Sins of our Mothers: Maternal Effects of Stress Explained

Most people know not to drink or smoke when pregnant - it hurts the baby! Nowadays, we are bombarded with even more warnings about how other aspects of our life - such as diet and exercise - could affect our most precious cargo. But what about your state of mind? Could your worrying about your fast-approaching deadline at work be stressing out your developing baby too? How (and why) will this embryonic stress affect the kid after their born?

Stress and You

To understand how stress affects our offspring, we need to first understand how stress affects us. Humans, just like almost every other animal you can think of, use chemicals called hormones to get things done. Hormones can be released in tiny amounts from whatever organ produces them and travel all throughout the body to effect change. One such hormone, often released in relatively large quantities in response to stress, is called cortisol. Odds are very good that anyone experiencing hardship (mental or physical) has high levels of cortisol running through their system.

Across species, cortisol (or whatever similar hormone an animal uses instead) has a myriad of long-term effects. Someone with too much cortisol for too long will probably experience an increased risk for depression, decreased immune function, increased weight gain, and even changes in their capacity to remember things!^1,2,3 Cortisol affects more than just your physiology. High levels of the hormone can change the way you express your DNA and, consequently, the way you react to stress.³

The idea that cortisol has wide-ranging effects on the body is nothing new, but the scientific community is finding increasing evidence that maternal stress can affect a baby long after they are born.

The School of Hard Knocks

When an embryo is developing inside a mother, the mother is transferring over a whole bunch of things to their offspring: nutrients, oxygen, and even hormones.⁴ In various species of animals, researchers have seen that offspring of stressed or depressed mothers look and act differently when compared to offspring from less stressed mothers.^4,5,6 Specific to our hormone of interest, babies from mothers with high cortisol also have elevated cortisol.⁷ Beyond the simple transfer of hormones from mother to embryo, offspring can also inherit the changes cortisol causes to the way DNA is expressed.^3,5 Altogether, this means developing offspring are changed by their maternal environment.

Figure 1. Corticosterone levels of vipers given all the water they could
 want (control) versus those deprived of water. Take-home message: 
water-deprived vipers have higher corticosterone levels.

Figure 2. Two measurements of growth in viper offspring from stressed 
(water deprived) and unstressed (control) mothers. Take-home message: 
babies from stressed mothers growth faster.

These changes embryos undergo can manifest in different ways. We can look at reptiles as examples because they are easy to study in this context. An experiment by Dupoue and his colleagues showed that environmental stress, such as not having enough water, does indeed raise the corticosterone (the reptile/bird version of cortisol) levels of vipers (Fig.1).⁹ This is important to show that natural events do in fact affect an animal’s stress response. The researchers go on to show that offspring from these stressed viper moms grow faster (Fig. 2).⁹ 
In a study by Sandrine Meylan and Jean Clobert, baby lacerta lizards from parents that received extra corticosterone were actually smaller and showed much slower sprint speeds (Fig. 3).⁸

Figure 3. The speed lacerta babies from different mothers can sprint at when 
their size is taken into account. Take-home message: babies from stressed mothers sprint slower.

Why do these studies show near opposite results? Turns out that the maternal effects of stress are complicated! The field of study investigating maternal effects is still evolving. New techniques are being developed every year and it’s clear that not every animal is affected the same way.

Lacerta lizard

Why Though?

A lot of the maternal effects of stress are considered by scientists to be detrimental at first glance.^3,4,6,8 However, when you step back and think about the context surrounding these changes to offspring, there are a few explanations that make sense.

To vastly simplify a complicated idea: poorly performing offspring may give their mom an edge when it comes to resources.^4,6,8,9 When a mother is stressed, they may allocate less energy into forming their embryos and said offspring won’t compete with their mother as well after they are born. 

Garter snake

Another big hypothesis that gains a lot of traction is that these changes are actually preparing the offspring to do well in a harsh environment.^3,4,6,8,9,10  For instance, in a well-studied species of garter snake, maternal stress seems to dictate how offspring live their life: live fast, die young or slow and steady.¹⁰ The authors posit that the stress the mother experiences is indicative of what type of life strategy would best serve the offspring.¹⁰

These theories might make sense for animals living in the wild, but what about humans? It can be hard to imagine why passing down conditions like post-traumatic stress disorder³ or depressive tendencies^5,7 could be a good thing. Perhaps this was adaptive in our evolutionary past, as in the aforementioned hypotheses. It is infinitely more difficult to investigate maternal effects in humans and we have only scratched the surface in the last decade or so. However, there is one thing everyone can seem to agree on regarding human pregnancy: just relax! For the baby’s sake.

Lacerta with babies

Author: Heather Neldner

References:

1.            Tataranni, P. A., Ennete, L.D., Snitker, S., Young, J.B., Flatt, J.P and Ravussin, E. 1996. Effects of glucocorticoids on energy metabolism and food intake in humans. American Physiological Society 271(2): E317–E325.

2.            Lupien, S. J., Maheu, F., Tu, M., Fiocco, A., and Schramek, T.E.. 2007. The effects of stress and stress hormones on human cognition: Implications for the field of brain and cognition. doi: 10.1016/j.bandc.2007.02.007.

3.             Yehuda, R. and Bierer, L. M.. 2007. Transgenerational transmission of cortisol and PTSD risk. Progress in Brain Research 167(February):121–135.

4.            Love, O. P. and Williams, T. D.. 2008. The Adaptive Value of Stress‐Induced Phenotypes: Effects of Maternally Derived Corticosterone on Sex‐Biased Investment, Cost of Reproduction, and Maternal Fitness. The American Naturalist, 172(4): 135–149.

5.            Oberlander, T. F., Weinberg, J., Papsdorf, M.,Grunao, R., Misri, S. and Devlin, A.M.. 2008. Prenatal exposure to maternal depression, neonatal methylation of human glucocorticoid receptor gene (NR3C1) and infant cortisol stress responses. Epigenetics, 3(2): 97–106.

6.            Bian, J. H., Du, S.Y., Wu, Y., Cao, Y.F., Nie, X.H., He, H., and You, X.B. 2015. Maternal effects and population regulation: Maternal density-induced reproduction suppression impairs offspring capacity in response to immediate environment in root voles Microtus oeconomus. Journal of Animal Ecology 84(2): 326–336.

7.            Field, T., Diego, M., Hernandez-Reif, N., Vera, Y., Gil, K., Schanberg, S., Kuhn, C., and Gonzalez-Garcia, A.. 2004. PRENATAL MATERNAL BIOCHEMISTRY PREDICTS NEONATAL BIOCHEMISTRY. International Journal of Neuroscience 114(8), pp. 933–945.

8.            Meylan, S. and Clobert, J.. 2004. Maternal Effects on Offspring Locomotion: Influence of Density and Corticosterone Elevation in the Lizard Lacerta vivipara. Physiological and Biochemical Zoology: Ecological and Evolutionary Approaches 77(3)

9.            Dupoue, A., Angelier, F., Brischoux, F., DeNardo, D.F., Trouve, C., Parenteau, C., and Lourdais, O.. 2016. Water deprivation increases maternal corticosterone levels and enhances offspring growth in the snake Vipera aspis. Journal of Experimental Biology 219: 658–667.

10.         Robert, K. A., Vleck, C. and Bronikowski, A. M.. 2009. The effects of maternal corticosterone levels on offspring behavior in fast- and slow-growth garter snakes (Thamnophis elegans). Hormones and Behavior 55(1): 24–32.

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