The Causes of SIDS (Things We Don’t Know about Pregnancy Series #19)

What causes SIDS, and why mention it with pregnancy?

Sudden Unexplained Infant Deaths (SUIDs) of infants under a year old occur unpredictably and don’t have an obvious cause. Of around 200 such deaths in the UK every year, around 80% are classified as SIDS – Sudden Infant Death Syndrome (also know as cot death). These are the deaths that can’t later be explained by suffocation, infections, or genetic disorders, even after autopsy.

Baby by Beth [CC BY 2.0], via Flickr

But what does this have to do with pregnancy?

Because SIDS happens to very little babies, many of the risk factors are linked to their mother’s health and what happens before and when they’re born.

Not all of them, of course.

The most commonly cited risk is sleeping position: babies on their backs are less likely to suffer stress complications, like restricted breathing or bedding entanglement, than babies placed on their fronts, sides, or co-sleeping.

There could also be a seasonal component, since more SIDS deaths occur in winter. However, this could instead be because parents use more bedding or babies are more likely to get sick.

And there are genetic factors. For example, boys are more likely to suffer from SIDS; one study cited a 50% male excess in SIDS per 1000 live births of each sex^[1]. There may be a racial component too (although it’s not clear from the literature whether this has been disentangled from socioeconomic components). Or inherited defects could play a part, such as in channelopathies, ion channels related to the contraction of the heart, which may explain 10-20% of SIDS^[2].

During pregnancy, maternal health is a key indicator for SIDS risk. Mums younger than 20, who smoke or take drugs, or get poor prenatal care are more likely to have babies that suffer from SIDS^[3]. They’re also more likely to have babies born prematurely (increasing their risk x 4) or underweight (increasing their risk by x 5.7 for 1000–1499 g babies versus 3500–3999 g babies)^[4]. An elevated risk is even seen in full term babies born before 39 weeks.

Some risk factors are a bit connected to pregnancy and a bit not, for example, minor illnesses (such as anaemia^[5]); these can be out of your control, but chances may be reduced by staying healthy during pregnancy (in the case of anaemia, eating tons of iron-rich food) and getting all the requisite vaccinations.

Why does SIDS happen?

We don’t know what causes SIDS, or why the risk goes away. The chances of it are zero at birth and zero shortly after 1 year old, peaking around 2-4 months old^[6].

Baby sleeping. Public Domain via TawnyNina (Pixabay)

Scientists think it might be due to a combination of developmental challenges and environmental stressors. This means that when the babies get stressed, their bodies do the wrong thing to respond to them. Or, rather, they’re delayed in developing good breathing, immune, cardiovascular, or temperature regulation. One example is “rebreathing”, where they have restricted air access (e.g. because they’re on their tummy) and so start breathing their own exhaled air again and again, until oxygen levels are too low and carbon dioxide too high. The brain should wake a sleeping baby and make them cry if this happens, but if it hasn’t developed properly yet, they won’t.

Reducing risk

You can reduce your baby’s risk of SIDS by sleeping them on their back, in their own space, with their feet at the end of the crib and the blanket below their shoulders, not smoking around them, and getting them vaccinated (which approximately halves risk)^{[7][8][9][10][11][12]}. Breastfeeding also seems to reduce the risk of SIDS^[13]. This is probably because it equips the newborn with more antibodies bequeathed by mum sooner, boosting their immune system.

Evidence even suggests pacifiers/dummies may help reduce the chance of SIDS (by 90%!) and eliminate the risk posed by soft bedding. Whilst scientists don’t know why this is, it’s probably because the bulky handle prevents the baby from burying their face in their bedding)^[7].

Is it time to worry about the next stage now then?

There are many unknowns when it comes to pregnancy, and over the next few months, I’ll be exploring more of them with you. Look out for my next blog post, which will be about cryptic pregnancies.

To read our full article on the things we don't know about pregnancy, check out our site. This article will be updated as we add posts across the coming months.

References
why don't all references have links?

[1] Mage DT, Donner EM (September 2004). The fifty percent male excess of infant respiratory mortality. Acta Paediatrica. 93 (9): 1210–5. doi:10.1080/08035250410031305.
[2] Behere SP, Weindling SN (2014). Inherited arrhythmias: The cardiac channelopathies. Annals of Pediatric Cardiology. 8 (3): 210–20. doi:10.4103/0974-2069.164695.
[3] Sullivan FM, Barlow SM (April 2001). Review of risk factors for sudden infant death syndrome. Paediatric and Perinatal Epidemiology. 15 (2): 144–200. doi:10.1046/j.1365-3016.2001.00330.x.
[4] Hunt CE (November 2007). Small for gestational age infants and sudden infant death syndrome: a confluence of complex conditions. Archives of Disease in Childhood: Fetal and Neonatal Edition. 92 (6): F428-9. doi:10.1136/adc.2006.112243.
[5] Giulian GG, Gilbert EF, Moss RL (April 1987). Elevated fetal hemoglobin levels in sudden infant death syndrome. The New England Journal of Medicine. 316 (18): 1122–6. doi:10.1056/NEJM198704303161804.
[6] Mage DT (1996). A probability model for the age distribution of SIDS. J Sudden Infant Death Syndrome Infant Mortal. 1: 13–31.
[7] Moon RY, Fu L (July 2012). Sudden infant death syndrome: an update. Pediatrics in Review. 33 (7): 314–20. doi:10.1542/pir.33-7-314.
[8] Müller-Nordhorn J, Hettler-Chen CM, Keil T, Muckelbauer R (January 2015). Association between sudden infant death syndrome and diphtheria-tetanus-pertussis immunisation: an ecological study. BMC Pediatrics. 15 (1): 1. doi:10.1186/s12887-015-0318-7.
[9] Mitchell EA, Stewart AW, Clements M (December 1995). Immunisation and the sudden infant death syndrome. New Zealand Cot Death Study Group. Archives of Disease in Childhood. 73 (6): 498–501. doi:10.1136/adc.73.6.498.
[10] Fleming PJ, Blair PS, Platt MW, Tripp J, Smith IJ, Golding J (April 2001). The UK accelerated immunisation programme and sudden unexpected death in infancy: case-control study. BMJ. 322 (7290): 822. doi:10.1136/bmj.322.7290.822.
[11] Hoffman HJ, Hunter JC, Damus K, Pakter J, Peterson DR, van Belle G, Hasselmeyer EG (April 1987). Diphtheria-tetanus-pertussis immunization and sudden infant death: results of the National Institute of Child Health and Human Development Cooperative Epidemiological Study of Sudden Infant Death Syndrome risk factors. Pediatrics. 79 (4): 598–611.
[12] Carvajal A, Caro-Patón T, Martín de Diego I, Martín Arias LH, Alvarez Requejo A, Lobato A (May 1996). [DTP vaccine and infant sudden death syndrome. Meta-analysis]. Medicina Clinica. 106 (17): 649–52.
[13] Goldwater, Paul N. A perspective on SIDS pathogenesis. The hypotheses: plausibility and evidence. BMC medicine 9.1 (2011): 64.">