Losing a child is a devastating experience which few people can imagine.

Yet for those working in specialist baby units, it is a common occurrence as they battle to help unborn babies and those born prematurely or with serious health problems.

The Scottish hospital units which cared for baby Jennifer Jane Brown, the premature daughter of Chancellor Gordon Brown and his wife Sarah, who died just ten days after she was born, has been praised by the couple.

Now, the work of a world famous specialist birth unit in London has been captured on camera in a new television documentary.

A BBC team was given unprecedented access to the unit, where they filmed the daily battles of a leading medic and his team to save the lives of unborn and newly-born babies.

The programme follows the moving stories of those for whom doctors at the Harris Birthright Research Centre for Fetal Medicine in King's College Hospital are the last hope.

The centre, opened by Diana, Princess of Wales in 1984, specialises in fetal medicine and surgery. The driving force behind it, Professor Kypros Nicolaides, was involved in the case of Mandy Allwood's doomed eight-baby pregnancy in 1996.

Nicolaides, a Greek Cypriot, is an expert in the field. He was the first doctor to perform laser surgery on a baby in the womb and pioneered an alternative to the high-risk tests for Down's Syndrome.

Each year, more than 15,000 couples from across the UK pass through the doors of the NHS unit, described by the senior midwife as "busier than Waterloo station", for routine scanning.

Most leave reassured their babies are developing healthily. Others, like Stacy and Steve Cork from Egham, Surrey one of several couples featured in the programme have to deal with the fact there are serious problems with the pregnancy.

The Corks have already lost three babies to a disease called allo immune thrombocytopenia, where the mother's immune system attacks the unborn child's cells responsible for clotting blood. Their unborn baby is suffering from the same disease.

The couple are faced with a choice between an early Caesarean section, risking an extremely premature delivery, or leaving the baby to grow but risking it haemorrhaging and dying in the womb.

Nicolaides, who works 18-hour days, says being a father of two young children means he emphathises with his patients.

"The patients are my life. They're my family. When a foetus has a problem it becomes a challenge to cure.

I'm constantly reminded of my own children and I see that foetus as a child to be."

Vanessa Ward and Roy Peters, from High Hurstwood in East Sussex, a young couple expecting their first child, are devastated when doctors alert them to a tumour on the baby's spine which is the same size as its head.

Vanessa's baby must be delivered by Caesarean section and if it survives, the tumour will be surgically removed a few days later.

These are just some of the patients for whom pregnancy is an emotional roller-coaster ride where anything can go wrong at any time.

The doctors are not always successful and some couples leave the hospital devastated their baby has not survived.

One of the most moving scenes in the two-part series is the baptism in the hospital of a newborn girl, hours before her death.

But there are happy moments, too.

One of the most touching is when senior midwife Pat Sorhaindo telephones patients with good news test results she knows will change their lives.

"You know you are not God but you do feel responsible," she says.

The second part of Life Before Birth can be seen on BBC1 on Wednesday at 9pm.