What's the most critical period of child development
From birth to the time enters first or second grade, a child learns about themselves and the world around them at incredible speed. At every turn, children encounter novel and sights, sounds and tastes – so every experience is also a teachable moment. This time of life, known as early childhood, is considered a critical window in a child’s developmental trajectory – or the overall path that a child’s development follows.
Early childhood development refers to the period between a child’s first 0 to 8 years where growth occurs across four domains, or sets of skills and abilities. These include the language, motor, cognition and social-emotional domains.
During this time, a child’s brain is acquiring new information at breakneck speed and has a high capacity for change (an ability known as neuroplasticity). The basic structure of the brain is formed primarily during the prenatal period and during early childhood, and the refinement of the networks inside the brain continue over the long-term .
A child’s long-term academic and life outcomes are heavily shaped by what occurs during their early life. The WHO  prescribes nurturing care as an integral part in improving the likelihood that a child will thrive. Nurturing care is defined as care that:
- is provided in a stable environment
- is sensitive to children’s health and nutritional (dietary) needs
- offers protection or safeguarding from threats
- provides opportunities for early learning
- involves interactions that are “responsive, emotionally supportive and developmentally stimulating” 
Lessons: Early childhood is generally defined as the time of life between ages 0 and 8 years old. It’s important to understand that this is considered a critical window of a child’s development, and can have a significant impact on a child’s academic and even professional trajectory – among other factors that are often taken to define positive later life outcomes.
##Defining early life adversity So what happens when a child isn’t provided nurturing care? Research suggests that later-life challenges related to health and wellbeing may sometimes be correlated to early childhood experiences. According to the CDC , stress during this critical period of development, can even disrupt the body’s own physiology, or normal bodily functions and processes. For example, stress in early life may impact the body’s neurological, metabolic and immunological sysstems.
Examples of early life adversity include:
- poverty or low socioeconomic status
- household food insecurity
- abuse or neglect
- chronic illness, developmental delay or a learning disability
- lack of access to high-quality early childhood education
Lessons: Exposure to early life adversity can have a decidedly negative impact on a child’s growth into adulthood.
The impact of stress in early life
As we’ve reviewed, clinical evidence shows that exposure to early life stressors can have an impact on how a child learns, how much they’ll earn as an adult and their long-term health outcomes .
Consider, for example, that:
- vocabulary skills by age 3 predict fluency of reading in third grade
- fluency of third grade reading predicts high school graduation rates
- high school graduates have increased earning potential and are less likely to have chronic diseases
- high school graduates are also more likely to report positive health outcomes overall
Lessons: Evidence shows that experiencing stress during early childhood can have a decisive impact on what happens in later life – including the likelihood of graduating and later professional success and earning power.
Addressing early life adversity
Given that early childhood constitutes such a critical window for child development, what can be done to identify children that are vulnerable and act upon this information? According to the CDC , three steps can be taken to ensure that children receive the help they require to reduce their chances of becoming even more high-risk for poorer long-term outcomes: screening to determine whether they’re exhibiting developmental delay, early-identification if children are experiencing challenges and linkage to support services.
Depending on a child’s age, there are several meaningful interventions – or ways to improve the impact of early life adversity or stress – that can help support the child and their family.
Interventions in the period before conception to birth :
- focusing on improving maternal health
- improving nutrition, micronutrients or iodine supplementation
- reducing maternal stress, rates of depression and mental disorders
Interventions from birth to age five:
- good-quality parental support
- promoting maternal-child attachment
- ensuring children have adequate access to micronutrients
- prevention of child abuse and neglect
- effective early-learning programmes, including high-quality child care
- providing children with a strong social safety net
To be clear, micronutrients are nutrients that are present as part of a good diet and help promote the normal growth and development of children. The word is sometimes used to describe vitamins and minerals in general.
Lessons: The CDC has identified three main steps that can be taken to address the impact of early life adversity. In addition, there are different meaningful interventions that can be introduced at various points in child development.
In 2016, the WHO (the World Health Organization) estimated that 250 million children, or 43% of children in low and middle-income countries, are unable to realize their full developmental potential. In most cases, this is due to adverse events in early life, as outlined above.
According to the Lancet, this is because of: a failure to apply emerging scientific knowledge on nurturing care to improve child development; and the failure to take action on a large-scale. Put simply, there is a great need for the healthcare and education communities to implement coordinated, organized interventions that ensure every child has a chance to succeed.
Lessons: Almost half the children in low and middle-income countries around the world are unable to realize their full developmental potential, which is in large part due to experiencing early life adversity.
-  Brain Development and the Role of Experience in the Early Years
-  WHO | Early Child Development
-  CDC Grand Rounds: Addressing Health Disparities in Early Childhood
-  Nurturing care: promoting early childhood development