Pregnancy & Baby 

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Amniocentesis – an invasive test

Dr Howard Lee describes what it is, who is offered it and what it does

Amniocentesis is a specialised diagnostic test that involves removing a small amount – usually about 2 tablespoons (20 mls) – of amniotic fluid (liquor) from the sac that surrounds your baby in the uterus (womb). This is so that the fluid, which contains hundreds of living cells from your baby’s skin, can be examined in a laboratory for changes. These can indicate – with a great degree of certainty – whether or not there are any serious abnormalities present in your baby.

What kinds of abnormalities can it detect and what are the reasons for the test?

• To diagnose any inherited disorders that your baby may be at risk of
• To count the number of chromosomes, to see if parts of them are missing. These chromosome tests can detect disorders such as Down’s syndrome.
• To decide on the ‘maturity’ of your baby’s lungs – if a Caesarean section is being considered; for instance, in cases of foetal distress and where early delivery is desirable.
• To check for the level of AFP (alpha-feto protein) – a chemical made by your baby’s liver. This is found in very high levels in the amniotic fluid if there are certain birth defects that cause a break in the skin – such as occurs in Spina-bifida, where the vertebrae of the back fail to close up, and Anencephaly, a condition in which the brain is incomplete or missing.
• To remove excessive amniotic fluid, or look for infection in the amniotic cavity.
• Your baby’s sex can be determined, and this is important when identifying gender-linked hereditary disorders.
• Can also reveal whether your baby is Rh-negative, but this test is usually performed later on in pregnancy.

Amniocentesis doesn’t detect every kind of abnormality – for example, it cannot tell you if your baby has a cleft palate or lip.

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