Defense Secretary Ashton B. Carter’s landmark decision Thursday to open all jobs to women in the military was greeted by many as a sign of changing times and a move toward true gender equality. But it also raises a related question: Will women be required for the first time ever to register for military drafts, and is it constitutional if they are not?

The Selective Service System has existed for decades, and is used to provide the U.S. military with enough manpower when it short-handed in a time of war. A variation of it was adopted in 1917, as the United States prepared to join World War I, and it has been updated over the years.

Selective Service laws, however, have never required women to subject themselves to the draft and face the prospect of being forced into military service. The current version of the Military Selective Service Act requires that virtually all men in the United States between the ages of 18 and 26 register, most within 30 days of turning 18. That includes non-U.S. citizens living in the United States, such as refugees. But women are entirely exempt.

The issue has come up on occasion since then-Defense Secretary Leon Panetta rescinded the longtime ban on women serving in ground combat units in January 2013. Panetta gave the services until this fall to make recommendations on how they wanted to proceed on implementing the full integration of women, but the Selective Service issue is unresolved.

Carter is required by U.S. law to submit a detailed analysis of legal implications if more jobs are opened to women when it comes to the Selective Service Act. White House Press Secretary Josh Earnest acknowledged that Friday, telling reporters the Obama administration will work with Congress to look at the analysis and determine if any additional reforms or changes to the act are necessary.

Sen. John McCain (R.-Ariz.) and Rep. Mac Thornberry (R.-Tex.) brought the issue up in a joint statement Thursday, saying they “look forward to receiving the [Defense] Department’s views on any changes to the Selective Service Act that may be required as a result of this decision.”

At the Pentagon, Carter said he did not know how the issue would be resolved, but acknowledged that the Selective Service issue is a “matter of legal dispute right now.” He added that “unfortunately,” it is subject to ongoing litigation. At least two lawsuits have been filed against the Selective Service System since the combat exclusion policy was appealed in 2013.

In one, a New Jersey girl sued saying the system was discriminatory because it did not consider women for the draft. In another, the non-profit National Coalition for Men did the same. The latter case is expected to be reviewed by the U.S. Court of Appeals for the 9th Circuit in Pasadena, Calif., on Tuesday.

A defense official, speaking on condition of anonymity due to the sensitivity of the issue, declined to elaborate on Carter’s position, but drew attention to prepared remarks he provided to the Senate Armed Services Committee in January when he faced a confirmation hearing for his current job.

“Given that the Armed Forces have waged the longest continuous conflict in our history with an All-Volunteer Force, and the fact that most military career fields are now open to women, a review of the military selective service act would be prudent,” Carter said at the time. “This is not solely a Defense issue, but rather part of a much broader national discussion.”

In those same remarks, Carter noted that the Pentagon was analyzing whether the Military Selective Service Act was constitutional in light of the Defense Department’s move to include women in more jobs. The law has been challenged previously, with the Supreme Court reversing the ruling of a lower court in 1981 in Rostker v. Goldberg with the high court saying that the Selective Service System could stand as it was because women were not allowed to join combat units.

“The existence of the combat restrictions clearly indicates the basis for Congress’ decision to exempt women from registration,” wrote Justice William Rehnquist in the majority opinion. “The purpose of registration was to prepare for a draft of combat troops. Since women are excluded from combat, Congress concluded that they would not be needed in the event of a draft, and therefore decided not to register them.”

Those combat restrictions, of course, no longer exist, raising the question whether the military is in violation of the Equal Protection Clause of the U.S. Constitution’s 5th Amendment, which Rostker v. Goldberg addressed.

The dissenting opinion in that case, written by Justice Byron White, said that he assumed “what has not been challenged in this case” — excluding women from combat roles — also did not violate the Constitution.

Granting that, White argued, it is evident that in a time of war that if all non-combat positions must be filled by combat-qualified troops, there would be “no occasion whatsoever to have any women in the Army,” either as volunteers or draftees.

“I perceive little, if any, indication that Congress itself concluded that every position in the military, no matter how far removed from combat, must be filled with combat-ready men,” White wrote. “Common sense and experience in recent wars, where women volunteers were employed in substantial numbers, belie this view of reality.”

Justice Thurgood Marshall also wrote a dissenting opinion saying he believed the majority decision “categorically excludes women from a fundamental civic obligation” in registering for the draft.

“Because I believe the Court’s decision is inconsistent with the Constitution’s guarantee of equal protection of the laws, I dissent,” he wrote.

Update: This post has been updated to include comments from White House Press Secretary Josh Earnest on Friday, Dec. 4.