WestCoast Midwives Client Handbook
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Prenatal Diagnostic Tests
Diagnostic testing may be offered to you (all are optional) to see if your baby has a birth defect.
Who is offered diagnostic testing?
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Women of “advanced maternal age.” This is considered to be pregnant women who will be 40 years
or older at the time of delivery. However, in twin pregnancies, amniocentesis will be offered to
women who are or will be 32 years or older at the time of delivery.
Couples who have received a positive maternal serum screening result.
Pregnancies initiated by intracytoplasmic sperm injection (ICSI).
Couples in which either person has had;
o Another child or stillbirth with a chromosome abnormality
o Another child with a neural tube defect such as spina bifida or anencephaly, or a close
relative (brother, sister, niece or nephew) with a neural tube defect
o Another child with a known or suspected genetic disorder, birth defect or developmental
delay
Couples in which an ultrasound test has shown particular abnormalities in this pregnancy.
Couples in which either person is known to have a chromosome rearrangement.
Couples in which one or both partners have a known or suspected genetic disease or birth defect.
Couples in which both partners are carriers or a genetic disorder, for example
o Thalassaemia (more common in Mediterranean, Asian and East Indian populations)
o Tay sachs (more common in the Ashkenazi jewish population)
o Sickle cell anaemia (more common in the black population)
o Cystic fibrosis (more common in the Caucasian population)
Women who have been exposed during pregnancy to certain drugs or other chemicals that may
cause birth defects. This exposure could include women who have taken acutane (acne drug) or
those who have used cocaine or alcohol heavily during pregnancy.
Couples with a family history of known or suspected genetic disorders, for example; Duchenne
Muscular Dystrophy, Haemophilia, Myotonic Dystrophy or Fragile X syndrome.
What tests are used?
• Chorionic villus sampling (CVS) (not available in Victoria)
• Amniocentesis
• Detailed ultrasound
What is chorionic villus sampling? (not available in Victoria)
This is a newer alternative to amniocentesis for prenatal diagnosis. Chorionic villi form a tissue surrounding the
amniotic sac and form the placenta.
CVS can be done as early as 10 weeks of pregnancy. The procedure involves inserting a narrow plastic tube
through the vagina and cervix (done at 10 – 12 weeks) or inserting a slender needle through the abdominal wall
(done at 10 weeks to term). As with amniocentesis, ultrasound is used to guide the catheter during the test. A
small sample of placental tissue is then removed by gentle suction. The tissue is grown in the laboratory and
examined under the microscope. Final results take approximately 2 –3 weeks.
Advantages
• CVS can detect chromosome abnormalities.
• As with amniocentesis, if there is a family history of a known problem, other special tests may be done on
the sample, however these have to be arranged in advance.
Disadvantages
• CVS cannot detect neural tube defects. Further testing with detailed ultrasound and MSS are offered for
this.
• Mild problems including cramp and spotting happen occasionally. Vaginal spotting (bleeding) is more
common after Trans-cervical CVS tests and is usually not serious.
• Approximately 1 in every 100 women who have CVS will have a miscarriage following the procedure.
WestCoast Midwives Client Handbook
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There is a possible increased risk of limb abnormalities in the baby following CVS (an increase from 6 per
10,0000 births without CVS to a risk of 9 per 10,000 births after CVS).
CVS results are sometimes difficult to understand. In such cases, an amniocentesis test is usually offered
to clarify the results.
What is Amniocentesis?
Amniocentesis is the most common test used for diagnosing a chromosome problem with the baby. It involves
removing a small amount of the fluid which surrounds the baby in the amniotic sac. The test is relatively painless
and is usually done after 15 weeks of pregnancy.
How is it done?
Amniocentesis is performed by an experienced obstetrician in a hospital on an outpatient basis. It involves
inserting a needle through the mother’s abdomen (not through the navel) into the amniotic sac to take some of the
fluid which surrounds the baby. Ultrasound is used to locate the baby and placenta. With the ultrasound picture on
the screen, the specialist finds the safest and easiest place to insert the needle. The needle is then carefully
guided to the selected spot and a small sample of fluid is slowly withdrawn through the needle. The fluid sample
contains cells that the baby has shed from its skin and bladder. These cells are grown in the lab and then
examined under a microscope. It takes 2 to 3 weeks before the results are available.
Advantages
• It is a diagnostic test (“yes or no” answer). It can detect Down Syndrome and other major chromosome
abnormalities as well as Spina Bifida.
• If there is a family history of a known problem, other special tests may be done on the sample, however
these have to be arranged in advance.
• It is considered a safe test for the mother.
Disadvantages
• Although not usually serious, mild problems including cramping, bleeding and slight leakage of amniotic
fluid happen occasionally in the mother.
• Although very rare, there is a small risk of injury to the baby.
• Approximately 1 woman in every 200 (0.5%) will have a miscarriage following the procedure.
What is ultrasound?
Ultrasound is helpful in giving important information about a pregnancy. It may be used early in a pregnancy (8-12
weeks) to find a normal heart beat, to identify twins or to predict the baby’s due date. It can be done at 10-14
weeks to provide a Nuchal Translucency measurement as part of a prenatal screen. A detailed ultrasound can be
done later in the pregnancy (18 weeks) to look for structural problems in the baby and to see if the baby is
growing normally. Ultrasound is also used during amniocentesis and CVS (see below) to locate the baby and
placenta during the tests.
What are the risks?
Ultrasound is not an x-ray. Long term follow up has shown no differences in growth or development between
those children whose mothers had a prenatal ultrasound and those who did not. Ultrasound safety has never
been studied by randomised controlled trial, however, so it is recommended that pregnant women have only
those ultrasounds that are medically necessary.