Prenatal screening for neural tube defects.

Neural tube defects are common birth defects with a prevalence of 4-15 per 10000 live-births. Prenatal diagnosis of neural tube defects has implications in obstetric management and many countries have successfully reduced their incidence by adopting nationwide preventive programmes. It is generally accepted that failure of the neural tube to close causes these defects. Periconceptional use of antiepileptic drugs such as valproate and carbamazepine and deficiency of folate is associated with a high incidence of these defects. Procedures for prenatal diagnosis include: (i) assessment of serum markers such as maternal serum alpha-foetoprotein and acetylcholinesterase activity; (ii) prenatal ultrasonography; and (iii) amniocentesis in selected cases. The levels of these two serum markers are increased in neural tube defects and a value of more than two multiples of the median is considered significant. The optimal time for serum screening is 10-18 weeks. In the case of serum markers it is important to apply normative data standardized for a laboratory and the duration of gestation. Antenatal ultrasonography is a simple, non-invasive and widely available test which has a similar sensitivity with lower false positivity compared to serum markers. Early diagnosis by ultrasound demands skill and experience in the procedure. Levels of amniotic fluid alpha-foetoprotein and acetylcholinesterase activity are elevated in neural tube defects. However, increased iatrogenic foetal loss is a disadvantage of this technique. Detailed counselling of the couple needs to be an integral part of the prenatal screening programme.