The 9th amendment has very little jurisprudence (binding court decisions, particularly from SCOTUS) behind it*. Most of what is available are in non-binding concurrences or dissents. It has very rarely been used to any legal effect, with the 14th amendment in particular providing the overwhelming majority of constitutional rights protections. Before about the 1980's one would be reasonably left with the impression that the amendment was completely forgotten and without effect (though note that the district court decision in Roe v. Wade used a 9th amendment argument, but was replaced with a 14th amendment one by SCOTUS).
The historical basis for the enactment of the 9th amendment was a fear that the fact that some rights are explicitly enumerated in the constitution would be used as a basis to deny (the existence of) other rights. So the amendment was intended to deny an "if they wanted to acknowledge and protect the right to X, they would have specifically listed it, just as they did with freedom of the press, religion, etc." argument. They also feared that such arguments would permit an expansion of federal powers, by offering up the implication that anything not denied to the federal government must implicitly be reserved to its power. This contradicted the intent of the constitution to restrict federal powers exclusively to specifically enumerated (or necessarily implied) ones. And that's the usual interpretation used by the judiciary.
This can take some repeating to really understand.
- The amendment is not interpreted as protecting or acknowledging any rights in any direct fashion.
- Rather it shields such potential rights from non-existence arguments that are based on the enumeration of other rights.
- The amendment's real purpose is to prevent expansion of federal powers beyond those expressly enumerated in the constitution.
- As a consequence (or in addition), the amendment does not protect such unenumerated rights from the constitution's enumerated powers.
The last point is specifically mentioned in the majority opinion for United Public Workers v. Mitchell
The powers granted by the Constitution to the Federal Government are subtracted from the totality of sovereignty originally in the states and the people. Therefore, when objection is made that the exercise of a federal power infringes upon rights reserved by the Ninth and Tenth Amendments, the inquiry must be directed toward the granted power under which the action of the Union was taken. If granted power is found, necessarily the objection of invasion of those rights, reserved by the Ninth and Tenth Amendments, must fail.
*However, in The Lost Jurisprudence of the Ninth Amendment, Kurt Lash argues that this is a pervasive but ultimately fallacious modern impression. He says in particular:
In fact, there is a surprisingly rich history of legal interpretation and
judicial application of the Ninth Amendment prior to Griswold [v. Connecticut (1965)]. Beginning in 1789 and extending to 1964, the Ninth Amendment played a significant role in some of the most important constitutional disputes in our nation's history, including the ratification of the Bill of Rights, the constitutionality of the Bank of the United States, the scope of exclusive versus concurrent federal power, the authority of the federal government to regulate slavery, the right of states to secede from the Union, the constitutionality of the New Deal, and the legitimacy and scope of the incorporation doctrine.