How smoke exposure in pregnancy can cause aggressive kids
This is part 3 of the “How to improve the success of your kids before they are even born” blog series. See Part 1 and Part 2.
If folic acid is the Austin Powers of womb environment, then Dr Evil has got to be cigarette smoke. O.K., O.K., there are plenty of other bad guys obviously – thalidomide, heroin, alcohol. The reason I am casting cigarette smoke as the bad guy here is that whilst you have total control about exposure to the others, passive smoking is harder to control. The US Department of Health and Human Services (2006) estimated that more than 126 million non-smoking adults continue to be exposed to second-hand smoke in the US. Some of these millions are likely to be pregnant. Thank goodness for the U.K. law banning smoking in bars and restaurants. This was great for winter time, but having had both pregnancies through spring and summer, it was a real pain to anticipate a delightful al-fresco meal in the fresh air, only to find, given the laws prohibiting smokers from dining inside that all the outdoor tables at pubs and restaurants were over-run with smokers. Worse still, I found that friends would continue to light up in front of me in social situations when I was pregnant. Most of the time, I would alert them to smoking and pregnancy but this usually meant they moved a few paces away and tried to blow smoke in another direction with varying degrees of success. I’m not a smoking killjoy. I enjoyed cigarettes in my youth, and drinking in other people’s smoke used to bring back good memories, but when I became pregnant this changed and my mind would shout “TOXIN, TOXIN” if ever smoke touched my nasal hairs. Sometimes, not to cause a scene I just grinned and bore it, but images of damaged children would always run through my mind.
Most everyone knows that you shouldn’t smoke in pregnancy, but I’ll bet you didn’t know why? Studies have shown that there are over 4,000 chemicals in cigarette smoke and more than 40 of these are known to be linked with cancer (Thielen et al., 2008; US Department of Health and Human Services, 2010). Many of these chemicals, including nicotine, are able to cross the placenta into the foetus’s blood stream where, due to differing mother and baby physiology, concentrations can build up to higher levels in the foetus than in the mother (Lambers & Clark, 1996). Every time you have one, that’s two fags for the baby. There is evidence that links exposure to smoking in the womb to an increased risk of spontaneous abortion, pre-term delivery, and the baby going on to develop asthma, allergies, breathing problems and even cancer in later adult life. If you are a pregnancy smoker, you know all this and are as immune to this as being presented with a photo of a messed up smoker’s lung in the doctor’s surgery when they suggested you quit. What may influence you and what were my main concerns, were with the lesser known associations of smoking: with higher levels of aggression, conduct problems, attention problems, impulsivity, memory, speech and language and intelligence (Knopic 2012). It shouldn’t really sway you more, but put it this way, if your child develops cancer in adult life, it’s a long way off and is largely their problem to deal with. If your child is prone to aggression, that’s going to be your problem in a matter of months to years! If you did already know that, then I bet you didn’t know how cigarette smoke does this?
If you do then you are well ahead of the scientists! Scientists are still hard at work on this, but their current hypotheses surround the fascinating ideas around foetal programming and epigenetics. The theory of foetal programming is attributed to David Barker. David studied a group of mothers who were pregnant during the Dutch famine of 1944-1945 and followed up on the consequences of the children who were in utero during the famine. These mothers suffered malnutrition and stress, the children exposed to maternal starvation and stress were found to have lower birth-weight which is of no surprise. However, despite the end of the famine and a return to plentiful food supply, these children went on to have worse adult outcomes than children conceived just after the famine ended. These included cognitive deficits and higher likelihood of type 2 diabetes, heart problems and obesity. Barker’s theory is that there is a mechanism for Darwinian adaptation to environment occurring right there in our swollen abdomens that has probably been responsible for the success of our species. He postulated in the 1980s and 1990s that the developing foetus could take on information from the mother’s womb environment and use this to “forecast” the state of the environment into which it would be born. It is then able to prepare itself by adapting to be able to best survive the predicted environment (generate a predictive adaptive response); in much the same way that we might reach for the umbrella, coat and wellies on hearing a weather forecast for rain. Thus, if the developing foetus finds itself in an environment in which nutrients are scarce (such as the Dutch Famine) it thinks to itself “Bugger, I’m in for it when I get out, if my mother can’t find enough food to eat, there’ll be no damn food for me!” It therefore adapts (programmes) itself to be small and thrifty (conserve and use minimal resources) in order to be able to best survive the harsh environment it will face on birth.
Through this mechanism our ancestors lived to survive droughts and famines long enough to reproduce and eventually generate us. The foetuses that did not adapt themselves in this way would have died in infancy and childhood not being resourceful enough in their physiology to withstand periods of deprivation. The problem occurs if the forecast is wrong as adaptations made at these early stages of development are generally permanent. In the Dutch famine group, conditions improved after 1945 and food became plentiful again. The small and thrifty babies turned into children and adults that continued to store everything they ate and basically became more obese and prone to obesity related diseases, compared to children from similar backgrounds born just a year or so later. This is the fool in his heavy overcoat, wellies and umbrella at the beach in 30 degree heat cursing Michael Fish’s dodgy weather report.
How the foetus is able to use womb environment information to affect changes in its development is thought to be by epigenetic effects. In essence, epigenetics is the study of which sections of DNA gets expressed. In my analogy of DNA as a large manual for human construction, epigenetics is the study of the post-it notes put into the manual to instruct the builder which instructions to follow and which to ignore. Scientists have discovered several ways in which these post-it notes can be inserted, one of which is by a process called “DNA methylation”. Smoking has been shown to affect this process and others implicated in epigenetics which activate changes in the placenta (which can then affect the development of the foetus) and foetal development directly (Knopic, 2012). Epigenetic changes can also be inherited and can continue to influence at least 2 generations of descendants (at least in insects-Matthews & Philips 2010), thus what you do in pregnancy now may not only affect your children, but also your grandchildren!
A foetus exposed to high levels of toxins (like cigarette smoke) might prepare itself by bolstering its immune defences. On being born to a toxin free environment, the hypersensitive immune system gives rise to asthma and allergic reactions, which are essentially immune responses to normal everyday surroundings. It is harder to understand the adaptive nature of the psychological associations of prenatal exposure to cigarette smoking (aggression, impulsivity), but it makes sense that if toxins are part of what can be described as a “hostile” environment, that the newborn baby and subsequent child should be prepared to look out for danger and fight its way in the world. Aggression, hypervigilence and risk-taking behaviour would be beneficial traits to aid survival of a toddler in an Armageddon situation, but probably not so beneficial in a 3-bed semi in Barnes.
I went to a Justin Timberlake concert when I was pregnant with Big Sis and she came out ready to dance! Does that count?
US Department of Health and Human Services. (2006). The health consequences of involuntary exposure to tobacco smoke: A report of the surgeon general. Atlanta, GA: Centers for Disease Control and Prevention.
Thielen, A., Klus, H., & Muller, L. (2008). Tobacco smoke: unraveling a controversial subject. Experimental and Toxicologic Pathology, 60, 141–156.
US Department of Health and Human Services. (2010). A report of the surgeon general: How tobacco smoke causes disease: What it means to you. Atlanta, GA: Centers for Disease Control and Prevention.
Lambers, D. S., & Clark, K. E. (1996). The maternal and fetal physiologic effects of nicotine. Seminars in Perinatology, 20, 115–126.
Knopic, V.S., Maccani, M.A., Francazio, S. & McGeary, J.E. (2012). The epigenetics of maternal cigarette smoking during pregnancy and effects on child development. Development and Psychopathology 24 (2012), 1377–1390.
Matthews, S.G., & Phillips, D.I. (2010). Minireview: Transgenerational inheritance of the stress response: A new frontier in stress research. Endocrinology, 151, 7–13.
Thought I’d have a go at a posting, fully aware I may be shot down by one who clearly knows a lot more about this than do I….
Surely a somewhat more prosaic interpretation of the smoking during pregnancy and associated behavioural issues would be that: we know that stressful environments are both conducive to starting smoking and make quitting harder, so (to a degree) smoking can be a ‘proxy marker’ of exposure to stress (or at least, feeling stressed), so that a simpler chain of causality would be that stress, or vulnerability to stress increases both the likelihood of smoking during pregnancy and (via genetics or environment) the extent to which any offspring will tend to experience stress and/or behavioural consequences.
This at least sounds simpler than a more exotic suggestion of behavioural epigenetics responsive to prenatal toxin exposure.
Thanks. Yes, sorry if it was not explained so clearly. I originally wrote this as a longer piece which flowed into talking about maternal anxiety in pregnancy. I split it to be shorter and more appropriate for a blog, so you will be treated to the anxiety in pregnancy part at a later date! As to smoking and cognitive and externalising deficits, it is quite a well established association I.e. has been found by many research teams, some of whom at least I am sure would have controlled for maternal anxiety, which would be pretty basic. If that doesn’t convince you, the association has also been found in mothers exposed to second hand smoke, which would go against your smoking as proxy for anxiety argument. DNA methylation caused by smoking has also been found so the exciting thing now is to try and link up DNA methylation patterns with outcome. The Knopic paper referenced is a good review, on which much of this is based if you are interested. I can not claim to be an expert in this field, but merely work in a department where much cleverer people than me are conducting really interesting research like this!
I think I have an intuitive problem with the implied determinism, which I realise you’re not actually doing and is my own (obviously non-scientific) prejudice. However, I’d be intrigued as to how they controlled properly for such things – also, for passive smoking, surely a lot of this is coming from friends or relatives, thus confounding toxins, genes and environment?
As a corollary, what of other environmental toxins? Is China soon to be overrun with more aggressive children as a consequence of rapid industrialisation, for example?
Thanks. All these studies are association studies so do not imply cause. You will see that I try to use non conclusive language of “may” and “could” or indeed “how it can” rather than, “it does”. Indeed the links between ADHD and smoking have been very much weakened by research in IVF studies where donar eggs have been used to disentangle genes from environment. However, this has not been the case (yet) for cognition and aggression, for which there are also animal studies pointing towards this. I dont think epigenetics adds anything to the determinism question, either you believe in the effect of genes or don’t. Epigenetics is actually anti-determinism as it implies that early environment (foetal) can adapt behaviour. To your point on Countries exposed to higher levels of environmental toxins, it is well known that some environmental toxins e.g. Lead, have neurocognitive effects; secondly slightly higher propensity to aggression does not “determine” adverse outcome as development is totally due to on-going gene- environment interplay. Many slightly “aggressive” children do exceptionally well in life if channelled into competitive sport, business, trading, banking etc…maybe aggression in Barnes might be useful after all…