No one has been prosecuted under the 2007 law, which state officials and clinic employees have said has resulted in less congestion outside the clinics.
The court which bars protests on the plaza outside its own building but allows them on the public sidewalks last considered abortion-clinic protest zones in 2000 when it upheld a Colorado law.
It seemed possible that there could be more than the five votes needed to strike down the law after Justice Elena Kagan said she was "hung up" over the size of the zone.
But it was hard to tell whether the court might also upend its 2000 ruling in support of the Colorado zone, which has been criticized by free-speech advocates for unfairly restricting protesters' rights.
That's because Chief Justice John Roberts, normally an active questioner, did not ask a single question of any of the three lawyers who argued the case.
Semicircles are painted outside Planned Parenthood clinics in Boston, Springfield and Worcester. Inside the line, protesters and supporters alike risk arrest.
One woman who shows up most Tuesdays and Wednesdays outside the Boston facility, Eleanor McCullen, 77, was at the court Wednesday, as was Marty Walz, the sponsor of the 2007 law in the Massachusetts House of Representatives and now the president and CEO of the Planned Parenthood League of Massachusetts.
McCullen said the law has made it harder, but not impossible, to persuade women to have their babies rather than abort.
In an exchange with Justice Stephen Breyer, lawyer Mark Rienzi started to make a point about being asked to stand back an additional 35 feet to make his argument on behalf of McCullen.
"I'd hear you," Breyer cut in.
"You might hear me," Rienzi said, "but I would suggest you'd receive it quite differently."
McCullen and other protesters at those clinics sued the state over its 2007 law setting up the buffer zone. Federal courts in Massachusetts have upheld the law as a reasonable imposition on protesters' rights.
Walking through the zone takes just a few seconds, Justice Ruth Bader Ginsburg said. "There's not much you're going to be able to do to have a conversation that will persuade people in seven to 10 seconds," she said.