Pregnant immigrants are missing out on pre-birth care because pressure is growing on the NHS to enforce immigration rules rather than provide care, a charity has claimed.
Doctors of the World UK said undocumented migrants and asylum seekers were being turned away from GP practices or received "huge bills", even after losing their babies, as the NHS was "asked to play the role of the UK Border Agency".
The charity said a crackdown on so-called "health tourists" - foreign nationals travelling into the UK to use the NHS - led to confusion, even though the pre-natal treatment was necessary and could avoid complications with a birth.
Dr Clare Shortall, report author, doctor and volunteer family clinic lead for Doctors of the World, said many of the women had been in the UK for four years and "do not fit the dubious categorisation of 'health tourist".
She said: "In no compassionate society would a health system bill a grieving woman for the loss of their child. Health trusts are increasingly being asked to play the role of the UK Border Agency and focus on checking documents rather than delivering care. This has to stop.
"Pregnant migrant women are also regularly sent letters from NHS trusts that have a very strong deterrent effect. Rather than mentioning their right to care, they ask for proof of entitlement, often directly mentioning the UK Border Agency. These sort of scare tactics need to stop.
"Both because the bills they face and the deterrent letters they receive, extremely vulnerable pregnant women are not getting the care they need, sometimes with tragic results."
The report, which surveyed the views of 35 women at a London drop-in clinic run by the charity, found two-thirds had not received antenatal care until at least three months into pregnancy, while more than half went without care for more than 20 weeks.
And two had received bills for four-figure sums even though they had lost their babies.
The women surveyed included victims of human trafficking, according to Phil Murwill, who runs Doctors of the World's clinic where the study was conducted.
He said: "By law antenatal care should always be provided, but we regularly see women who've been denied access to primary care, putting both mother and child at risk.
"We've seen extremely vulnerable women who've been trafficked into the country who've accessed antenatal care, but who've then received threatening letters from hospitals demanding repayment for thousands of pounds that they simply don't have."
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Commenting on the report, Cathy Warwick, chief executive of the Royal College of Midwives, said: "We have real concerns that the aggressive pursuit of charging migrant women for medical care may deter them from accessing maternity care.
"I fear that these women could fall through the cracks and only find their way into the health system when it is too late - if at all.
"Women from these groups are often already in poorer health, have poor pregnancy outcomes and these steps could have negative consequences for their health.
"Our view is very clear: Midwives should not act as gatekeepers to the maternity services. They owe a duty of care to all pregnant women who seek care from them and, they should provide care to all pregnant women irrespective of the woman's ability to pay.
"We urge service providers to exercise compassion and sensitivity when dealing with migrant women, especially when they have suffered the tragic loss of their baby."
A Department of Health spokesperson said: "International visitors are welcome to use the NHS, provided they pay for it - just as UK families do through taxes.
"With the NHS busier than ever, our plans to recoup these costs better will help keep services sustainable.
"As a key part of our existing plans, GPs must give appointments to vulnerable patients and women will never be refused hospital maternity care if they haven't paid in advance."