Roe v Wade had three trimesters

He was trying to explain how Roe applied different standards of abortion rights based on the trimester of pregnancy due to a shifting balance between state interests and the individual's interests. Most importantly, the court held you were entitled to get an abortion without needing any reason in the first trimester. Casey changed this to a standard based on fetal viability.

This didn't have anything to do with the question, which was whether should be any limits on abortion based on the stage of development—as several states have none. Nothing in Roe or Casey prevented states from restricting the late-term abortions.