By Lizzie Wortley, ST7 Paediatrics

Meaning the time from conception to approximately 2 years of age, the first 1000 days is when the body goes through its most rapid period of growth and development.  Influences at this time – be they molecular, cellular, systemic or environmental – have profound impacts on the outcomes of said body in a way that can still be measured decades later.

Yes, what happens in utero could determine what you die of (1)

Hello Geriatricians, want to have a chat to the paed?

It hangs out alongside Adverse Childhood Experiences as a concept model, but these really deserve some time of their own, and so we’ll discuss them more in another piece.

How is it so powerful? 

In the space of (approximately) 1000 days humans go from one to trillions of cells.  That is quite the growth curve!

So during that period of rapid cell growth & complex structural and pathway development the potential to “code-in” errors, blips or unhelpful responses is more likely to occur than at other times.   

The developing foetus, and young babies are designed to survive.  Remember the grasp reflex? It’s a handy examination tool now, but it allowed the baby ape to cling to its parents’ back as they moved around. We don’t really need it now and other innate skills, like our ability to adapt our biology to our environment, also have variable levels of helpfulness.  It’s good if your carer is stressed by a tiger. You want them to remember you are there (you cry) and protect you or take you with them when they flee.  You then learn that tigers are stressful, and you are primed to respond quickly in similar situations in the future.  The parent who isn’t stressed and running from the tiger gets eaten by the tiger.  So does their non-mobile infant.  To literally “succeed in life” a child learns to run from the tiger.  

I have no idea who first started the tiger analogy.
We are always discussing tigers

However, sometimes this adaptive process isn’t all to the good. There aren’t that many tigers near where I live (Zoo aside).  You may have heard of “toxic stress”.   It’s when chronically raised cortisol and adrenaline levels cause long term damage. There are lots of models and theories behind this, including Halfon and Hochstein’s 2002 Life Course Development Framework describing “critical” and “sensitive” periods within each system (notably hormonal, genetic, neurological and immunological). (1)  

During certain periods of rapid growth, cells are more sensitive to biological factors which are influenced by the external environment (e.g. during a domestic violence incident the adrenaline/ cortisol systems are activated, the heart rate is raised, the amygdala is triggered etc).  

These factors can determine how the focus of a system is set up (i.e. how reactive one may be to stressors) and then that system is “set” for life (2).  The critical and sensitive is important because at certain time frames the impact of an event, or ongoing situation, has a bigger effect than at others – and that is why we all come out differently, no matter how similar our origins (even in the same family).  

What ends up making it stick for life is this:

  • Genetic
    • acetylation of DNA, and telomere length changes cements the stress impact
      •   affects life length, disease outcomes and potentially crosses generations
  • Neurological
    • an increase in one area of brain development with decrease in other areas eg. increased amygdala size, fewer connections to prefrontal cortex  
      • ie more space to look for danger, less central control on what they then do about that
  • Immunological
    • inappropriate activation of the immune system 
      • more AI disease, but also more infectious disease

Later on, beyond this initial period, changes are still being made, but the underlying structure of the genetic code, hormonal pathways, immunological pathways and brain structure is founded within this rapid period of growth and “programming”.  

The best way I have ever found to conceptualise this is to think about The Hulk.  Although “created” by a single explosion, his back story is of a difficult childhood (domestic abuse and the death of his mother) – the explosion only gives form to an “anger” that already existed (so it is described by marvel).  

And although we see Dr Bruce Banner develop better control over the times he transforms into the Hulk, he will always have the Hulk as part of him, and when life is extremely overwhelming or emotional, he often appears.  

This is how it rerates to children who are raised in stressful environments (from whatever cause) – they can learn to manage most of their emotional reactions to stressors, but in the immediate aftermath of an event, the first response will be “Hulk, Hulk, Hulk” and it takes work to overcome that.

All analogies are reductive, and this one is not without its faults (it’s stigmatising and inaccurate to reduce unhappy childhoods and violence to more violence later), but it gives a clear visualisation of how environment and circumstance can result in behaviour and consequences that are in conflict with a person’s own intentions, goals and desires.  

This is important, because that’s more likely to make education and work environments more difficult to navigate.  If something happens that gets your heart pumping, it’s harder to control the angry/aggressive or avoidant response that may be interpreted as inappropriate by friends, teachers and colleagues.  That in turn means you may be labelled as difficult and not receive the help and support you actually need.  Or it is acknowledged but managed inappropriately.  Poor educational attainment and trouble maintaining relationships in any form all contribute to social inequalities and the cycle is perpetuated.  

So do any of us have any influence over our own lives?

Of course we do, but maybe not in the way we assume as a society.  For those that like “life is a race” analogies we need to remember that we are not all standing on the same starting line.  Not only are some behind the starting line, but they might be carrying a bag of bricks, have their legs tied together, wearing a blindfold (you get the picture).  But whilst it’s harder, it doesn’t mean they are going to fail.

the wonderful oxytocin – also known as the “baby brain builder”

Because the second thing to remember is that probabilities of an increased risk of disease, even on an individual level, are not the same thing as actually having a disease or social outcome.  There are plenty of protective factors in life too, and many a person has overcome great adversity to live the lives they want, feel fulfilled and “succeed” in whatever definition you may be using. 

Which is why it’s really important that we don’t make this all negative, and we absolutely don’t use our new found knowledge to parent blame, point fingers or mum bash.  It’s hard to understand and process this stuff because of the many layers of our own personal experience we bring – all as children, many as parents and more as siblings, daughters, sons and family members.  

What next?

This piece is a huge oversimplification of what is a nuanced, dynamic, multifaceted and intricate process.  The difficulties of determining what’s causation, correlation, bias and confounding within the evidence is huge, and no one study or review neatly presents us with answers when making decisions and having discussions.   

Influenced by Figure 1 of Pearce et al ADC paper 2019. (3)
From tiny acorns: a representation of how environment impacts development at a micro and macro level

What we know is that generally chronic stress is bad.  It has an impact in utero and there are protective factors that exist.  I think a lot of us can instinctively agree with that (which I agree is not how evidence based medicine works).  The problem is in the daily working around all this in reality.  There is no one recipe for making a little human.  It can seem overwhelming.  What initially may feel like power in knowledge can fast become fear in the vastness of possibility for every action.  The important thing for us to remember is that we need to be willing to have the conversation about what may be going on in someone’s life and sit in the uncomfortable space.  .  

That is hard.  

We all know that is hard.  Listening when someone needs to speak, even if we don’t know what to do next.  Perhaps we could start with one family a day/week (click here to see Max Davie and my take on this).  I’m talking to myself here too, the times where one know you’ve shut a conversation down, or cut it off – next time I/you let that flow.  Then we’ll take it very slowly from there.  

Note 1:

Googling first 1000 days leads to WHO unicef piece on nutrition (which is vital), however a number of charities and websites are funded by food and formula companies and have 1st 1000 days in the title, so watch out where you get your information from.  

Note 2:

This piece cannot cover everything and they will not always get their language right.  We are always trying to get better, and love learning how we can do that. 

We are slowly trying to build up on these themes and there will be more resources coming. The next pieces should be on Protective Factors and Adverse Childhood Experiences which further develops the toxic stress models across the early life course.  Please send feedback so we can make our work stronger and better.  @paediatricFOAM @lizziewortley


1.        Halfon N, Hochstein M. Life Course Health Development: An Integrated Framework for Developing Health, Policy, and Research.

2.        Moore DT, Arefadib MN, Deery DA, West MS. The First Thousand Days AN EVIDENCE PAPER. Melbourne; 2017. 

3.        Pearce A, Dundas R, Whitehead M, Taylor-Robinson D. Pathways to inequalities in child health. Arch Dis Child. 2019;104(10):998–1003. 

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