Stressed expectant mothers “60% more likely” to have babies with health problems, warns a Daily Mail report, duly accompanied by a picture of a pregnant women sleeping peacefully.
While the Mail has whipped up a storm in a teacup, a more appropriate picture may have been a woman about to have her house blown down by a hurricane in Texas. This is because the paper’s headline was based on a research report called “Weathering the storm: hurricanes and birth outcomes” that has been circulated for comment and discussion before its potential publication.
This rather unconventional study did not actually measure stress levels in pregnant women. Instead, it measured how close women living in Texas were to the eye of severe storms or hurricanes and made the rather astonishing assumption that this correlated with their levels of stress. The researchers found that those living within 30km of a storm during pregnancy were 60% more likely to have babies who experienced complications than those living further away. The researchers make the potentially error-fraught assumption that the majority of stress was caused by the hurricane threat and not from other possible sources such as relationship breakdown or losing one’s job. A more direct measure of stress (for example, by questioning the women using validated assessment tools) would have addressed the main, but not only, limitation of this study.
This study focused on the effect of being close to severe storms and hurricanes on the health outcomes of newborns – something not obvious from the media coverage. Despite the current wet and windy Wimbledon weather, infant health problems due to the threat of hurricanes remains an unlikely source of concern for most British women. While a link between stress in pregnancy and poorer health outcomes for newborns is plausible, this study alone provides little convincing evidence to support it.
Where did the story come from?
The study was carried out by researchers from Princeton University and was funded by the MacArthur Foundation and the Eunice Kennedy Shriver National Institute of Child Health and Human Development.
The study was published as a National Bureau of Economic Research working paper. Working papers are circulated for discussion and comment purposes and have not been peer-reviewed, as is the case with official National Bureau of Economic Research publications.
The Mail did not make it clear that stress during pregnancy was not measured directly by this study. Nor was it obvious that the research was not applicable to most UK women, as it was based on Texan women in the path of severe storms or hurricanes during their pregnancy – conditions rarely experienced in the UK, even in June 2012.
What kind of research was this?
This was an ecological study that compared birth outcomes of women in Texas who lived near the path of a hurricane to those who lived further away. The proximity to hurricanes was used as an indirect measure of stress.
The authors says that a growing body of previous research suggests that stressful events in pregnancy can have negative effects on birth outcomes. The authors highlight extreme weather events as an unpredictable and unusual source of stress during pregnancy, particularly the fear of being in the path of a hurricane, as well as the damage and disruption that follow hurricanes.
What did the research involve?
The researchers analysed the effects of severe storms and hurricanes on birth outcomes in Texas over the period 1996 to 2008. They examined birth records for women living in Texas between 1996 and 2008, including detailed information on new mothers’ ethnicity, date of birth and residential addresses.
The researchers identified mothers who lived in the path of all major tropical storms and hurricanes (defined as those that caused more than $10million of damage). They did this by linking their addresses to a publically available storm database that included details of the latitude and longitude co-ordinates of the storm path and the storm type for each day of severe weather.
By linking both sets of data they calculated the distance to the closest point on the storm path so that they could compare mothers who lived in the path of the hurricane to those who lived further away. For comparison, they split the women’s place of residence into those that lived within 100, 60 and 30km of the storm paths. The eye of a storm is usually around 30-60km across and the area immediately surrounding the eye is where the most severe damage is likely to have occurred, and was therefore assumed by the researchers to be the most stressful area to have lived in.
The researchers wanted to calculate whether a child was exposed to a hurricane during the first, second or third trimester of the mother’s pregnancy. To do this they used information on:
- the children’s dates of birth
- the number of weeks of pregnancy before birth
- the dates of storm occurrences
Mothers who lived more than 100km away from the storm paths were excluded, and the sample was limited to single births only as twin/multiple births are more prone to complications. This gave a sample of 485,048 mothers, of whom 3,430 lived less than 30km from a hurricane or tropical storm during pregnancy.
The study analysis sought to exclude women who may have moved away from the storms during pregnancy. The researchers also did many types of sub-analysis to measure the effect of the mother’s behaviour (such as smoking) on the outcomes for their newborns.
What were the basic results?
The researchers found that 5% of newborns had what the authors described as ‘abnormal conditions’ and 13% had complications. The three most common abnormal conditions reported were:
- meconium aspiration syndrome. This is when a newborn breathes in a mixture of meconium (early foetal faeces) and amniotic fluid around the time of delivery. Meconium is sometimes expelled into the amniotic fluid before birth, or during labour, often in response to foetal distress. If the baby inhales meconium it can cause respiratory problems
- assisted ventilation
- being on a ventilator for more than 30 minutes
In contrast with most previous studies, the researchers found little evidence of a relationship between exposure to a stressful event during pregnancy (the proxy measure of exposure to a hurricane) and length of gestation (duration of pregnancy) or birthweight. However, mothers living within 30km of the hurricane path during their third trimester were reportedly 60% more likely to have a newborn with abnormal conditions. They were also 30% more likely to have complications during labour and/or delivery.
The authors stated these results were ‘robust to changes’ in other variables, such as estimates of how many women may have moved away from the storm path during pregnancy and individual maternal behaviours during pregnancy such as smoking, weight gain and the use of antenatal care.
How did the researchers interpret the results?
The authors conclude that, “although we do not directly measure stress, our results are supportive of the idea that stressful events in pregnancy can damage the health of the foetus”.
They add: “However, our results suggest that the effects may be subtle and not readily apparent in terms of widely used metrics such as birthweight and gestation.”
This study provides indirect evidence that stress induced by living close to a storm or hurricane during pregnancy may increase the chance of women living in Texas having complications after birth and problems during labour and delivery. However, pregnant women should not batten down the hatches just yet. The following limitations of the research should be considered:
Hurricanes are an indirect measure of stress
The authors clearly state that the research does not measure stress directly. Instead, they assumed that a person’s proximity to a hurricane would predict their stress levels. This assumption is likely to contain significant error. Stress levels in response to the threat of a hurricane may vary significantly between women and we cannot say that every pregnant woman living near a hurricane path will be stressed and potentially influence the health of their baby. This study also does not tell us whether living close to the path of a hurricane increases stress levels or to what extent. A direct measure of stress levels would have addressed this severe weakness in study design.
The authors state that, in principle, hurricanes could subject pregnant women to negative conditions including injury, disruptions in the supply of clean water, inadequate access to safe food, exposure to environmental toxins, interruption of healthcare, or crowded conditions in shelters. However, in the US, with the notable exception of hurricane Katrina, such direct threats to health from hurricanes affect relatively very small numbers of people. The authors consider that the primary threat to pregnant women in the path of a hurricane is the stress generated by the fear of the hurricane, as well as by the property damage and disruption that follows it. This is a big assumption and may not be realistic.
Other sources of stress were not measured
The study did not assess the effect of other sources of stress on newborn outcomes, such as moving house, losing a job or a family bereavement. We cannot assume that all stress experienced during pregnancy was due to worry about severe adverse weather. It is likely that overall stress levels will be substantially affected by individual circumstances.
Differences in mothers other than proximity to hurricane
The authors state that the most striking difference between mothers living near storm paths during pregnancy and other mothers is that they are less likely to be black and more likely to be non-Hispanic white. They are also more likely to be aged less than 20, less likely to be high-school dropouts, and less likely to be married than other mothers. These differences may significantly influence pregnancy outcomes, blurring the link between the hurricane proximity and newborn health. The authors made some attempt to measure the influence of these differences, but residual effects are likely to have remained.
Paper not peer-reviewed
This study was published as a working paper, which means it has not yet been scrutinised by independent experts in the field. The peer-review process ensures that the study methods and findings are properly challenged to give an idea of the reliability and believability of the results.
Only relative risks reported
The figures of 60% and 30% increased risk in the results section are relative differences comparing those living within 30km to those living further afield. The absolute differences in risk between these groups have not been reported. Absolute differences usually give a more real and intuitive indication of the chance of a harmful event occurring. The study reports that 5% of newborns have abnormal conditions and 13% had complications overall, but these figures are not broken down by proximity to the storm. This basic information would be expected in most published papers and may be suggested by those submitting comments on this working paper before it is potentially published.
In summary, this study focused on the effect of being close to severe storms and hurricanes on newborn health outcomes – something not obvious by the media coverage. While a link between stress in pregnancy and adverse outcomes in the newborn is plausible, this deeply flawed study alone provides little convincing evidence to support it.
- Currie J and Rossin-Slater M. The National Bureau of Economic Research. Working Paper No 18070: Weathering the Storm: Hurricanes and Birth Outcomes. Issued in May 2012.