How to decide on antenatal tests for chromosomal abnormalities and other conditions

How to decide on antenatal tests for chromosomal abnormalities and other conditions

Chromosomal abnormalities that cause disability in babies

Most babies are born healthy and without disability.

In about 1 in 25 pregnancies, an unborn baby has a chromosomal abnormality or other condition that leads to disability. These conditions and disabilities vary a lot, from mild to severe. Some mean that the pregnancy miscarries in its early stages. Some can affect the baby's physical and/or intellectual development and chances of surviving the pregnancy or early weeks of life.

About antenatal tests for chromosomal abnormalities that cause disability

Antenatal tests can pick up some disabling conditions in pregnancy, including some caused by chromosomal abnormalities and inherited genes. It isn't possible to test for all disabling conditions in pregnancy.

Different kinds of tests for chromosomal abnormalities are done at different stages of pregnancy. The earliest you can have tests for chromosomal abnormalities is about 10-12 weeks of pregnancy.

These different kinds of tests can be screening tests or diagnostic tests.

Antenatal screening tests work out whether your baby has a high risk or low risk of having certain chromosomal abnormalities or other conditions that cause disability. Even if your baby has a 'low risk', this doesn't rule out the possibility that your baby has the condition.

If you get a high-risk result from screening tests, you can choose to have follow-up diagnostic tests. Diagnostic tests can give you a yes or no answer - yes, your baby has a condition, or no, your baby doesn't have a condition.

If you have a family history of a certain condition, you might choose to have some genetic tests before trying to get pregnant. These tests can often be arranged through a genetic counselling service.

Your choice: finding out about chromosomal abnormalities

Having antenatal testing in pregnancy is your choice.

Many people want to know whether their baby has a disability, so they can prepare practically and emotionally before the birth. Others might want to think about the option of ending the pregnancy.

Other parents choose not to have tests in pregnancy. This can be because they wouldn't choose to end the pregnancy even if there were abnormalities, or they want nature to take its course. Others are worried that tests could harm their baby or cause a miscarriage.

Deciding on antenatal tests for chromosomal abnormalities

To help you decide about antenatal tests it might be worth thinking and talking with your partner or a support person about some of these issues:

  • How do you feel about having tests? Anxious? Stressed?
  • If you get a high-risk result from a screening test, might you go on to have a diagnostic test?
  • Would you prefer just to have a diagnostic test, without having screening tests first?
  • If a diagnostic test shows that your baby does have a condition that causes disability, what might you do?
  • Do you want information that might make you think about ending your pregnancy?
  • Are you comfortable knowing that, if you don't have tests, it's most likely that your baby won't have a disability - but that there is a very small chance that your baby will have a disability?
  • Are you comfortable knowing that not all conditions can be tested for and that the tests are often for specific conditions, not for every disability?

You might also want to discuss questions and concerns with your GP, obstetrician or midwife. Here are questions you could ask and talk about:

  • What is my risk of having a baby with Down syndrome or other chromosomal abnormalities?
  • What conditions other than Down syndrome can be diagnosed? What would these conditions mean for my child?
  • Is the test safe for me and my baby? Is there a risk of miscarriage or other complications?
  • What will the testing cost?
  • How long will it take to get the results back?
  • What does 'low risk' and 'high risk' mean for me and my baby?
  • How much time do I have to decide about tests?
  • Where can I get more information about these conditions and tests?

Your decisions about antenatal testing shouldn't affect the quality of care you get in pregnancy.

Your choices about what to do if you have an increased risk shouldn't affect quality of care either. You have a right to good and supportive care no matter what decisions you make. If you don't get it, you have a right to speak to the staff about this or to make a complaint to the health service.

Sometimes asking difficult questions about antenatal tests for chromosomal abnormalities can help you and your partner decide about having the tests. The Murdoch Children's Research Institute's prenatal decision aid has more information about the decision-making process.

Chromosomal abnormalities

For normal human development, we need 46 chromosomes (23 from each parent) in all of the cells in our bodies.

Chromosomal abnormalities happen when there are missing or extra chromosomes or the chromosomes are changed in some way. These chromosomal abnormalities often cause conditions associated with physical and/or intellectual disability.

Down syndrome is an example of a condition caused by a chromosomal abnormality. People with Down syndrome have three copies of chromosome 21 instead of the usual two. This causes intellectual disability and other physical and learning challenges.

Another example is sex chromosome abnormalities. The sex chromosomes are the X and Y chromosomes. Girls normally have two X chromosomes, and boys normally have one Y and one X chromosome. An abnormal number of sex chromosomes might affect development and lead to learning difficulties, although sometimes there are no obvious problems.

Many chromosomal abnormalities cause miscarriage. Some chromosomal abnormalities will most likely cause the death of the baby soon after birth - for example, Edwards syndrome and Patau syndrome.

Gene abnormalities

A baby inherits half of its genes from each parent.

A gene abnormality is when a gene is changed or damaged. Sometimes a change in just one gene can cause a serious medical condition or disability.

Sometimes one or both parents can carry and pass on gene abnormalities that cause physical and/or intellectual disability. Other times gene abnormalities aren't inherited. Instead the changes to the gene happen in the egg or sperm at the time of conception.

If you have a family history of a particular genetic condition, or you have another child with the condition, there's a higher chance that your baby will have the condition.

Cystic fibrosis and thalassaemia are examples of conditions caused by a single gene abnormality.

Other conditions that cause child disability

Other conditions that can be picked up in antenatal tests include physical abnormalities like spina bifida, and some major heart, stomach or kidney problems. These conditions can be diagnosed unexpectedly at an ultrasound, which is one of the routine tests in pregnancy.