawyer-referrals
NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney in your area who regularly practices in the subject matter which your question is about. You should develop an attorney client relationship with the lawyer of your choice so that your communications will be subject to the attorney client privilege and have the other benefits of a professional relationship. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific matter as partially described in the question.
You mentioned your partner gave birth just before you were married--were you married within 300 days of the child's birth? If so, the child is a child of the marriage because the presumption of parentage written into MA law applies regardless of the parents' genders/biological sexes in Massachusetts and there is no need for an equity action to establish parentage.
If this isn't the case, I would recommend bringing your agreement to an attorney to discuss the details of your case in a bit more detail. Parentage has the potential to get complicated, and its worthwhile to take the time to go through things in detail and understand the strengths and weaknesses of your position and your options for moving forward.
As a side note--my read on the case law in general is a bit different than everyone else's--I don't believe a separate equity action is necessary, and further, there is actually case law stating that filing a separate equity action while a divorce is pending between the parties is inappropriate.