Four Republican legislators in New Hampshire will introduce a bill that would ban abortion at “15 days” gestational age, according to a copy of the bill they prefiled.

The bill, which is unlikely to pass, amounts to an outright abortion ban. Gestational age is calculated from the first day of the woman’s period; at 15 days gestational age, a fertilized egg — if it exists yet — has most likely not implanted in the uterine wall. Implantation is the point when pregnancy begins, according to the American College of Obstetrics and Gynecology.

Doctors often estimate that women ovulate around two weeks after their period begins — at about 14 days gestational age — and fertilization is thought to occur shortly afterward, though these times vary. A fertilized egg is nonviable until it implants on the uterine wall, which is thought to typically occur a week or more after fertilization.

The bill is the latest in a string of abortion restrictions Republicans have pursued since the Supreme Court overturned the constitutional right to an abortion last year. More than a dozen states have banned abortion completely or do not have facilities where people can obtain an abortion, NBC News reported this year.

New Hampshire currently restricts abortion after 24 weeks, except in cases of fatal fetal abnormalities or in certain circumstances to preserve the life and health of the mother. The proposed bill maintains those same exceptions.

New Hampshire Gov. Chris Sununu, a Republican who signed the 24-week ban into law, has promised to protect access to abortion and called himself “pro-choice.”

State Rep. John Sellers, a Republican co-sponsor of the bill, said he wasn’t sure why the bill’s author had targeted 15 days, but said he would support any restriction on abortion.

“I believe life begins at conception,” he said, pointing to fertilization.

State Democrats slammed the bill.

“In a state where voters overwhelmingly believe that reproductive health decisions should be made solely between patient and medical provider, this legislation is absurd. Stopping this bill isn’t enough; it must be completely renounced,” said state Rep. Alexis Simpson.