Morality seeks the best good and the least harm for everyone. In the question of abortion there are two interested parties, the baby and the mother. In
Roe v. Wade, the Court made a reasonable decision. In the first trimester, the fetus is not yet developed enough to be considered a "person", that is, a thinking and feeling entity with an interest in its own life. In the third trimester, the baby has a stake in the outcome, because it can experience and suffer harm, and could, if delivered and cared for, survive outside the womb.
In practice, the CDC's Abortion Surveillance System FAQs indicate that nearly all abortions fall within the first trimester: "The majority of abortions in 2019 took place early in gestation: 92.7% of abortions were performed at ≤13 weeks’ gestation; a smaller number of abortions (6.2%) were performed at 14–20 weeks’ gestation, and even fewer (<1.0%) were performed at ≥21 weeks’ gestation. "