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 1 tablespoon (15 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 genetic abnormalities present in your baby. It also accurately determines whether your baby is a boy or a girl.

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 ensure the correct number are present, and whether or not any parts are missing. These chromosome tests can detect disorders such as Down's syndrome (Trisomy 21) in which an extra chromosome number 21 is present.
• To decide on the 'maturity' of your baby's lungs if an early Caesarean section is being considered; for instance, in cases of foetal distress.
• To check for the level of AFP (alpha-feto protein) - a chemical made in 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 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.
• To determine your baby's sex, for instance when identifying gender-linked hereditary disorders.
• To reveal whether your baby is Rh-negative, although 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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