(Reuters) - Mississippi's Republican governor signed one of America's strictest abortion bills on Thursday banning women from obtaining an abortion once a fetal heartbeat is detected, which can often occur before a woman even realizes she is pregnant.

Dubbed the 'heartbeat bill,' this is the second legislative attempt in less than a year aimed at restricting abortions in a state with a single abortion clinic.

In a tweet earlier this week, Governor Phil Bryant thanked the state's legislature for "protecting the unborn" by passing the bill and sending it to him for his signature.

The Mississippi law joins a wave of similar Republican-backed measures recently introduced in Iowa, Kentucky, Tennessee and Georgia.

Conservative Republican proponents say these bills are intended to challenge Roe v. Wade, the U.S. Supreme Court's 1973 landmark ruling that women have a constitutional right to an abortion.

U.S. states are jostling for a showdown on abortion rights in 2019, with all eyes on the conservative-dominated Supreme Court.

Just last November, a U.S. federal judge struck down a Mississippi law banning most abortions after 15 weeks, ruling that it "unequivocally" violates women's constitutional rights.

The new Mississippi bill prohibits the abortion of a fetus with a detectable heartbeat, before the point where a woman may be aware she are pregnant.

It also states that any physician who violates the restriction is subject to losing the license to practice medicine.

The law makes exceptions for women whose health is at extreme risk. It is a victory for anti-abortion groups, but abortion rights advocates have promised to pursue legal action to overturn it.

"This ban is one of the most restrictive abortion bans signed into law, and we will take Mississippi to court to make sure it never takes effect," Hillary Schneller, staff attorney at the global abortion rights advocacy group Center for Reproductive Rights, said in a statement.

"This ban — just like the 15 week ban the Governor signed a year ago — is cruel and clearly unconstitutional."

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A fetus that is viable outside the womb, usually at 24 weeks, has widely been considered the threshold in the United States to prohibit an abortion.

Last week, a federal judge blocked Kentucky's fetal heartbeat abortion law. An Iowa judge overturned that state's heartbeat law in January after declaring it violated the state's constitution.

(Reporting by Gabriella Borter; Editing by Nick Carey and Richard Chang)