The primary symptom of microcephaly is having a head size that is much smaller than that of other children of the same age and sex.

Head size is a measure of the distance around the top of the child's head (circumference). Using standardized growth charts, health care providers compare the measurement with other children's measurements in percentiles.

Some children just have small heads, with a measurement that falls below an established value for children of the same age and sex. In children with microcephaly, head size measures much smaller than average for the child's age and sex.

A child with more-severe microcephaly may also have a sloping forehead.

Chances are your health care provider will detect microcephaly at your baby's birth or at a regular well-baby checkup. However, if you think your baby's head is small for the baby's age and sex or isn't growing as it should, talk to your provider.

Microcephaly usually is the result of a problem with brain development, which can occur in the womb (congenital) or during infancy. Microcephaly may be genetic. Other causes may include:

• Craniosynostosis (kray-nee-o-sin-os-TOE-sis). Early fusing of the joints (sutures) between the bony plates that form an infant's skull keeps the brain from growing. Treating craniosynostosis usually means an infant needs surgery to separate the fused bones. This surgery relieves pressure on the brain, giving it enough space to grow and develop.
• Genetic changes. Down syndrome and other conditions may result in microcephaly.
• Decreased oxygen to the fetal brain (cerebral anoxia). Certain complications of pregnancy or delivery can impair oxygen delivery to a baby's brain.
• Infections passed to the fetus during pregnancy. These include toxoplasmosis, cytomegalovirus, German measles (rubella), chickenpox (varicella) and Zika virus.
• Exposure to drugs, alcohol or certain toxic chemicals in the womb. Any of these may affect fetal brain development during pregnancy.
• Severe malnutrition. Not getting enough nutrients during pregnancy may hurt fetal brain development.
• Uncontrolled phenylketonuria (fen-ul-kee-toe-NU-ree-uh), also known as PKU, in the mother. PKU hampers the mother's ability to break down the amino acid phenylalanine and may affect brain development of the fetus during pregnancy.

Some children with microcephaly achieve developmental milestones even though their heads will always be small for their age and sex. But depending on the cause and severity of the microcephaly, complications may include:

• Developmental delays, including speech and movement
• Difficulties with coordination and balance
• Dwarfism or short stature
• Facial distortions
• Hyperactivity
• Intellectual delays
• Seizures

Learning your child has microcephaly can raise questions about future pregnancies. Work with your health care provider to determine the cause of the microcephaly. If the cause is genetic, you may want to talk to a genetics counselor about the risk of microcephaly in future pregnancies.

To determine if your child has microcephaly, your provider likely will take a thorough prenatal, birth and family history and do a physical exam. Your provider will measure the circumference of your child's head, compare it with a growth chart, and remeasure and plot the growth at future visits. Parents' head sizes also may be measured to determine whether small heads run in the family.

In some cases, particularly if your child's development is delayed, your health care provider may order a head CT scan or MRI and blood tests to help determine the underlying cause of the delay.

Except for surgery for craniosynostosis, there's generally no treatment that will enlarge your child's head or reverse complications of microcephaly. Treatment focuses on ways to manage your child's condition. Early childhood intervention programs that include speech, physical and occupational therapy may help to maximize your child's abilities.

Your health care provider might recommend medication for certain complications of microcephaly, such as seizures or hyperactivity.

When you learn your child has microcephaly, you may feel many different emotions. You may not know what to expect, and you may worry about your child's future. The best remedy for fear and worry is information and support.

• Find a team of trusted professionals. You'll need to make important decisions about your child's education and treatment. Create a team of health care providers, teachers, therapists and other professionals who can help you evaluate the resources in your area and explain state and federal programs for children with developmental disabilities.

  Your child may require care from several medical specialties, including pediatrics and developmental pediatrics, infectious diseases, neurology, ophthalmology, genetics, and psychology.

• Seek other families dealing with the same issues. Your community might have support groups for parents of children with developmental disabilities. You might also find online support groups.

If you've just learned your child has microcephaly or you suspect your child's head is too small, you're likely to start by seeing your pediatrician. However, in some cases, your pediatrician may refer you to a pediatric neurologist.

Here's some information to help you and your child get ready for the appointment, and what to expect from the doctor.

What you can do

Before your child's appointment, make a list of:

• Symptoms, including any that seem unrelated to the appointment
• Key personal information, including any major stresses or recent changes in your child's life
• Any medications, including vitamins, herbs and over-the-counter medicines that your child is taking, and their dosages
• Questions to ask your child's doctor to make the most of your time

You may want to ask about small head size or delayed development. If you're worried about your child's head size, try to get the hat sizes or measure the head circumferences of several first-degree relatives, such as parents and siblings, for comparison.

Take a family member or friend along, if possible, to help you remember the information you're given.

For microcephaly, some basic questions to ask your doctor might include:

• What's likely causing my child's symptoms?
• Other than the most likely cause, what are other possible causes for my child's symptoms?
• Does my child need additional tests? If so, do these tests require special preparation?
• What's the best course of action?
• What are the alternatives to the primary approach you're suggesting?
• Is there a treatment that will return my child's head to a more typical size?
• If I have additional children, what are the chances they'll have microcephaly?
• Are there brochures or other printed material that I can have? What websites do you recommend?

Don't hesitate to ask other questions.