At common law, a child born to the wife during the marriage is presumed to be the husband's child, as determined by law. This well-settled concept is the "presumption of lawful paternity", and assigns to the husband complete rights, duties and obligations as to the child, regardless of whether he is the biological parent or not. The presumption, however, can be rebutted by evidence to the contrary, at least before a court issues a formal adjudication of paternity in the husband's favor, or a duty of support is established by a decree of divorce, annulment, or legal separation. Jurisdictions differ widely on whether, when, and under what circumstances a judgment establishing paternity or a support obligation founded on the presumption can be set aside on the grounds that the husband was not in fact the father.
In the case of an unwed mother a man may come forward and accept the paternity of the child, the mother may petition the court for a determination if she can identify the likely candidate(s) or paternity can be determined by estoppel over time.
Where paternity of the child is in question, any party with an interest may ask the court to determine paternity, of one or several potential fathers (called putative father) based initially upon sworn statements and then upon testimony, including but not limited to:
In some jurisdictions the courts have also declared the male who acts as the child's father to be the father through the equitable operation of an estoppel. Once the child has been declared to be a son or daughter and has lived with the man for a period of time, the court may cloak the putative father with all of the obligations of parenthood letting the biological father, "off the hook" so to speak.
Please remember in calculating the date of conception that the human gestation period is roughly 9 months, less 7-11 days.
A child born to the husband and wife prior to the marriage usually becomes a child of the marriage when the marriage is performed. This was the old rule from canon law, but at common law retroactive legitimacy was not conferred on a child by the marriage of the parents after the child was conceived; a matter of some import when inheritance was at issue. The different rules have largely been abrogated in the United States by a series of judicial decrees or legislation curtailing or diminishing the disabilities of bastardy.