Expanded carrier screening

Expanded carrier screening is a blood test that looks at many genes at once to see whether someone carries a silent genetic change that could cause disease in a baby if both parents (or the mother, for some X‑linked conditions) are carriers. It focuses mainly on autosomal recessive and X‑linked recessive conditions, which often have serious health consequences for affected children.​

What is expanded carrier screening?

• For autosomal recessive conditions, the test looks at genes where a child is affected only if both parents carry a faulty copy and both pass it on; carriers are usually healthy.​

• For X‑linked recessive conditions, the test usually looks at genes on the X chromosome in women; if a woman is a carrier, there is a 50% chance each son will be affected and a 50% chance each daughter will be a carrier.​

Examples of conditions detected

• Common autosomal recessive conditions on many panels include cystic fibrosis (lung and digestive problems), spinal muscular atrophy (progressive muscle weakness and breathing difficulty), and some haemoglobin disorders like beta‑thalassaemia and sickle cell disease.​

• Common X‑linked recessive conditions include Duchenne muscular dystrophy (severe progressive muscle weakness) and haemophilia A or B (bleeding tendency due to lack of clotting factor).​

Cystic fibrosis

• Cystic fibrosis causes thick, sticky mucus in the lungs and digestive system, leading to repeated chest infections, difficulty gaining weight, and long‑term lung damage.​

• It is lifelong, but modern treatments such as physiotherapy, antibiotics, and newer CFTR‑modulator medicines can significantly improve health and survival.​

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Spinal muscular atrophy (SMA)

• SMA affects the nerves that control muscles, causing muscle weakness and wasting; in severe forms, this can lead to difficulty sitting, moving, or breathing independently in infancy or childhood.​

• Newer disease‑modifying treatments can improve strength and survival, especially when started early, but many children still have long‑term disability.​

Beta‑thalassaemia

• Beta‑thalassaemia is a blood disorder where the body makes too little normal haemoglobin, leading to severe anaemia, tiredness, poor growth, and bone changes.​

• Children with the severe form often need regular blood transfusions and medicines to remove excess iron; some may be helped by bone marrow transplant.​

Sickle cell disease

• Sickle cell disease causes red blood cells to become stiff and “sickle” shaped, which can block blood vessels, causing painful crises, infections, and damage to organs over time.​

• Treatment focuses on preventing crises (for example with hydroxyurea), managing pain, and preventing complications; some patients may be eligible for curative bone marrow or gene‑based therapies.​

Duchenne muscular dystrophy (DMD)

• DMD is a severe muscle‑wasting condition, usually affecting boys, with muscle weakness starting in early childhood and progressive loss of the ability to walk, followed by breathing and heart problems.​

• Steroids, heart and lung support, physiotherapy, and emerging gene‑targeted treatments can slow progression, but most affected individuals still have shortened life expectancy.​

Haemophilia A and B

• Haemophilia A and B are bleeding disorders caused by low levels of clotting factors VIII (A) or IX (B), leading to easy bruising, joint and muscle bleeds, and prolonged bleeding after injuries or surgery.​

• Regular infusions or newer long‑acting or gene‑therapy‑based treatments can greatly reduce bleeding episodes and protect joint health, allowing many patients to live active lives.

Chance of finding at‑risk fetuses

• In non‑consanguineous couples (not related), most people are carriers of at least one recessive condition, but the chance that both partners carry the same rare condition is relatively low (estimated at 1-2%); expanded carrier screening increases the chance of identifying the small percentage of couples at risk of having an affected child.​

• In consanguineous couples (related by blood), the chance that both partners carry the same recessive gene change is higher, so the likelihood of finding an at‑risk fetus for an autosomal recessive disorder is significantly increased compared with unrelated couples.​

When to perform the test

• Ideally, expanded carrier screening is done before pregnancy, so couples can understand their reproductive risks and consider options such as natural conception with prenatal diagnosis, IVF with preimplantation genetic testing, the use of donor gametes, or adoption.​

• If it has not been done beforehand, it can still be offered in early pregnancy, usually in the first trimester, so that if both parents are carriers of the same condition, there is time to discuss diagnostic tests such as chorionic villus sampling or amniocentesis.​

Advantages of testing pre‑pregnancy or in first trimester

• Before pregnancy, results can be considered calmly and privately, with the widest range of choices, without the time pressure and emotional stress that can occur once a pregnancy is advanced.​

• In the first trimester, early testing still allows for timely diagnostic procedures and more options for ongoing pregnancy care, planning for a child with special needs, or, where legally and personally acceptable, considering termination of an affected pregnancy.​

International recommendations

• Several international professional bodies now support offering expanded carrier screening, especially for serious conditions: for example, the American College of Obstetricians and Gynecologists (ACOG) and the American College of Medical Genetics and Genomics (ACMG) state that carrier screening for a panel of conditions can be offered to all women or couples planning a pregnancy or already pregnant.​

• Other organisations, such as the European Society of Human Genetics (ESHG) and various national genetics and obstetric societies, also acknowledge the role of expanded carrier screening and provide guidance on offering it in a responsible, informed‑consent‑based manner.

• Singapore currently does not have a recommendation on this issue yet.

Contact us if you do want expanded carrier screening.