Amniocentesis is usually done around the 16th week of pregnancy, but can be performed later. Under ultrasound guidance, a fine needle is passed through the mother’s abdomen into the uterus and a small sample of the amniotic fluid surrounding the baby is collected. The fluid contains cells from the baby which can be grown in culture in a laboratory and used to look at the baby’s chromosomes. Occasionally the fluid may be tested for a particular problem, such as an infection.
Most women do not describe the procedure as painful but it can feel uncomfortable and it is not unusual to feel some period-like cramping pains afterwards. It is a good idea to take it easy for 48 hours after the procedure and avoid strenuous physical activity such as heavy lifting.
QF-PCR and FISH are molecular tests that can be performed on the sample to provide a rapid but accurate diagnosis of Down’s syndrome and two other rare but serious chromosomal syndromes called Patau’s and Edwards’ syndromes. Results take on average three working days. All hospitals can offer this service through the laboratories they use, but some will ask you to pay a fee.
A full karyotype means the laboratory use cells from the sample to look at all the baby’s chromosomes under a microscope. They check for any major changes in the chromosomes and can tell the baby’s sex. It is a longer process than the rapid tests and so results can take up to three weeks.
Very occasionally the sample of amniotic fluid taken cannot be used by the laboratory to obtain an accurate result. If this happens you will be offered a repeat procedure.
Microarray (sometimes called array CGH) is an advanced method of genetic testing of a sample from CVS or amnio. It can detect copy number changes in a baby’s chromosomes. This means it looks for where there are deletions (bits missing) or duplications (where there are extra bits) in the baby’s DNA that would not be identified through the full karyotype. It is now sometimes used instead of or as well as karyotyping.
Why do some women miscarry after amniocentesis?
Approximately one out of every 200 women who has an amniocentesis will have a miscarriage. It is difficult to give a definite reason why this happens.
Most of those few women who have a miscarriage due to the procedure have it fairly soon afterwards, within around 72 hours. There is still a risk of miscarriage for up to two weeks after the amniocentesis, but it is extremely unlikely to happen.
If the healthcare professional performing the procedure is skilled at doing it, this can help to minimise the risk. This skill comes with experience so you can ask how often the doctor who is going to do your amniocentesis does the procedure. The Royal College of Obstetricians and Gynaecologists recommends that a healthcare professional should do at least thirty procedures a year to retain his or her competency.
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