First born child of a father. (Not the first born child of a mother). Because ..
15 If a man have two wives, one beloved, and another hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated:
16 Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn:
17 But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his.
Inheritance Rights of the Firstborn – (Deuteronomy 21:15-17) KJV
And yet we know of fathers who have abandoned their first born child, to be raised by other people, or by the mother alone, without any form of child support.
Well, God provides anyway, and God will deal with that “biological father”, make no mistake.
God can hear an unwed mother’s cries and her child’s too and protects them both