The birth of Rekha's second daughter should have been one of the happiest days of her life. Instead, she lay on the bed of her home on the outskirts of Delhi, the newborn child on the floor, screaming in terror as her mother-in-law poured paraffin over her.

This was her punishment, the older woman said, preparing to strike a match: Rekha had failed again to deliver a son and it would be better for everyone if she were dead. Suddenly the door burst open and her neighbours rushed in, roused by the frantic screaming. They bundled Rekha and her daughter out of the house, never to return.

In a country where boys remain prized and having a daughter is considered by many to be a curse, they were lucky. Many are not so fortunate. India has banned pre-natal scanning to determine the sex of a baby and made aborting a child as a result of such a scan punishable with five years in prison. Poster campaigns urge Indians to 'save the girl child'.

Yet the latest birth figures tell a story. In the state of Punjab, only 798 girls were born for every 1,000 boys. Haryana was next up the list with 819, followed by Chandigarh with 845 and Delhi, the national capital, came in fourth with 868. The Delhi-based Centre for Social Research, which recently surveyed the worst-affected parts of Delhi, estimates that 10 million girls have been lost to female foeticide in India over the past 20 years.

Most alarming for those monitoring the figures is the fact that the gap appears to be widening. Today the national average for births is 933 girls to 1,000 boys; in 1991 it was 945. 'The low numbers in a state like Delhi tells us the enormity of the situation,' said Anju Dubey Panbey, of the Centre for Social Research. 'In India today, if you are blessed with a son you are almost revered, and if you are the mother of daughters you are made to feel guilty and your status in your family goes down. It is very, very disturbing.'

Despite every effort to change perceptions, she said, many Indians simply do not want daughters, who are still seen as a financial burden because of the matrimonial dowry demanded by a groom's family. 'People don't want girls, because they have to worry about their safety and security and they have to pay to get them married off. People say bringing up a daughter is like watering a neighbour's plant,' she said.

The answer, for many, is to turn to the ultrasound clinics which display large notices warning that they are prohibited by law from carrying out scans to determine the sex of a baby - but which will do it if the price is right.

The Observer spoke to a number of medical practitioners and women who had attended such clinics. They revealed that one common trick involves using the form which the law requires to be signed to affirm that no sex determination has taken place. If the scan shows the foetus is male, the form will be signed in blue; if it is a girl the signature will be in red. Other clinics hand the family a blue sweet for a boy, a pink one for a girl. They can then arrange a quick abortion.

A scan costs about 2,000 rupees (£30). Those who cannot afford those prices turn to the unqualified midwives who proliferate in the poorer areas and who rely on less scientific methods. Some use crystals, others claim to be able to discern the sex from the way the baby is lying in the womb: a boy if the left side of the stomach is larger, a girl for the right.

There are plenty of such midwives in the industrial squalor of Bawana in Haryana state, on the north-western fringes of Delhi. Most of the residents were resettled there five years ago after being forced out of an illegal slum in the capital; many still spend a significant proportion of their income travelling back into Delhi every day to work as domestic staff. The area is pitifully poor and few feel they can afford the luxury of daughters. It is easier to pay the 500 rupees the midwives demand for a makeshift abortion.

This is where Rekha still lives. Dressed in a cheap blue-and-white sari, she sits nursing the one son she managed to bear before she was driven out. She talks about the beatings, how her mother-in-law asked her to kill her first daughter after she was born. 'They used to say they would throw me out and get another girl who could have more sons,' she said. Her story is extreme, but the pressure placed on her is not unusual. Munni, a neighbour in her early thirties, said she had already had one daughter and two sons when she found out she was pregnant a month ago. She and her husband, an electrician, could not afford a daughter, she said.

'I was afraid that it would be a girl child. I went for a lab test and they were asking 5,000 rupees, and I could not afford that, but the doctors said it looked as if I was carrying a girl, so I had an abortion. I went to one of the clinics near here to have it done. For any mother a daughter is not a burden, but for the family it is,' she said.

Another neighbour, Sheila Devi, said she was also worried after her first daughter was born. When she became pregnant again, she paid for a scan and was told that she was carrying a girl, so she went to a clinic a little way away and had an abortion. She now has three daughters, the youngest three months old, and one son. 'We wanted a boy, which is why we kept trying, but we ended up with three girls,' she said. 'My in-laws were creating a lot of problems because I had so many girls. They said it is my fault because I'm the one having girls all the time.' With demand apparently rising, those clinics prepared to flout the law are doing brisk business. The Centre for Social Research study found that more than 400 breaches of India's Pre-Conception and Pre-Natal Diagnostic Techniques Act had been reported, and a maximum fine of 40,000 rupees did not appear to be proving much of a deterrent.

The Observer visited a number in the Burari area of Delhi which are facing prosecution. The area is littered with small clinics offering scans. The Nanak Hospital displays a board outside warning that sex selection is prohibited, but the man who runs it, Dr Harkiran Singh, is facing prosecution for carrying out an illegal abortion.

The hospital is a grubby building by the side of the road, with a reception area crammed with women and children. In his tiny consulting room, Singh insisted he was resolutely opposed to sex selection - 'for me it is a strict no, it shouldn't even be in your mind' - but was unable to explain why he had been arrested and bailed. 'I think it was bad luck,' he said.

But he had heard of other clinics that were doing it. 'Whoever is desperate will go from pillar to post to find it and there may be places that will do it,' he said. 'My friends say that they are getting these inquiries. In the cities it is more difficult, but elsewhere it is not so hard to find.'

It is not just the backstreet clinics that stand accused. The large and modern Jaipur Golden Hospital in Rohini, Delhi, has been named in a case brought by a paediatrician, Dr Mitu Khurana, who claims she was taken there by her in-laws and scanned against her will. Once they discovered she was carrying a girl, they tried to make her have an abortion. She has lodged an action against her husband, his family and the hospital.

Dr Ashish Chandra, a senior administrator at the hospital, admitted that the hospital had destroyed the paperwork relating to the case but denied any wrongdoing. Instead he complained that the authorities were going after the wrong people. 'The law is well-intentioned, but the problem is in the implementation. The big hospitals like this follow the rules scrupulously, but what about the outside areas, where there is no regulation? There is an unregulated market,' he said. 'The authorities should be following up the people who are doing the ultrasounds, the one-man bands, because it is easier for them to do it.'

Earlier this year, India's Prime Minister, Manmohan Singh, condemned female foeticide as 'inhuman, uncivilised and reprehensible'. Yet the sheer scale of the problem appears to be thwarting the government's stated intention to tackle it. The High Court in Delhi recently took Google and other internet firms to task for running ads for sex selective abortion. But no sooner does one door shut than another one opens. Before scans were widely available, unwanted girls tended to be killed shortly after birth. With the arrival of scans, the focus switched to abortions.

Now those with enough money can go one step further by using IVF treatment to ensure that they get a male child. Fertility expert Dr Shivani Sachdev Gour, of Delhi's Phoenix Hospital, said she was regularly approached by wealthy families who offered 500,000 rupees or more to ensure that their next child was a boy. 'They put tremendous pressure on you for sex selection for embryos. They want you to implant a male child. It is illegal to select embryos on the basis of sex, but they are determined to get it done and money is not a problem for them. People are blatant. I tell them it is illegal, but they say [they] know that.'

She said that although her own hospital rejected such approaches others did not. 'I know there are clinics in Delhi that do this. I know of a few places where the charges are five or 10 times what a normal scan would cost, but still people are doing it and there is a market for it. I just don't know what to say about my doctor colleagues who are doing it. I just don't know what to say.'

'He threw me out to remarry someone who can give him a son'

The pregnant mother forced to flee her husband

Dr Mitu Khurana, 32, had been married only two months when she became pregnant with twins in January 2005. It was the start of a nightmare that has seen her become the first woman in Delhi to start legal proceedings against her husband for trying to force her to abort their babies.

Dr Khurana says her husband, Dr Kamal Khurana, and his family were determined that she should not give birth to girls. She said that when she was admitted to the Jaipur Golden Hospital in Rohini on an unrelated matter, her husband and mother-in-law persuaded the staff to carry out an ultrasound scan to determine the sex of the babies.

'They didn't say anything to me, but afterwards it was clear that my husband and my in-laws knew that I was carrying girls,' she said. 'After that, they began badgering me to have at least one of them killed. They told me we could not bring up two girls, we would not be able to afford to get them married.'

Her husband maintained that as a priest had told him he would only father one son, the girls could not be his. He demanded a DNA test. Dr Khurana gave birth to the girls, Guddu and Pari, two months prematurely on 11 August, 2005. She maintained that, when one of the girls was four months old, her mother-in-law tried to push the child down the stairs.

Faced with open hostility from her husband and his family, she eventually left her husband in March. 'He threw me out of the house because he wants to remarry someone who can give him a son,' she said. She has filed a complaint, which is being investigated under India's Pre-Conception and Pre-natal Diagnostic Techniques Act. But she said she had struggled to have her allegations taken seriously.

'Even as an educated woman I am pushed around,' she said. 'But my daughters are now my biggest source of happiness, and I am proud that I have saved them.'

She already had two girls - then a third pregnancy led to tragedy

The mother who died after forced abortion

Nirmala Devi had given birth to two daughters when she became pregnant again this year. She died on 21 October after undergoing an abortion, allegedly forced on her by her husband and his family.

According to a report filed to the police by her brother, Surender Singh, Nirmala had been under pressure for some time to produce a male child. Singh told the police that her husband, Prem Singh, and his family had regularly beaten her since their wedding in 2003.

When they discovered she was pregnant again, they forced her to undergo an ultrasound test at a clinic in Noida, in the state of Uttar Pradesh, to determine the sex of the three-month-old foetus. The test apparently revealed that the child was a girl and she was taken to a hospital where an abortion was carried out. She failed to regain consciousness and died later at her home, where she had been taken by her husband.

Police have registered a case for murder and placing a woman under pressure to abort a female foetus. A police spokesman said they were investigating claims that she had been harassed to the point where 'she succumbed to their torture and died'.

Prem Singh, his mother Govindi Devi, father Dugar Singh and sister Bhagwati Devi have all been arrested in connection with the allegations.

Prem Singh claims that she demanded an abortion and that she underwent the procedure voluntarily.

'My husband beat me a lot and my mother-in-law tortured me

Teenage mother of two daughters tortured by in-laws

Rekha was eight when she was married. Now 18, her first daughter was born when she was just 13. She now lives with her mother in Bawana, an industrial area in Haryana state on the north-western edge of Delhi.

Although she also bore her husband one son, it was not enough for him or his family. They were angry that she was burdening them with the two daughters she gave birth to. 'It started when my first daughter was born,' she said. 'My husband beat me a lot and my mother-in-law tortured me.' They would hit me with sticks and anything they could get their hands on.

'When I had my first girl my mother-in-law wanted me to kill her once she was born. When the second girl was born, the day she was born, my mother-in-law was asking me to leave the house and I would not leave. She came into the room I was in. I was on the bed. She poured [paraffin] on me and she had matches and she was threatening to light it. The baby was sleeping on the floor when she did this.

'I started screaming and crying and the neighbours ran in and they said to me that it was better that I leave the house because I would be killed if I stayed. My mother-in-law just said it would be better if I were dead.'

She left the house that day and has not been back there since.