The bill puts Iowa, a mostly rural state in the midwest that famously swings in elections, in the forefront of restricting abortions.
Iowa's governor signed a bill that is now considered the most restrictive abortion law in the country.
The law bans most abortions after a fetal heartbeat is detected, which can happen within six weeks of the start of pregnancy. There are exceptions for rape and incest.
Although Reynolds reportedly acknowledged the likely prospect of a legal challenge, she stood by her decision.
The conservatives' effort to restrict abortions is part of a bigger picture countrywide where anti-abortion activists are pushing for tougher legislation to protect fetuses.
"The bold pro-life action taken by the Iowa legislature reflects growing national pro-life sentiment and restlessness under the extreme status quo imposed by Roe v. Wade", Marjorie Dannenfelser, the president of pro-life group Susan B. Anthony List said in a statement.
In the past, abortions were banned after 20 weeks.
"We will challenge this law with absolutely everything we have on behalf of our patients because Iowa will not go back", said Suzanna de Baca, president and CEO of Planned Parenthood of the Heartland, in a statement.
State Republicans have said such a case could go to the Supreme Court.
Critics argued the bill would ban abortions before some women even know they're pregnant.
But the Supreme Court in 2016 rejected appeals by both North Dakota and Arkansas to preserve similar "heartbeat" laws, which had been struck down by lower courts.
In Iowa, the same Republican-majority Legislature passed a 20-week abortion ban previous year. Still, Republican lawmakers indicated they would push for more restrictions this session.
Reynolds said in a statement that she recognizes the controversy, and expects lawsuits to follow.