About an Amniocentesis

An amniocentesis involves using ultrasound to guide a fine needle into the fluid that surrounds a fetus. A small sample of the fluid is then drawn off into a syringe and sent for testing.

What do I do before and after the test?

The procedure requires no special preparation and takes about 20 minutes. After resting in our recovery room for about 5-10 minutes, you can go home to rest for at least 24 hours. Ideally, try to arrange for someone else to drive you home and stay with you for a little while afterwards. You should book time off work the next day and try to arrange for help if you have small children or animals to look after.

When is amniocentesis done and who will do the test?

Amniocentesis is performed after 15 weeks of pregnancy. All amniocentesis tests are carried out in our centre by Dr Bryan Beattie, Consultant in Fetal Medicine. Dr Beattie leads the specialist NHS Fetal Medicine Unit in Cardiff and has performed over a thousand amniocentesis procedures.

What are the risks of amniocentesis?

The main risk is a miscarriage, which occurs in about 0.5% (1:200) pregnancies following amniocentesis. There is a huge variation in miscarriage rates between different consultants and Dr Beattie’s measured rate is lower than the average, at around 1 in 300 pregnancies (0.33%).

There are also risks of abdominal cramps and vaginal bleeding – usually these are mild and do not cause any problems (you can safely take paracetemol for pain relief). If there is heavy bleeding, severe pain, fever, vaginal discharge or vaginal fluid loss you should always contact your midwife or local hospital immediately.

Why do you need to know my blood group?

Women who are Rhesus Negative need an injection of anti-D within 72 hours to protect their baby against the development of antibodies in the mother which can cause anaemia and jaundice in the fetus.

What tests are done on the fluid?

There are 3 main chromosome tests applied to the fluid:

  • PCR – this works in 98% of cases to give a result within 3 days for Down Syndrome (Trisomy 21), Edwards Syndrome (Trisomy 18), Patau Syndrome (Trisomy 13) and if requested the sex of the baby
  • A karyotype – takes about 14 days and will identify most of the rarer chromosomal problems not picked up by the PCR
  • An Array CGH – will identify up to 10% more chromosomal abnormalities that would not be picked up by a karyotype, this is the most accurate test and ideally we would advise this in all cases

We also test for the dF508 cystic fibrosis genes which are responsible for about 85% of cases of cystic fibrosis.

Sometimes the amniocentesis may fail to give a result, give a result which is difficult to interpret or an unexpected result and if so it will be explained to you what implications this may have for the baby.

How will I get the results?

This will be discussed at the time of the amniocentesis but will usually be by phone followed by a report posted to you, the hospital and your GP.