Surrogate McKenna West is gearing up for a massive legal fight over the baby she carried ... with her attorney saying they're prepared to take the battle all the way to the U.S. Supreme Court.

West's attorney, Lincoln Wilson, told the New York Post they're challenging a California court order giving the baby's biological parents custody, arguing West should be recognized as the mom because she gave birth in Texas.

Wilson says his side believes the California judgment is invalid and, if they don't prevail in the lower courts, “we'll take it up to the US Supreme Court if we have to.” He maintains once the California order is tossed, West is the mom under Texas law.

The declaration comes just hours after we reported West currently has no decision making authority over the newborn.

We obtained a temporary restraining order barring West from making medical decisions for the boy, taking possession of him, representing herself as his parent or guardian, removing him from the hospital or interfering with the biological parents' access to him.

As we reported ... West entered into a surrogacy agreement with the California couple, which included a provision allowing them to request termination if a fetal anomaly was discovered.

The baby was later diagnosed with hypoplastic left heart syndrome, a serious congenital heart defect. West refused to terminate the pregnancy and traveled to Texas to give birth, setting off the legal showdown.

The TRO remains in effect until at least August 25, when West has been ordered to appear in a Dallas court.