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Abortion could become illegal in more than half the states if a future Supreme Court overturned the 1973 Roe v Wade decision, according to a new report from the Center for Reproductive Rights.

"More than 37 million women in 33 states are at risk of living in a state where abortion could become illegal," the group said.

The report was issued to observe the 44th anniversary of the Supreme Court decision.

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Donald Trump said during the presidential campaign that he would appoint "pro-life justices" to the Supreme Court.

If the court voted to overturn Roe, "it will go back to the states, and the states will then make a determination."

Pro-choice activists hold signs in front of the U.S. Supreme Court on Jan. 22, 2014 on Capitol Hill in Washington, D.C. Alex Wong / Getty Images

Based on an analysis of party control in state legislatures and governorships, as well as existing laws and constitutional provisions, the report said 22 states were at the highest risk of banning abortion. Most are in the central and southern US.

Four of them — Louisiana, Mississippi, and North and South Dakota — have laws in place that would automatically make abortion illegal if Roe v Wade were to be overturned.

Eleven states were classed as an intermediate risk. And in the remaining 17, abortion rights were said to appear secure.

22 high risk states: WI, ID, ND, MI, SD, IN, OH, UT, MO, KY, WV, AZ, KS, AR, TN, SC, OK, LA, MS, AL, GA, TX 11 moderate risk states: NH, IL, WY, IA, PA, RI, CO, NE, VA, DE, NC 17 states where the right appears secure: ME, VT, WA, MT, MN, NY, MA, OR, NV, NJ, CT, CA, MD, NM, HI, AK, FL

"We cannot go back to the days before Roe, when some women put their lives on the line when they needed to end a pregnancy," said Nancy Northup, the Center’s president.

President Trump has pledged to nominate a replacement promptly for Justice Antonin Scalia. Any nominee who opposed the Roe decision would have the same view Scalia did.

But Trump might need to put two more appointees on the court in order to make overturning Roe a realistic possibility.

Sen. Jeff Sessions, Trump's nominee for attorney general, said in his confirmation hearing that while he opposes abortion, "It is the law of the land, it has been so established and settled for quite a long time and it deserves respect. And I would respect it and follow it."