This is from a great website by the University of Michigan. I have shared this with parents of children I worked with. It is a primer on developmental delays for parents.
Developmental Delay
What is developmental delay?
Developmental Delay is when your child does not reach their developmental milestones at the expected times. It is an ongoing major or minor delay in the process of development. If your child is temporarily lagging behind, that is not called developmental delay. Delay can occur in one or many areasfor example, gross or fine motor, language, social, or thinking skills.
Developmental Delay is most often a diagnosis made by a doctor based on strict guidelines. Usually, though, the parent is the first to notice that their child is not progressing at the same rate as other children the same age. If you think your child may be slow, or seems behind, talk with your child's doctor about it. In some cases, your pediatrician might pick up a delay during an office visit. It will probably take several visits and possibly a referral to a developmental specialist to be sure that the delay is not just a temporary lag. Your child's doctor may use a set of screening tools during regular well-child visits.
The first three years of a child's life are an amazing time of development...
...and what happens during those years stays with a child for a lifetime. That's why it's so important to watch for signs of delays in development, and to get help if you suspect problems. The sooner a delayed child gets early intervention, the better their progress will be. So, if you have concerns, act early.
What causes developmental delay?
Developmental delay can have many different causes, such as genetic causes (like Down syndrome), or complications of pregnancy and birth (like prematurity or infections). Often, however, the specific cause is unknown. Some causes can be easily reversed if caught early enough, such as hearing loss from chronic ear infections, or lead poisoning.
http://www.med.umich.edu/yourchild/topics/devdel.htm