Pregnancy and baby

Screening tests for abnormalities in pregnancy

What happens at a scan and what will they tell me?

Media last reviewed: 13/04/2012

Next review due: 13/04/2014

Checks for abnormality

Most expectant parents worry at some time during pregnancy that there may be something wrong with their baby. Some people find that talking openly about their fears helps them to cope. Others prefer not to think about the possibility that something could be wrong.

Some women worry because they are convinced that if something does go wrong it will be their fault. You can increase your baby's chances of being born healthy by following the advice about your health during pregnancy, including information on a healthy diet, exercise, smoking and drinking. However, there are certain problems that can't be prevented, either because the causes are not known, or because they are beyond your control.

How common are abnormalities?

Of all the babies born in the UK, 97% are healthy and 1% will be born with abnormalities that can be partly or completely corrected. However, about 2% of babies will have a major abnormality that cannot be corrected. Screening tests help to pick up potential problems and allow appropriate action to be taken.

If you are particularly concerned – perhaps because you or someone in your family has an abnormality or a disability – talk to your midwife or doctor as soon as possible. They may be able to reassure you or offer you helpful information and tests to check for abnormalities, which can be done during pregnancy.

If you have previously had a baby with an abnormality or disability, talk to your midwife or doctor to see if you need any additional care during this pregnancy.

What abnormalities will I be screened for?

You'll be routinely offered screening tests for:

  • spina bifida
  • Down's syndrome
  • thalassaemia

And also possibly:

  • sickle cell anaemia (if you are at risk)
  • Tay-Sachs disease (if you or your partner are Jewish)
  • cystic fibrosis (if there is a family history)

What screening tests are used?

The screening tests offered during pregnancy are either ultrasound scans or blood tests, or a combination of both. Ultrasound scans may detect structural abnormalities, such as spina bifida. Blood tests and scans can help to detect chromosomal abnormalities, such as Down’s syndrome.

Find out about screening for Down’s syndrome.

Down’s syndrome is caused by an abnormal number of chromosomes. Chromosomes are the structures within every cell of a person’s body, and carry the individual genetic code or "instructions" to make that person. Conditions such as cystic fibrosis and achondroplasia (a form of dwarfism) are caused by abnormalities within the chromosomes (a "mistake" in the genetic code).

You’ll be offered an anomaly scan when you are around 18-20 weeks pregnant.

Screening tests for spina bifida and Down's syndrome

You'll be offered screening tests that can detect structural abnormalities, such as spina bifida, which is an abnormality of the spine, or some chromosomal abnormalities, such as Down's syndrome. Different maternity units may use different tests, but all tests will meet national standards. You can discuss the tests and what they mean with your midwife. 

Screening tests can:

  • reassure you that your baby is likely to be born healthy
  • give you time to prepare for the arrival of a baby with special needs
  • allow you to consider the termination of an affected pregnancy

Tests can also provide valuable information for your care during pregnancy. However, no test can guarantee that your baby will be born without an abnormality. No screening test is 100% accurate, and some abnormalities may be undetected before the birth.

Screening for inherited disorders

Some diseases or conditions are inherited from one or both parents. These include cystic fibrosis, haemophilia, muscular dystrophy, sickle cell disorders and thalassaemia. If you, your baby's father or any of your relatives has an inherited condition, or if you already have a baby with an abnormality, talk to your doctor.

You may be able to have tests early in pregnancy to check whether your baby is at risk, or is affected. Ask your GP or midwife to refer you to a clinical geneticist (a specialist in inherited diseases) for advice. Ideally, you should do this before you get pregnant or in the early weeks of pregnancy.

Sickle cell disorder

Sickle cell disorder and thalassaemia major are serious, inherited blood disorders. They affect haemoglobin, a part of the blood that carries oxygen around the body. People who have these conditions will need specialised care throughout their lives. There are other, less common, haemoglobin disorders, but many of these are not as serious.

People with sickle cell disorder:

  • can have attacks of very severe pain
  • can get serious life-threatening infections
  • are usually anaemic
  • need medicines and injections when they are children and throughout their lives

People with thalassaemia major:

  • are very anaemic
  • need regular blood transfusions
  • need injections and medicines throughout their lives

For all these reasons, it is strongly recommended that you are screened. However, you can choose not to be tested and your choice will be respected.

How are sickle cell and thalassaemia passed on?

Sickle cell and thalassaemia are inherited disorders that are passed on from parents to children through faulty haemoglobin genes. People only have these disorders if they inherit two unusual haemoglobin genes: one from their mother and one from their father. People who inherit just one gene are known as carriers (referred to as having a trait). Carriers are healthy and do not have the disorder. However, if a carrier has a baby with someone who is also a carrier, or who has one of the disorders, there is a chance that their baby could inherit a disorder.

Who can be a carrier?

Anyone can be a carrier. However, it is more likely that you will carry the unusual gene if you or your ancestors came from places where malaria has been common. This is because being a carrier can help protect people against malaria. This means you are more likely to be a carrier if your ancestors came from the Mediterranean (for example, Cyprus, Italy, Portugal and Spain), Africa, the Caribbean, the Middle East, India, Pakistan, Bangladesh, Sri Lanka, South America or south and south-east Asia.

Screening for sickle cell and thalassaemia

Screening involves a simple blood test. The best time to have the test is before you are 10 weeks pregnant. All pregnant women are offered a blood test for thalassaemia. You will not always be offered a blood test for sickle cell anaemia. You may be given a questionnaire to find out where your family – and the family of your baby’s father – come from. If this shows you are at low risk, you may not be offered the blood test for sickle cell. You can always ask for the test if you're worried you might be at risk.

If the blood test shows that you are a carrier, the baby’s father will be invited for a test. If he is also a carrier, your baby may inherit a disorder, may be a carrier, or may not be affected. There is a helpful diagram on page nine of the NHS leaflet Screening tests for you and your baby, which explains the risks in more detail. This leaflet is available in other languages, including Arabic and Greek.

These tests early in your pregnancy give you the chance to talk to a counsellor and find out more about the disorders and the care available. If you want to, you can have another test to confirm whether your baby has one of the disorders (see amniocentesis and chorionic villus sampling).

If the test shows that you are a carrier, there is a chance that other family members could also be carriers. You may want to encourage them to ask for a test, especially if they are planning to have a baby.

Although people who are carriers of sickle cell anaemia are healthy, they can experience some problems in rare situations where their bodies might not get enough oxygen (for example, when having an anaesthetic before an operation or if they go deep-sea diving). Knowing that you are a carrier can help you manage these situations. However, people who carry thalassaemia or other unusual haemoglobin genes do not experience these problems.

Why should the baby’s father have a test?

Babies can only inherit the disorders if both parents carry the unusual gene. So if you are a carrier, it is important to find out whether the baby’s father is also a carrier. If he is not available or does not want to have a test, you may be offered another test to find out whether your baby has sickle cell anaemia or thalassaemia.

If you and the baby’s father both carry the gene for sickle cell disorder, thalassaemia or another haemoglobin disorder, for each baby you have there is:

  • a 25% chance that your baby will not be affected (that is, they will not have or carry a disorder)
  • a 50% chance that your baby will be a carrier
  • a 25% chance that your baby will have a disorder

If both you and the baby’s father are found to be carriers, you will be offered a test to confirm whether your baby is affected. This test is either an amniocentesis or a chorionic villus sample.

Other inherited conditions

Cystic fibrosis

Cystic fibrosis is an inherited disease that affects vital organs in the body, especially the lungs and digestive system, by clogging them with thick, sticky mucus. The sweat glands are usually also involved. The disease is inherited, and both parents must be carriers of the gene variation for a baby to be born with cystic fibrosis. Testing is offered in pregnancy if there is a family history of conditions such as cystic fibrosis or Duchenne muscular dystrophy.

Tay-Sachs disease

Testing for Tay-Sachs disease should be offered if you or your partner is of Ashkenazi Jewish origin and you consider yourself or your partner to be at risk.

Deciding whether to have tests

When you're deciding whether or not to have a test, think about what you might do if the test suggests that your baby has an abnormality. If a screening test suggests a higher chance of a chromosomal abnormality, you'll be offered diagnostic tests, such as amniocentesis, which will give a more definite diagnosis. These diagnostic tests carry a small risk of miscarriage, so you may decide not to have them. Discuss the issue with your partner, midwife, doctor, or friends to help you decide what's right for you.

Page last reviewed: 19/01/2015

Next review due: 19/01/2017


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The 2 comments posted are personal views. Any information they give has not been checked and may not be accurate.

Kathryn at NHS Choices said on 31 January 2014

Dear Froggit61,

You will find information on B-strep in pregnancy in our article on 'Infections in Pregnancy':

Best wishes,
Kathryn Bingham, NHS Choices editor

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Froggit61 said on 21 August 2013

This page has no information on B-strep during pregnancy.
Women are tested for this through a urine sample but are not told they are being tested and therefore have no information on this condition. If the baby is born while the mother is positive B-strep, the baby is at high risk of dying.
Please add B-strep info to this page to save young mother's heartache!

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