Finding fetal abnormalities may be the result of screening tests carried out on most or all pregnant women, or as the result of specific diagnostic tests performed to detect specific conditions. Prenatal diagnosis is important as it will identify babies who might need medical or surgical treatment before or soon after birth. In addition, it may also detect severe abnormalities for which parents might decide to have a therapeutic ABORTION.

ULTRASOUND scanning is probably the most widely used diagnostic tool in obstetric practice. It can detect structural abnormalities such as SPINA BIFIDA and CLEFT PALATE and even cardiac and renal problems. A series of scans can assess whether the baby is growing at a normal rate; ultrasound may also be used to assist with other diagnostic tests (e.g. AMNIOCENTESIS – see below).

Tests on the mother's blood can also diagnose fetal abnormalities. Alphafetoprotein (AFP) is produced by babies and ‘leaks’ into the AMNIOTIC FLUID and is absorbed by the mother. In spina bifida and other neural-tube defects there is increased leakage of AFP, and a blood test at 16 weeks’ gestation can detect a raised level which suggests the presence of these abnormalities.

The triple test, also performed at 16 weeks, measures AFP and two hormones – HUMAN CHORIONIC GONADOTROPHIN and unconjugated OESTRADIOL – and is used in diagnosing DOWN'S SYNDROME.

Amniocentesis involves inserting a needle through the mother's abdominal wall into the uterus to remove a sample of amniotic fluid at 16–18 weeks. Examination of the fluid and the cells it contains is used in the diagnosis of Down's syndrome and other inherited disorders. The test carries a small risk of miscarriage.

Chorionic villus sampling may be used to diagnose various inherited conditions. A small amount of tissue from the developing PLACENTA is removed for analysis: this test has the advantages of having a lower incidence of miscarriage than amniocentesis and is carried out at an earlier stage (9–13 weeks).

Analysis of a blood sample removed from the umbilical cord (cordocentesis) may diagnose infections in the uterus, blood disorders or inherited conditions.

Direct observation of the fetus via a viewing instrument called a fetoscope is also used diagnostically and will detect structural abnormalities.

Most tests have a recognised incidence of false positive and negative results and are therefore usually cross-checked with another test. Counselling of the parents about prenatal tests is important. This allows them to make an informed choice about whether or not to ask for the pregnancy to be terminated. (See PREGNANCY AND LABOUR.)