Are the first 1001 days of life really the most important?

Have you ever wondered why the earliest days of a child’s life are considered so crucial? It’s more than just a phrase; the first 1001 days—from the moment of conception to a child’s second birthday—form the most significant window of opportunity for human development.

During this remarkable time, a baby’s brain grows at an incredible rate, forming over a million neural connections every single second. The experiences, relationships, and environment surrounding a child during this period lay the foundation for their future physical health, mental wellbeing, and capacity to learn.

How do we know about the importance of the first 1001 days?

Over the past 50 years, researchers have learned a great deal by studying children who grew up in extremely difficult environments. One well-known example comes from Romania and Russia, where many babies were abandoned and raised in large public orphanages. These orphanages often had very few caregivers looking after many infants and toddlers, which meant the children received very little attention, interaction, or emotional care.

It later became clear that these settings exposed children to severe levels of sensory and emotional deprivation—now referred to as pervasive neglect. While heartbreaking, these situations gave psychologists an important opportunity to understand the long-term effects of extreme neglect, as well as what happens when children are later placed in more nurturing environments.  

U.S. researchers who followed the development of these children through the Bucharest Early Intervention Program showed that children who remained in the orphanages experienced serious delays in thinking and learning, along with difficulties in social and emotional development. Many also developed mental health challenges and long-term impairments.

The researchers also found some hopeful results. Children who were removed from the orphanages and placed into foster care showed improvement in certain areas, though not in all. Many continued to struggle with attachment, emotional regulation, anxiety, and overall intellectual functioning.

One of the most important findings was the timing of intervention. Children who were moved into foster care before the age of two showed the strongest recovery and the best long-term outcomes.

So why are the first 1001 days so important?

A child’s very early years play a powerful role in shaping how their brain develops. While genes provide the basic starting point, it’s a child’s experiences—what they see, hear, feel, and do—that truly build the foundation for future learning, behavior, and health.

During this critical time, the brain is growing at an incredible speed. Billions of brain cells, called neurons, communicate with one another by sending electrical signals. As they connect, they form pathways that act like the brain’s wiring system. The more these pathways are used, the stronger they become.

A child’s everyday experiences and environment help decide which connections are used most often. Connections that are used repeatedly grow stronger and more permanent, while those used less often naturally fade away in a process called pruning. Over time, this helps the brain develop strong networks that support emotions, movement, self-control, thinking, language, and memory.

Which aspects of the environment are the most important?

After a baby is born, they experience all kinds of different environments at home. But in those first few years, the most important part of that environment is their parents or main caregiver. What really matters is how the baby and caregiver interact with each other.

Experts at the Centre on the Developing Child at Harvard call these important back-and-forth moments “serve and return” interactions. It’s a bit like a game of catch—when a parent smiles, talks, or gestures (the “serve”), the baby responds in some way (the “return”), and then the parent reacts again. These little exchanges might seem simple, but they’re incredibly powerful. They help build the baby’s brain by triggering the release of special chemicals and hormones that support healthy development.

The importance of these interactions was demonstrated by Edward Tronick and his team who introduced the “still face” experiment at a child development conference. In this simple but powerful study, a mother first interacts naturally with her baby—smiling, cooing, and responding to the baby’s gestures and sounds. Then, she suddenly becomes expressionless and stops responding altogether.

The baby’s reaction is immediate and striking: confusion turns to distress as the child tries everything to re-engage the parent.

This experiment vividly showed how deeply babies rely on responsive, engaged interactions with their caregivers. Tronick’s “still face” study profoundly changed how we understand early relationships—and why those first emotional connections are so crucial for healthy social and emotional development.

You can watch a powerful demonstration of the still face experiment in this short video.

It all begins in pregnancy!

When we imagine a baby in the womb, we imagine her to be peaceful, disconnected, unaffected by the world around her, quietly floating, like an astronaut in space. Yet nothing could be further from the truth. The womb is a vital place for human development, where nature meets nurture, where genes are expressed, and where babies and their brains are grown.

How all this happens is directly connected to outside influences, perhaps most important being the mother. New research has shown that a mother’s stress level can have a significant impact on her unborn baby. The emotional state of a woman in pregnancy can affect the development of her baby in the womb and actually could have long-term effects on her child. 

For example, research has shown that unborn babies can be affected by chronic anxiety in the mother, the mother’s cigarette and alcohol intake, and the mother’s feelings about the unborn baby. The child could be more likely to have emotional problems, to have behavioural problems, to be slower at learning.

So why does all this matter?

Research shows that the first 1001 days of a child’s life are a crucial time. During this period, the right support can make a real difference in giving every child the best possible start, especially for families facing challenging circumstances.

In many countries, this understanding has led to the development of programmes for parents living in poverty during pregnancy and the early months after birth. These visits are designed not only to support parents, but also to help improve their baby’s future opportunities and wellbeing.

That’s why, it’s important that everyone around babies — including family members, early years practitioners, childminders, nursery staff, and healthcare professionals — understands how important the first 1001 days are. By working together, they can help ensure that every baby has the strongest possible start in life.

Jane Barlow is the Professor of Evidence-Based Intervention and Policy Evaluation at the Department of Social Policy and Intervention (DSPI) at the University of Oxford. Her research focuses on evaluating the mental health of mothers and babies.

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