Even if born alive, a baby with this neural tube defect usually dies within hours, writes Dr Muiris Houston, Medical Correspondent
Anencephaly, meaning the absence of the greater part of the brain and of the skull bones, is an example of a neural tube defect (NTD). Other NTDs include spina bifida, where part of the spinal cord protrudes through an unfused vertebrae and cephalocoele, the protrusion of part of the brain through a bony defect in the skull.
Open neural tube defects occur in two to five babies per 1,000 pregnancies. Taking folic acid before conception has been shown to reduce the risk.
The neural tube, which develops into the brain and spinal cord, is usually completely formed and closed by the end of the third week of intrauterine life.
Neural tube defects are the commonest form of congenital central nervous system malformations.
In anencephaly, the most severe form of NTD, there are no cerebral hemispheres and no development of the vault in which the brain resides. The condition is uniformly fatal; many children with the condition are stillborn, but if born alive the baby usually dies within hours.
A detailed ultrasound looking for foetal abnormalities is usually carried out at between 18 and 20 weeks of the pregnancy, at which time a severe NTD such as anencephaly is readily identifiable.
A maternal blood sample to measure a substance called alphafetoprotein (AFP) can also be used to screen for NTDs. If it is raised then an invasive procedure called amniocentesis, in which a sample of foetal cells is taken from the amniotic fluid surrounding the baby, may be performed.
If the foetal AFP is also raised there is a 99.5 per cent chance of NTD being diagnosed.
In the UK, many specialist units no longer use the AFP test, relying instead on ultrasound to detect a neural tube defect. It is estimated that, faced with a serious congenital abnormality, between 80 per cent and 90 per cent of parents in Britain opt to terminate the pregnancy.