Amniocentesis is a prenatal diagnostic procedure where fetal cells from the amniotic fluid are obtained and used to determine whether a fetus’ chromosomes are normal or whether there is a chromosome abnormality present.
Amniocentesis is usually performed after the membrane has fused to the uterine wall, at approximately 15-16 weeks.
Amniotic fluid is the fluid in the uterus that surrounds the fetus. It is mostly made up of the fetus’ urine. As with our skin cells, fetal skin cells are constantly being shed. The amniotic fluid is rich in fetal skin cells and fetal cells lining the lungs and the bladder.
During amniocentesis, a small amount (about 15-20 mls) of amniotic fluid (which contains these fetal cells) is removed under ultrasound guidance and sent to a pathology department, where a specialised medical scientist, or cytogeneticist, will culture (grow the cells) and then examine them through a microscope.
A short term result is usually available in 24-48 hours and in 10-14 days we will get a long-term culture or microarray result.
Who is offered this test?
Amniocentesis offered to patients
who are high risk for chromosome abnormalities.
This may include:
A high risk Combined First Trimester Screen for Down Syndrome or other Syndromes
A high risk Non-Invasive Prenatal Screen (NIPS)
An anomaly detected at the 20 week morphology scan
There has been a previous pregnancy with a chromosomal abnormality or other genetic disorder
There is a known inherited genetic disorder in the family of either the mother or father of the fetus for example cystic fibrosis, thalassaemia or sickle cell disease.
How is the amniocentesis performed?
Using an ultrasound probe for accurate guidance and to ensure a safe distance from your baby, the Specialist will pass a fine needle through your abdomen and uterus into the amniotic fluid.
A small sample of the fluid surrounding the baby is removed using a syringe. The needle is then taken out and your baby is observed on ultrasound. The amniotic fluid, which contains some of the baby’s cells, is sent to the laboratory for testing.
Occasionally, for fewer than 7 in every 100 women having amniocentesis, not enough fluid can be taken at the first attempt and the needle has to be re-inserted. This is can be due to the position of your baby. If a second attempt fails, you will be offered an appointment to have the procedure again on another day.
What are the risks of an amniocentesis?
Every pregnancy carries a risk of miscarriage. As amniocentesis involve putting a needle through the wall of the womb, there is a small additional risk of a miscarriage due to injury or infection in the womb. The additional overall risk of miscarriage from amniocentesis is approximately 1:1000. In other words, up to one extra woman in every 1000 who have amniocentesis under ultrasound guidance after 15 weeks will miscarry.
You may notice some ‘spotting' of blood and cramping for a few hours afterwards. This is normal and it is safe to take paracetamol.
If you experience any unusual symptoms immediately after the test, such as feeling shivery (as if you have flu), fluid loss, bleeding or contractions, then you should seek advice.
Two sets of results will be available; the short term result is usually available within 1-2 days which examines the 5 most common abnormal chromosomes including Down Syndrome.
The long term result takes around 10-14 days and looks at all chromosomes in greater detail. One of our Doctors at Wellington Ultrasound or your referring Doctor will discuss the short term results with you over the phone within 2-3 days of you having the test.
The long term results will be discussed with you by your Doctor, over the phone with one of our Doctors or at your next appointment with us. If the result is abnormal our Specialists are here to help, providing further information and counselling.
Care following an amniocentesis
Some women have mild cramping and a small amount of vaginal loss, either bleeding and/or amniotic fluid, after the procedure. This usually resolves without treatment over 24 to 48 hours. If it persists or is heavy, please seek medical advice.
It is recommended women avoid strenuous activities and sexual intercourse for 48 hours following a amniocentesis. Paracetamol is safe to take in pregnancy and may be required following the procedure.
If your blood group is Rh (sometimes called rhesus) negative, you will be recommended an injection of anti-D immunoglobulin after the procedure to prevent you from developing antibodies against the baby’s blood cells.