Pregnancy tests amniocentesis

Amniocentesis is a prenatal procedure that your doctor may recommend during pregnancy. It checks for fetal abnormalities (birth defects) such as Down syndrome, cystic fibrosis or spina bifida. In most cases the results are normal.

What amniocentesis is#

Amniocentesis is usually performed between 16 and 20 weeks of pregnancy. By around this time the developing baby is suspended in roughly 130 ml of amniotic fluid, which the baby continually swallows and excretes. A thin needle is used to withdraw a small amount of this fluid from the sac surrounding the fetus.

The fluid is examined to obtain information about the baby, including its sex, and to detect physical abnormalities such as Down syndrome or spina bifida. Amniotic fluid samples can also be DNA tested to identify a range of genetic disorders, such as cystic fibrosis and fragile X syndrome.

Amniocentesis is generally only offered to women thought to be at higher risk of having a baby with a birth defect. Discuss with your doctor or obstetrician whether the test is right for you.

What the test can detect#

Amniocentesis can detect a number of chromosomal and other birth abnormalities in a developing fetus, including:

• Down syndrome
• neural tube defects, such as spina bifida
• genetic disorders identified through DNA testing of the fluid sample, such as cystic fibrosis and fragile X syndrome

Who may be offered amniocentesis#

As a woman grows older, the chance of having a child with Down syndrome rises significantly, from about one in 2,000 at age 20 to about one in 100 at age 40. Women who may be candidates for amniocentesis include those who:

• are older, where age increases the risk of a chromosomal abnormality
• have an abnormal result from a serum screening blood test or an ultrasound examination
• have a family history of chromosomal abnormalities, such as Down syndrome
• have already had a child with a chromosomal abnormality
• are known to be carriers of a genetic disorder
• have a partner with a family history of a genetic disorder or chromosomal abnormality

Before having amniocentesis, the woman and her partner are usually counselled about the risks of the procedure.

The procedure#

The whole appointment takes about 90 minutes. The main steps are:

• The woman lies down, and the position of the fetus and placenta is checked with an ultrasound scan.
• Once the doctor is sure of a safe spot, they swab the belly with antiseptic and inject a local anaesthetic into the skin.
• Using a long, thin needle, the doctor withdraws about 15 to 20 ml (roughly three teaspoons) of amniotic fluid. This takes about 30 seconds.
• The fetus is checked afterwards to make sure all is well.

Your doctor will advise when to expect the results. In some cases this may take up to three weeks. You may be asked to wait around 20 minutes before going home. Most women find amniocentesis painless, though it is sensible to rest for an hour or so afterwards.

After the procedure#

Most women feel fine afterwards and no change to normal routine is needed, but it is generally wise to take it easy for the next couple of days.

Possible side effects and complications include:

• slight bruising at the injection site
• mild discomfort
• a small amount of vaginal leakage of fluid, which in most cases slows and stops within two days
• infection, which may cause symptoms such as fever
• miscarriage, which can occur in a small number of women undergoing the test; exact figures are hard to calculate because some miscarriages would have happened regardless of the procedure
• very rarely, the needle accidentally touching part of the baby’s body

If the mother is Rh negative, she may form antibodies that attack the baby’s red blood cells. To prevent this, an Rh negative mother is given Rh(D) immunoglobulin (anti-D).

Amniocentesis rarely causes lasting harm to either the mother or the baby. Complications are very rare. However, if you experience unusual symptoms, such as bleeding from the vagina, fever or persistent leakage of fluid, seek medical attention promptly.

Key points#

• Amniocentesis is only performed on women thought to be at higher risk of delivering a child with a birth defect.
• Before having amniocentesis, the woman and her partner are usually counselled on the risks of the procedure.
• An Rh negative mother is given Rh(D) immunoglobulin (anti-D) to prevent antibodies forming against the baby’s red blood cells.
• Amniocentesis rarely causes harm to either the mother or her baby in the long term.
• If you experience unusual symptoms, such as bleeding from your vagina, seek medical attention promptly.

Where to get help#