Attempts to stop or 'reverse' medical abortions may cause women to bleed out dangerously, a new study suggests.

Six states in in the US have enacted laws that require doctors to inform patients, incorrectly, that there is medication to stop or 'reverse' a medical abortion if they should change their mind after taking the first of the two doses involved.

But there's not science to prove that doing so - by administering a high dose of the hormone, progesterone - is effective, let alone safe.

Researchers at the University of California, Davis, set out to test just that, but had to stop their trial short when three developed such severe bleeding that they had to be taken to the emergency room via ambulance.

Though their study was unable to gather enough data to say with certainty whether or not progesterone can stop an abortion, it was enough for them to warn that lawmakers should not mandate what amounts to 'human experimentation.'

After attempting to test claims that that medication abortions can be 'reversed' after women take the first dose of mifepristone (pictured) if they take a high dose of progesterone led to dangerous bleeding for three women, UC Davis doctors urged that not taking the second drug may be dangerous

'For now, the level of evidence is still inadequate to support or refute the benefits and risks of progesterone treatment to stop a medical abortion after taking mifepristone,' said study lead author Dr Mitchell Creinin, an OBGYN at UC Davis Health.

'Laws should not mandate counseling or provision of any treatment that claims to reverse abortion when both its efficacy and safety are unclear.'

As anti-abortion sentiments have swelled int the US, so has the number of states with laws making it more difficult for women to terminate pregnancies by a several approaches.

In addition to the controversial 'heartbeat bans,' effectively making abortion illegal as early as six weeks into a pregnancy, Arkansas, Idaho, Kentucky, Nebraska, South Dakota and Utah have all enacted laws to require doctors to tell their patients about purported 'reversals' for medication abortions before prescribing the drugs.

North Dakota and Oklahoma have enacted similar laws, but they've been put on temporary holds by court orders.

In these states, lawmakers require providers to tell women that if they change their mind after taking the first drug involved in a medical abortion, mifepristone, their abortion can be stopped in place and 'reversed.'

A medical abortion involves a dose of mifepristone, followed by a dose of a second drug, misoprostol, to be taken 24 to 48 hours later.

Used as instructed, in combination, the drugs are approved as safe and effective for aborting pregnancies within their first 70 days by the Food and Drug Administration (FDA).

But women in these states are advised that a high dose of progesterone - a sex hormone involved in menstruation as well as in the retention of an embryo in the uterine lining in the earliest days of pregnancy - will stop the pregnancy.

The theory is that flooding the body with progesterone will block the mifepristone's abortive effects on receptors for progesterone.

But the only documentation of this phenomenon are anecdotes and case studies of instances in which the pregnancy did end. There were no standardized studies to test the administration of progesterone against a placebo.

To test this, the UC Davis team set out to recruit 40 women who had decided to have surgical abortion, but were okay with waiting a couple of weeks after discovering they were pregnant to get the procedure.

They also consented to take the first dose of a medical abortion, mifepristone, but not the second, with either progesterone or a placebo taken afterwards instead.

Planned followup visits were set for two weeks after they took that first dose to see if the women were still pregnant and assess any other changes.

Quickly, the study took a turn for the worst.

Of the first 12 women who took mifepristone, four bled so severely they had to be rushed to the hospital.

Unbeknownst to the women, one had been given progesterone, and two had received placebos.

That small sample wasn't enough to determine anything about whether it was possible or safe to continue their pregnancies.

But it was enough of a safety concern for the researchers to shut the whole trial down.

'Women who use mifepristone for a medical abortion should be advised that not following up with misoprostol could result in severe hemorrhage, even with progesterone treatment,' said Dr Creinin.