The developmental milestones are a set of goals or markers that a child is set to achieve from infancy to childhood. They are categorized into 5 domains: gross motor, fine motor, language, cognitive, and social-emotional and behavioral. Knowing and identifying the developmental milestones can help the pediatrician and healthcare providers to recognize delayed development, and early identification would allow for timely referral to the appropriate services.

• Six months: Likes mirrors; rolls over; begins to say consonants while babbling; brings things to mouth.
• Nine months: Fear of strangers; sits without support, crawls, pincer grasp; understands “no,” points with a finger, says “mama” or “baba;” plays “peek-a-boo.”
• Twelve months: Puts out arm or leg when dressed, cries when familiar people leave; pulls to stand, cruises; responds to simple commands, makes gestures, puts things in a cup and removes them, bangs things together.
• Eighteen months: Plays simple pretend (like with a doll), kisses/hugs familiar people, walks alone, walks up steps, eats with utensils, says several individual words, says “no,” points to one body part, scribbles with crayon, marker, or pen.
• Two years: Begins playing with other children; stands on tiptoes, kicks a ball, throws a ball overhand; two to four-word sentences, points to things in a book, strangers can understand 50% of language; stacks four or more blocks, follows two-step instructions.
• Three years: Dresses/undresses self, copies others, takes turns; walks up and downstairs with one foot per stair, runs easily; strangers can understand 75% of language; stacks six or more blocks, turns pages in a book, pushes buttons and turns knobs.
• Four years: Likes to play with others, more imaginative play; hops on one foot, can stand on one foot for two seconds, cuts with scissors; can recite a poem or sing songs, understands basic grammar; identifies some colors and numbers, draws a person with two to four body parts.
• Five years: Differentiates between real and pretend, wants to be like friends; can stand on one foot for 10 seconds, can somersault; easily understood by others, tells stories, uses future tense; counts to 10, draws a person with six body parts, prints some letters and numbers.[1]

To understand the workflow of assessing the developmental milestones and diagnosing developmental delays, there are 2 concepts to apply in the clinical setting:

• Surveillance: an ongoing assessment of the child’s development done by eliciting concerns from parents, obtaining an adequate history, and observing the child during the encounter [2] It is done at every well-child care visit, and it can be performed by using an age-appropriate checklist of milestone records. Special attention must be had at the 4 to 5-year visit prior to the start of school. 
• Screening: It is done for better identification of children that may need further evaluations.[1] The importance of screening depends on identifying delays and disabilities at a young age (<2 years of age) to implement appropriate interventions sooner rather than later.[2] The American Academy of Pediatrics recommends screening, using a validated tool, at ages 9, 18, and 30 months. Some of the tools used are the Denver Developmental Screening Test, Ages, and Stages Questionnaires.

When evaluating a child, it is important to take into consideration the gestational age at birth, as premature infants have a higher risk of long-term neurodevelopmental disabilities. In order to assess the normal growth and development of an infant born premature, the clinician must adjust his chronological age to the appropriate gestational age, and adjust the milestones to the expected by the corrected gestational age. For example, a baby is born at 32 weeks, and they are 8 weeks premature based on a full-term baby born at 40 weeks gestation. One would expect this 32-week old baby to reach their milestones 2 months behind their chronological age. 

The developmental milestones are constantly evolving as the child ages. Isolated delays in development can be difficult to pick up by the clinician if the delay is mild; another cause of delay in development is if the parent reports a normal development in any given area and the clinical is not able to assess it during the visit. That is why continuous surveillance is important because it gives the clinician the opportunity to note when delays are noted and if the child is able to catch up.

Developmental delays can be specific (present in one area), or global (present in greater than 2 areas). Children can present initially with developmental delay in a specific area, and this could affect the skills in other areas, thus progressing to global delay.[1] Of the 5 areas of development, one could argue the most important area to surveil and to recognize any delays is speech and language skills, as it is an important predicting factor for literacy level and cognitive skills in childhood and adulthood, and benefits the most from earlier interventions.[3][4]

Once a delay in development is identified, the clinician must try and find a cause for the delay. A child with a speech delay may have an underlying hearing problem. Children with visual problems may have a delay in motor development. Genetic and metabolic conditions are a likely cause of global developmental delays. Children with cerebral palsy will also present with global delays. Work-up should include hearing screening, thyroid function testing, lead testing, and a microarray.

When delays are due to an organic cause, sometimes the specific cause can be treated. Whether or not a cause is identified, children with delays qualify for Early Intervention, which are programs that can help the child catch up to development typical for their age. These interventions may include speech therapy, physical therapy, and occupational therapy. The earlier delays are identified and addressed by early intervention, the better the success of the interventions.[5]

In disorders such as autism, one may begin to see deviations in development, especially in the social/emotional and verbal areas, as early as six months of age. The importance of detecting this early on is that the earlier a child has interventions in place, such as speech therapy for verbal delays, the more likely a child is to become more functional, in this case, have better communication skills than they would have without the speech therapy.  The older a child gets without the detection of these delays, the therapies, while still useful and worth the effort, may become less effective. Unfortunately, many children with an autism spectrum disorder do not receive a diagnosis until after age 5. Many pediatric offices use screening tools, the most well studied of which is the Modified Checklist for Autism Screening in Toddlers (M-CHAT), and subsequent variations of this screening tool.[6]