Did you know that you most likely never completely left your biological mother’s body? That your cells crossed the placenta while you were growing and probably stuck around in various parts of her body for a while – decades, perhaps for ever?

No, it’s not the premise of a zombie film. The phenomenon is called fetomaternal microchimerism, and in the past decade scientists have come up with some incredible theories about what the cells might be doing.

During pregnancy, cells are exchanged between the mother and foetus via the placenta. These fetal cells have been found to remain in multiple areas of the maternal body – organs and tissues including the liver, heart, brain, lungs and blood – and crucially in areas of disease and damage. Some studies point to a potentially benevolent role in repair – the presence of fetal cells in healed caesarean section wounds suggest they are beneficial. But they’re also found at sites of disease, and may play a detrimental role in maternal health, for example in the pathogenesis of autoimmune diseases and pre-eclampsia.

Certainly, this transfer of cells has profound implications for maternal biology and health. It also troubles the philosophical idea of us as self-contained individuals. The intricate exchange creates what the geneticist Dr Diana Bianchi has called a “permanent connection which contributes to the survival of both individuals”.

Did you know, also, that in pregnancy, some hormones increase in levels by 200 or 300 or even 1,000 times? And that some hormones are unique to pregnancy? Me neither.

The new science of pregnancy and motherhood is showing us just how dramatic the hormonal fluctuations are, as well as how seismic the cardiac, immunological, haematological, renal and respiratory changes – and their lifelong impacts on the body.

Did you know that if you have been pregnant and given birth, your brain will have changed shape in multiple areas? Changed actual shape?

In a landmark study published in Nature Neuroscience in 2017, researchers led by Elseline Hoekzema, a neuroscientist from the Netherlands, and Erika Barba-Müller, a neuroscientist working in Spain, provided evidence for the first time that pregnancy renders pronounced, consistent changes in brain structure.

This synaptic reorganisation and fine-tuning, it is thought, make the brain more efficient and streamlined in what it needs to do to care for a baby. Or as the neuroscientist Jodi Pawluski puts it, “to make sure we, and our child, survive parenthood”.

Did you know that the impact of pregnancy on the brain is comparable to that of adolescence? Neither did I.

Before I became pregnant with my first child in 2016, I did not know that my body, my brain, my self would undergo any lasting, let alone significant, changes. I thought that the hormonal impact of pregnancy was a one-time, transient event, to enable the baby to grow, and then, once out, I’d “bounce back”. Not a big deal, basically.

What I saw portrayed about early motherhood had no bearing on what I experienced, which was that I myself changed. I was altered, rewired, metamorphosed. I was also – and this was a shock – vulnerable, just like our baby.

At the time, I thought I must be imagining it. It is no surprise that I thought this: we knew virtually nothing about the maternal brain until the 2010s and our knowledge of the maternal experience is still staggeringly minimal.

The new science of motherhood shows us what many feel: that becoming a mother is more of a big deal than western society allows. In fact, after childhood and adolescence, there is no other time in a human’s life that entails such dramatic psychological and physical change.

The word that changed everything for me was “matrescence”. It means the process of becoming a mother and it’s a concept that exists in most societies on Earth, with special rites and rituals to hold and support the new mother. The neglect of this transition in western societies has devastating consequences but, in its illumination, critical social possibilities.

In the UK, as many as 20% of women develop a mental health problem in pregnancy or the first year of motherhood; these include mild and moderate to severe depression, anxiety, PTSD and psychosis.

The likelihood of depressive episodes can be twice as high during the period of matrescence, compared with other times of a woman’s life. This number rises for women of colour, those in disadvantaged socioeconomic groups who face systemic health inequalities, and women who have experienced loss. Suicide is the leading cause of death for women in the perinatal period between six weeks and one year after giving birth in the UK.

The neural plasticity of the maternal brain during this period is one of the reasons this is a vulnerable time. The dramatic increase in hormone levels throughout pregnancy and then the dramatic decrease when the placenta is expelled is another.

This new frontier of discovery could have an enormous impact on the health and wellbeing of mothers and their infants. The more scientists know about the physiological, endocrine and neural changes of pregnancy and early motherhood, the more we will know about how these processes can trigger mental illness, and how we can improve postpartum care.

But as a society, there is much more we need to do for care-givers and vulnerable people. Because our current approach is inadequate.

In the first year or so of my matrescence, I felt ashamed by how stressful and bewildering I found aspects of my new life. But as my research continued for my book, and I met and talked with other mothers who were similarly blindsided, I began to feel angry.

I think of the woman I met with a clitoral tear, and the one with severe anxiety, and the one with episodes of psychosis, and the one with rectal prolapse, and the one with autoimmune disease, and the one with fistula, and the one with suicidal ideation, and wonder: why are we sending a high-risk group of people off to spend an unknown period of time at home alone, where they must look after vulnerable infants and recover from the trauma of giving birth without adequate investment in healthcare, or much sleep, or non-hostile public spaces or transport, or social support? Instead, we give them a shedload of impossible cultural expectations and myths, including the imperative to enjoy every minute. Are these the actions of a responsible or functional society?

The crisis of modern motherhood, combined with the new science, is a damning indictment of the current social paradigm. It lays bare structural injustices, such as chauvinism and racism in maternal care, and the failures of predatory capitalism and the nuclear family. And it reminds us of our entangled, interdependent, cellular origins: that we are interconnected and need each other, even if our current systems of thought and power want us to believe otherwise.

• Lucy Jones is the author of Matrescence, Losing Eden and The Nature Seed