Microcephaly: what it is, causes and what it can mean for a child

Microcephaly means a head that is smaller than expected. The Brain Charity describes it as a rare, lifelong condition and explains that it can take two forms. In primary microcephaly a baby is born with a head smaller than average. In secondary microcephaly the head is the expected size at birth but then stops growing.

A small head is the defining feature, not the whole story. The head is a reflection of how the brain has grown, so what matters for a child is the reason the head is small and how the brain has been affected. The Brain Charity says people with mild microcephaly will often only experience a smaller head, while others need care for associated problems.

The Brain Charity says there are a few potential causes, including infections during pregnancy such as the Zika virus, and genetic abnormalities. UK NHS pages on infections in pregnancy add detail about which infections can harm a baby.

It is also true that for many children no single cause is found, and the UK pages used here do not say how often that happens, so they should not be read as a complete list of causes.

The NHS says Zika virus is mainly spread by mosquitoes found in some parts of the world, that for most people it is mild and not harmful, but that it can cause problems if you are pregnant. It lists problems during and after pregnancy such as loss of the baby before birth, premature birth, stillbirth, problems with the baby's brain, hearing and eyesight, and the baby having an unusually small head.

The NHS says the mosquitoes that carry Zika virus are not found in the UK, and that it is found in parts of South and Central America, the Caribbean, the Pacific islands, Africa and Asia. It says you should consider avoiding travelling to a country with a risk of Zika virus if you are pregnant or planning a pregnancy in the near future.

The NHS says cytomegalovirus is a common virus that is usually harmless, but that it sometimes causes problems in babies. Congenital CMV is caused by passing on an active CMV infection to your unborn baby during pregnancy. Most babies with congenital CMV do not have any symptoms, but it can cause a rash, jaundice, a low birth weight, a smaller head than usual (microcephaly), seizures, hearing problems, eye problems and problems with the liver and spleen.

The NHS says babies born with congenital CMV may have tests to check their kidneys, liver, brain, eyes and hearing, and regular follow-up appointments until they are around 5 years old. Antiviral medicine may be used to treat babies diagnosed with congenital CMV after they are born. It weakens the virus and lowers the chance of serious problems but does not cure the infection.

To lower the chance of getting CMV in pregnancy, the NHS advises washing hands with soap and water, especially after changing nappies, feeding young children or wiping their nose, washing toys, avoiding sharing food, cutlery and drinking glasses, and avoiding kissing young children on the mouth. It says there is currently no vaccine.

The Brain Charity lists symptoms that can be associated with microcephaly: seizures, balance issues, hearing loss and vision problems. Depending on the severity of the microcephaly, some babies will need treatment and care to manage these symptoms and other health problems that may arise. People with mild microcephaly will often only experience a smaller head.

The range is wide, which is why families are often told that the size of the head on its own does not predict how a child will be. The specialist team looks at the whole child: how they are developing, whether they have seizures, how they hear and see, and the cause if one can be found.

The sources used here do not set out a diagnostic pathway for microcephaly, so this section describes what they do say. In its craniosynostosis guidance the NHS says to see a GP if you are concerned about the development of your child's head in terms of its shape and size, and says the GP will examine your child's head and may take some measurements to see if it is an unusual size for your child's age, and may refer to a specialist centre for tests such as X-rays or scans.

If your baby is diagnosed with congenital CMV, the NHS says tests of the kidneys, liver, brain, eyes and hearing and regular follow-up appointments may be arranged. A GP, health visitor or paediatrician can explain what is planned for your child and why.

The Brain Charity says there is no treatment for microcephaly, but that early intervention can improve the person's quality of life. In practice, care focuses on the problems that arise, such as seizures, hearing and vision, and on supporting the child's development and the family.

The Brain Charity offers practical help, emotional support and support for carers, friends and family, including for people with microcephaly. It also lists a Facebook support group for parents of children with microcephaly and another UK charity, Cerebra, which supports children with neurological conditions.

Speak to a GP, health visitor or paediatrician if you are concerned about the size or shape of your baby's head, or about their development. The NHS says that if you are pregnant and have a new rash, or have been in close contact with someone who has rubella, you should call your midwife, maternity unit or GP immediately. For Zika, it says to ask for an urgent GP appointment or get help from NHS 111 if you have recently travelled to a country with a Zika risk and you or your partner are pregnant or become pregnant within 3 months of returning.

If you are unsure whether a symptom needs urgent attention, NHS 111 can advise.

On the NHS, a small head is usually noticed at the newborn examination or at routine checks, then assessed by a paediatrician who can arrange tests and refer on to specialists for any associated problems such as seizures, hearing or vision. Support services for children and families are usually coordinated through the paediatric team, health visitors and local children's services.

Some families also see a private paediatrician or paediatric neurologist for a second opinion or faster access to tests. The tests and the supportive care described here are the same, and sharing results between the private and NHS teams helps keep one clear plan.