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Down’s syndrome can be identified through prenatal screening in pregnancy or confirmed after birth through chromosomal testing. This guide explains the full diagnostic pathway, including the NHS combined test, NIPT, amniocentesis and postnatal karyotype analysis.
Down’s syndrome can be identified at two stages: prenatally, during pregnancy, and postnatally, after the baby is born. Prenatal identification involves a sequence of screening and diagnostic tests. Screening tests estimate the probability that a pregnancy has Down’s syndrome but do not provide a definitive answer. Diagnostic tests, such as amniocentesis and chorionic villus sampling, analyse the chromosomes directly and provide a definitive result but carry a small risk of miscarriage. After birth, a karyotype analysis on a blood sample from the baby confirms the diagnosis and identifies the chromosomal type.
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The NHS offers a combined screening test between 11 and 14 weeks of pregnancy. This test combines a blood test measuring certain proteins with an ultrasound scan that measures the fluid at the back of the baby’s neck, known as the nuchal translucency. The results are combined with maternal age and other factors to calculate an individual risk figure. A result of 1 in 150 or higher is considered high chance, prompting an offer of a diagnostic test.
NIPT analyses fragments of fetal DNA in the mother’s blood and can detect Down’s syndrome with high accuracy. The NHS offers NIPT to women who receive a high-chance result from the combined test, rather than proceeding directly to invasive diagnostic testing. Private NIPT is also available from 10 weeks gestation. NIPT is a screening test rather than a diagnostic test: a positive result should be confirmed by a diagnostic test before a decision is made.
Amniocentesis is a diagnostic test that analyses chromosomes in a sample of amniotic fluid taken from around the baby. It provides a definitive chromosomal result. The test is typically offered from 15 weeks gestation. Amniocentesis carries a risk of miscarriage of approximately 0.5 to 1 percent, which means it is not offered routinely but only to women who have received a high-chance screening result or who have other clinical reasons for the test.
CVS is a diagnostic test that analyses chromosomal material from the placenta. It can be performed earlier in pregnancy than amniocentesis, typically from 11 weeks gestation, but carries a slightly higher miscarriage risk of approximately 1 to 2 percent. CVS provides a definitive chromosomal result. Like amniocentesis, it is offered to women with a high-chance screening result or other clinical indications.
Where Down’s syndrome has not been identified prenatally, clinical suspicion following birth prompts chromosomal testing. A blood sample is taken from the baby and a karyotype analysis is carried out, which analyses all 46 chromosomes and confirms whether an extra chromosome 21 is present. FISH testing provides a faster preliminary result within 24 to 48 hours. Full karyotype results are typically available within a few days and confirm the type of Down’s syndrome.
Whether a diagnosis is received prenatally or after birth, families should be given the news in a sensitive and balanced way, with both parents present where possible. The Down’s Syndrome Association helpline provides immediate support, information and connection to other families. Following a postnatal diagnosis, the paediatric team will arrange the health checks required for newborns with Down’s syndrome and refer the family to early intervention and specialist support services.
For related information see our articles on Down’s Syndrome Pregnancy Screening and NIPT for Down’s Syndrome.
Down’s syndrome is diagnosed through chromosomal testing: either prenatal diagnostic testing such as amniocentesis or CVS, or postnatal karyotype analysis on a blood sample from the baby. Screening tests during pregnancy estimate the probability of Down’s syndrome but do not provide a definitive diagnosis. Only chromosomal analysis can confirm the diagnosis and identify the type of Down’s syndrome.
A screening test estimates the probability that a pregnancy has Down’s syndrome but does not provide a definitive answer. The NHS combined test and NIPT are screening tests. A diagnostic test analyses chromosomes directly and provides a definitive result. Amniocentesis and CVS are diagnostic tests. Screening tests carry no risk to the pregnancy; diagnostic tests carry a small risk of miscarriage. A positive screening result is typically followed by an offer of a diagnostic test.
It is possible for Down’s syndrome to be missed at birth if the physical features are mild or if the clinical team does not suspect the condition. If parents or carers have any concerns about their child’s development after birth, they should raise these with their GP or health visitor. The developmental profile associated with Down’s syndrome, including low muscle tone, developmental delays and the characteristic physical features, will typically lead to investigation and diagnosis in the early months or years of life.
Visit our Understanding Down’s Syndrome hub for more guides on causes, diagnosis, education, health conditions and independent living.
The information in this article is provided for educational purposes only and is not intended as medical advice. If you have concerns about Down’s syndrome or any health condition, speak to a qualified healthcare professional. SENDhelp Education Limited accepts no responsibility or liability for any loss or damage arising from reliance on this content. Any links to third-party websites are provided for convenience only and do not constitute endorsement of their content.
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