In the context of family law, paternity refers to the legal responsibility of fatherhood conferred on a parent. Paternity is a uniquely gendered issue due to the nature of biological reproduction. While there can be in doubt as to who is the mother of a child, the father can be more dubious if, for example, there is adultery or other circumstances that muddy the waters of paternity. 

The reason paternity is so important has to do with what responsibilities a father has. Whoever is assigned paternity has an obligation to support the child. This could include paying child support if the couple is not married or divorces after the birth of the child. Additionally, establishing a legal father allows the child to collect an inheritance, life insurance, and other benefits from the legal father.

While paternity can be established as soon as the child is born, if not sooner, paternity can be established after the child is born as well. In general, paternity can be established at any point before the child reaches 18, though some states may extend this to 21. 

This means that as long as the child has not reached the cutoff age, paternity can still be established. While it is possible to establish paternity after a child is born, it can become more complicated the longer it is put off.

If both parents are willing to submit to blood testing, a paternity test can be performed as early as the 7th or 8th week of pregnancy. In general, this type of early paternity testing will only be necessary if the potential father denies their paternity.

If a child has already been born, then paternity tests can be performed using non-invasive DNA collection techniques. Most often, this will manifest as a buccal swab, where the inside cheek of the mother, father, and child are taken and compared.