An influx of expectant mothers facing murder charges for the deaths of their unborn children in the U.S. has sparked outrage among women's rights activists.

Thirty eight states have introduced foetal homicide laws to protect pregnant women and their unborn children from violent attacks by third parties.

But they are increasingly being used to threaten and prosecute women over the outcome of their pregnancies, says the National Advocates for Pregnant Women (NAPW).

© FacebookBei Bei Shuai was charged in March with murder in the January 2 death of her three-day-old daughter, Angel Shuai
Campaigners say Rennie Gibbs is one such case. Gibbs was just 15 when she became pregnant, but her child was stillborn at 36 weeks in December, 2006.

After prosecutors discovered she had a cocaine habit, she was charged with the 'depraved-heart murder' of the baby, which carries a mandatory life sentence.

The case has sparked a firestorm, with Gibbs being the first woman in Mississippi to be charged with murder connected to the loss of her unborn child.

There is no evidence drug abuse was related to the baby's death.

Under Mississippi law it is a crime for any person except the mother to try to cause an abortion.

© Getty ImagesThirty eight states have introduced foetal homicide laws intended to protect pregnant women and their unborn children from violent attacks by third parties
However, anti-abortion groups have tried to amend the state constitution by setting up a state referendum that would widen the definition of a person under the state's bill of rights to include a foetus from the day of conception.

Gibbs' lawyer, Robert McDuff, called the accusations 'unprecedented' and quite extreme.'

He questioned the state supreme court: 'If it's not a crime for a mother to intentionally end her pregnancy, how can it be a crime for her to do it unintentionally, whether by taking drugs or smoking or whatever it is.'

His client, however, is not alone.

Bei Bei Shuai, is charged with murder in the January 2 death of her three-day-old daughter Angel Shuai.

© AlamyActivists say a new murder statute in Mississippi that makes women accountable for the outcome of their pregnancies infringes on the rights of expectant mothers
The 34-year-old attempted suicide on December 23 - at 33 weeks pregnant - by taking rat poison after she was dumped by her boyfriend.

She gave birth on December 31 and in March she was charged with murder and attempted foeticide. She remains in custody in an Indianapolis prison, without bail.

Lynn Paltrow of the campaign National Advocates for Pregnant Women (NAPW) said cases like theirs are infringing on women's rights.

She told the Guardian: 'Women are being stripped of their constitutional personhood and subjected to truly cruel laws. It's turning pregnant women into a different class of person and removing them of their rights.'

While the phenomenon is not new, it appears to be growing, Paltrow said.

NAPW estimates there have been up to 300 women arrested for their actions during pregnancy.

In 2006, Alabama introduced a 'chemical endangerment' law to protect children whose parents were cooking methamphetamine in the home.

Forty cases have since been brought to court under the statute.

Amanda Kimbrough is one of those women. Her foetus was diagnosed with possible Down's syndrome and doctors suggested she consider an abortion. She declined and the baby was delivered by caesarean section prematurely in April 2008 but the child died within minutes.

Six months later, she was arrested and charged under the statute, accused by prosecutors of taking drugs during pregnancy.

She has denied the claims and awaits an appeal ruling; if she loses, she will face a ten-year prison sentence.

She told the Guardian: 'I'm just living one day at a time, looking after my three other kids. They say I'm a criminal, how do I answer that? I'm a good mother,' she said.

The mothers on trial are receiving support - a reported 70 organisations have filed testimonies on Gibbs' behalf to protest her prosecution. On brief states to treat 'as a murderer a girl who has experienced a stillbirth serves only to increase her suffering.'

Another briefing from a group of psychologists argues there is a misconception about addiction, particularly involving mothers-to-be.

It states that Gibbs did not take cocaine because she had a 'depraved heart' or to 'harm the foetus but to satisfy an acute psychological and physical need for that particular substance.'

Paltrow says the backlash in Gibbs' case shows a hint of what women could face in the state if abortion rights were overturned - and if a state referendum is set up by anti-abortion groups.

'In Mississippi the use of the murder statute is creating a whole new legal standard that makes women accountable for the outcome of their pregnancies and threatens them with life imprisonment for murder,' she said.