Sitting there in the courtroom, never caressing the bump under her pink teenage top the way expectant mothers do, she is composed beyond her years.

Only occasionally does she extend the index finger of her right hand to discreetly scoop away an escaping tear.

When the lawyers' discourse veers towards the crude minutiae of her foetus's condition, she and her boyfriend leave the courtroom, quietly.

It is the way she speaks to her mother, with whom she shares a high-boned beauty, that best demonstrates her maternal instincts. The sketchy information that may be published indicates a troubled family history. Her father left a long time ago, her mother has trouble coping, though "is good when there is a crisis", according to her daughter's affidavit, and there is a younger sibling whom Miss D loves greatly.

On Thursday morning, as the former attorney general, Eoghan Fitzsimons, commenced a full day's exposition of Miss D's case to Judge Liam McKechnie and more than 20 other lawyers, her mother started to shake and sob. "Stop crying, " her daughter whispered sternly, as if their roles had been reversed.

"Stop it." Watching the exchange, one prayed that, some day, this girl will have a child of her own to mother.

She had wanted this baby. She had already bought the nappies, though the birth was not due until the autumn.

How excited she must have felt on Monday 23 April, setting off for the hospital with her boyfriend for the first glimpse of their baby on an ultrasound scan.

They would have blended in with the other expectant parents, though she was only newly 17 and he, betrayed by kind eyes under a harsh haircut, was not many years older.

The nurse brought them to a cubicle and smeared gel on Miss D's bump. The ultrasound screen flickered like a magic show and there appeared the grainy outline of their growing baby, all 16 weeks and one day old. "Look, there's a leg, " the nurse would have been saying, "and there's an arm." But Miss D was looking and what she saw was that her baby "had no head". The nurse left to summon a consultant. "Your baby has anencephaly, " they were told. Together, they ran out of the hospital, the doctor's words screaming in their ears, about their baby having no brain and being born dead, or dying soon afterwards.

That night, she left the B&B where she had been lodging under a HSE interim care order, and moved into her boyfriend's mother's house. She went on the internet to learn more about what was happening to her. Wikipedia would have informed her that, if born alive, her baby would be blind, deaf, unconscious and devoid of the thinking part of the brain, and that the remaining brain tissue would not be covered by bone or skin, exposing it to rapid infection. She would have read that some clinicians advise withholding nutrition and hydration on the grounds that euthanasia is morally and clinically appropriate. And she would have learned that 95% of women who discover they are carrying an anencephalic baby have an abortion.

By Thursday, her unviable foetus had become a prisoner of the state in her womb and she a prisoner in her country.

Her social worker, R, warned that she would be arrested if she attempted to go to Britain for a termination and that, if she took a legal case challenging his decision, it would be "a media circus". R phoned and wrote to a garda superintendent notifying him that Miss D was to be prevented leaving the jurisdiction, eliciting a reply that the garda had no power to stop her in the absence of a court order. Another letter was sent to the Passport Office declaring that the HSE, in loco parentis of Miss D , did not consent to a passport being issued in her name. It was the nightmare ayatollah's moral police state, caricatured 15 years ago during the X case, come horribly true for another teenage girl.

"I was shocked at how they [the HSE] ignored my wishes, " she said in her affidavit to the court. "It makes me feel very vulnerable in their care."

Miss D, supported by her boyfriend, his mother and her own mother in her wish to terminate the pregnancy (without financial assistance from the state), phoned a solicitor for an appointment but R also made an appointment for her to see a psychiatrist at the same time. The HSE believed that, if she was suicidal, she would qualify for an abortion under the X case ruling and an application could be made to the district court under section 47 of the Childcare Act for authorisation to leave the jurisdiction. It had in mind an incident on 31 January last, when Miss D took 10 tranquillisers at her home, a dramatic gesture she describes not as a suicide bid but an attempt to get her mother's attention, culminating in her mother leaving the house and the girl calling an ambulance for herself.

To Miss D, who can no longer bear to look at herself in the mirror, it seemed a Hobson's choice: either be committed to a psychiatric hospital or get arrested trying to leave the country.

"I want to arrange my personal affairs with as much dignity and as little personal trauma as possible, " she pleaded in her affidavit.

The psychiatrist assessed her as showing "no evidence of any psychiatric symptomology", thus, in the HSE's view, shutting the door on an abortion.

The broader X case criterion for an abortion, a risk to the pregnant girl's life, was not considered. Yet an expert report read to the court on Thursday referred to a "potentially life-threatening thrombosis" resulting from an anencephalic pregnancy.

According to her senior counsel, Eoghan Fitzsimons, former legal adviser to the government, Miss D could have applied for a termination in an Irish hospital, based on the state's defence in a case brought against Ireland in the European Court of Human Rights.

An Irish woman had an abortion in England after one twin she was carrying died in her womb and the other was diagnosed with a terminal condition;

she sued under the European Convention of Human Rights but lost the case when lawyers for Ireland argued that she had not exhausted the Irish courts, which would have entertained the hypothesis that termination should be provided in a case where the foetus was unviable.

Pointing out that, in Miss D's case, "Ireland and the attorney general believe she cannot be prevented from travelling for an abortion", Fitzsimons argued that the legal impasse "is entirely unnecessary". He added: "My client is faced with five legal teams and she has to endure this process."

Throughout, Judge McKechnie has been compassionate and conscious of the urgency. When the hearing resumes tomorrow for an extraordinary bank holiday sitting, Miss D will have been pregnant exactly 18 weeks and one day.

In the courtroom, her body clock eclipses the hands of time turning on the wall clock. She did not attend on Friday, sparking a rumour that she was unwell.

The ever-deepening tragedy for her is that she is in the legal care of a state whose own fitness to act as a parent has been called into question. As the High Court heard on Friday, the HSE is the temporary parent to hundreds of children. For Miss D, whose anencephalic pregnancy poses the risk of hysterectomy, there is the awful danger that she will never get the chance to fulfil her promise as a mother.