Down syndrome (Down's syndrome): what it is, symptoms, causes and support

Cells normally carry two copies of each chromosome. In Down's syndrome there is an extra chromosome, which is why it is also called trisomy 21. The NHS says you usually get the extra chromosome by chance, because of a change in the sperm or egg before you are born.

The Down's Syndrome Association says both "Down's syndrome" and "Down syndrome" are used interchangeably, with the apostrophe form more common in the UK. Mencap prefers "Down syndrome" because the condition is named after the physician who described it, not someone who had it. Both organisations ask for person-first wording, such as "a person who has Down's syndrome".

Mencap says people with Down syndrome will typically have some level of learning disability and characteristic physical features. A child may take longer than other children their age to reach certain milestones and develop certain skills, and may need ongoing support for different aspects of life as an adult.

The NHS stresses that people with Down's syndrome have their own personalities, things they like and dislike and things that make them who they are. The level of ability differs from person to person.

There are health problems associated with Down syndrome, such as heart problems and difficulties with sight and hearing. Mencap says these will not affect everyone.

The NHS says the extra chromosome change does not happen because of anything anyone did before or during pregnancy. Mencap says that in the majority of cases it cannot be inherited and occurs by chance at the time of conception.

The NHS says that in almost all cases it does not run in families. It also says your chance of having a baby with Down's syndrome increases as you get older, but anyone can have a baby with Down's syndrome. If you want to know more, speak to a GP, who may be able to refer you to a genetic counsellor.

The NHS says that if you are pregnant you will be offered a screening test to find your chance of having a baby with Down's syndrome. It is offered between weeks 10 and 14 of pregnancy and involves an ultrasound scan with a blood test, which can be done at the same time as the 12-week scan.

The NHS pregnancy screening page says screening for Down's, Edwards' and Patau's syndromes (the combined test) usually happens between 11 and 14 weeks. If that was not possible, the quadruple blood screening test is offered between 14 and 20 weeks.

Screening can tell you if your baby has a lower or higher chance of a condition, but it cannot tell you for certain. If you have a higher chance, you can have further tests.

The NHS says it is your choice whether or not to have any screening test. If you choose not to, you can change your mind and have them later, although some tests must be done at certain times.

The Down Syndrome Medical Interest Group (DSMIG) says people with Down syndrome on the whole do not have medical problems that differ from the general population, but some conditions are overrepresented. Most are treatable, and if undiagnosed they add a preventable burden.

DSMIG sets out a minimum standard of basic medical surveillance for the UK and Republic of Ireland, covering the areas below.

Down's syndrome is lifelong and there is no treatment that removes it. Care focuses on health checks, treating associated conditions, development and support.

The NHS says children with Down's syndrome should have regular check-ups with a paediatrician or GP. It says it could help to see a speech and language therapist, a physiotherapist for walking if there is low muscle tone, an optometrist or hearing specialist, and an occupational therapist for development.

Day-to-day, the NHS suggests praising new learning, speaking clearly and calmly, showing how to do something instead of just giving instructions, setting routines and using communication aids such as Signalong, Makaton or PECS. It also says to look out for changes in mood or behaviour, because they may not be able to tell you if something is wrong.

For education, the NHS says many children go to mainstream schools, and that children will get special educational needs support. If more is needed, you can apply for an education, health and care (EHC) plan.

Mencap lists what helps children grow into adults who can choose how they live: access to good healthcare, early intervention programmes, good relationships and family life, education and support groups.

The NHS says people with a learning disability often have poorer physical and mental health than other people, and that this does not need to be the case. Anyone aged 14 or over on their doctor's learning disability register can have a free annual health check once a year. You do not have to be ill.

The NHS says the check includes a physical check-up with weight, heart rate and blood pressure, possibly a urine or blood test, a medicines and vaccination review, and questions about things more common with a learning disability, such as epilepsy, constipation, swallowing problems and eyesight or hearing problems. Agreed actions go in a health action plan.

Practices do not have to offer the service, but you can ask. If they say no, the NHS says to ask your local community learning disability team. You can also ask for reasonable adjustments, such as changes that make an appointment easier to attend.

Regular reviews are the main safeguard, so keep up routine hearing, eyesight and health checks. The NHS says they may not be able to tell you if they are unwell, so look out for changes in mood or behaviour and mention them to a GP.

For a sudden severe problem, such as difficulty breathing, collapse or a seriously unwell child, call 999. For urgent but non-emergency advice, use NHS 111.

On the NHS, care is organised through antenatal screening, newborn checks, paediatric and cardiology services, community learning disability teams, therapy services and the GP annual health check. These are free at the point of use.

Some families also use private services, for example for a second opinion or a faster developmental assessment. If you do, share the results with your GP so they join the NHS record and surveillance schedule.