Causes of maternal death in Spain
In this study, we found that the main causes of maternal death in Spain during the study period were haemorrhage, complications of hypertensive disorders, infection/sepsis and amniotic fluid embolism.
Several cross-sectional studies have evaluated the main causes of maternal mortality in high income countries such as ours, as well as in impoverished countries. The most relevant study published to date has been the 2015 Global Burden of Disease (GBD) Study, in which a global and regional review of data from 186 countries during the period of 1990–2015 identified the eight main causes of maternal death. This analysis showed that only ten countries achieved the millennium development goal of reducing maternal mortality and identified haemorrhage as the main cause of death . Other relevant causes of maternal death identified in this study were maternal sepsis, hypertensive disorders of pregnancy, obstructed labour and uterine rupture. Global reductions in maternal mortality have not yet reached the initially proposed value of 75%; the GBD study reports reductions of close to 30%. In 2015, more than 250,000 women died during pregnancy or immediately after delivery, most of them from preventable causes.
During the 2011–2013 period in the United States, the maternal mortality rate was 17 deaths per 100,000 live births, higher than the reported rate in Spain during 1999–2015. Data from the United States show an increase in risk with increasing maternal age and differences according to ethnic origin, with a 3.4-fold increase in risk among non-Hispanic black women. In this study, the main causes of maternal death were cardiovascular disease (15.5%), obstetric haemorrhage (11.4%), pulmonary thromboembolism (9.2%), amniotic fluid embolism (5.5%), hypertensive disorders of pregnancy (7.4%) and maternal infection (12.7%) . In the United Kingdom the main cause of maternal death identified in 2012–2014 was cardiovascular disease, with a specific rate of 2 deaths per 100,000 live births .
Our data reveal that cardiovascular disease was among the main causes of maternal death in our country (although not the most important), with 46 cases (17% of total maternal deaths). Cardiovascular deaths included 18 cases of thromboembolism, 7 cases classified as indirect causes of circulatory diseases and 21 cases of specific circulatory causes.
Maternal deaths are generally underestimated in countries of the European Union and in the USA, with up to 40–60% inaccurate records of the causes of death, which may explain differences in the reported causes of death in published series [27, 28].
Maternal mortality in Spain and differences by region
Our results show a very low maternal mortality rate compared with countries similar to ours, such as Norway, which has one of the lowest rates in Europe with 7.2 deaths per 100,000 live births .
Even so, we found very significant differences between regions of Spain, with the Basque Country having the lowest mortality rate and Ceuta having the highest risk of death after adjusting for year and maternal origin. These data differ from those previously published by Luque Fernandez MA et al. , who showed that Andalucia had the highest risk of maternal death adjusted for origin and maternal age. Our study did not take maternal age into account as an adjustment variable, but our study period was longer by 9 years.
There are notable differences in the public health infrastructure of each region in our country in terms of economic expenditure and level of provision of health services, and these have been exacerbated since the 2008 crisis. The crisis may have diminished the quality of care in the most disadvantaged areas of Spain and jeopardized equity between territories. Similar patterns have been observed in other territories of the European Union . Thus, all countries should increase their awareness of the problem and establish national and local initiatives to reduce inequalities between citizens.
The excess risk of maternal death in immigrants
The data in our study show an excess risk of maternal death among immigrant women in our country. These results are consistent with findings from the existing literature, showing significant differences in maternal morbidity and mortality according to racial/ethnic origin. In the USA, these differences can be extreme .
Possible factors that may contribute to the increased risk of maternal death in certain populations include a higher prevalence of cardiovascular diseases and obesity as well as poorer management of maternal health . Differences in the proportion of perineal lacerations , use of tobacco and alcohol , gestational diabetes , type of anaesthesia used during caesarean  and ectopic pregnancy  have also been observed when comparing groups by maternal origin.
Some authors have identified hospital factors that may be involved in the unequal distribution of maternal death by ethnic groups. Howell et al. studied morbidity in New York City and found poorer perinatal outcomes among non-Hispanic black women compared with non-Hispanic white patients. This analysis concluded that the poorer obstetric outcomes could have been avoided to some extent in a healthcare centre that typically serves white women during labour . In this regard, disparities at the institutional level should be documented and addressed in such a way that factors that worsen equitable care among pregnant women and that are potentially avoidable can be identified.
Pedersen GS et al. examined whether an increased risk of maternal mortality exists among migrants in Western Europe including 13 studies with more than 42 million women and 4995 maternal deaths. This systematic review showed that immigrant women in Western European countries have a doubled risk of maternal death when compared with indigenous born women .
Strengths and limitations of the study
This study has several strengths. First, it is the most recent analysis of maternal death in our country with data provided by each region and has the longest period of observation among such studies. Additionally, we were able to adjust for relevant sociodemographic variables when quantifying the risk of maternal death in our country and identified the most important causes of maternal death.
Our analysis also has several limitations. We did not have data for very important adjustment variables such as maternal age or the presence of obesity and/or maternal cardiovascular factors that increase the risk of maternal death. We also did not have sociodemographic adjustment variables that would provide a better profile of each pregnant woman, such as the level of fluency in Spanish, duration of living in our country, educational level, degree of social integration, degree of deprivation, access to financial risk protection/health insurance scheme or family structure. This information is generally not recorded nationally or it is variably recorded across regions in our country.
Another important limitation was that we did not include deaths from any obstetric causes occurring after 42 days but less than one year after delivery and deaths from sequelae of obstetric causes occurring one year or more after delivery as recommended in the monitoring of the Sustainable Development Goals (SDGs).
In addition, we did not show data on the level of health service provision in each region or the per capita health investment made by each region. This information could help to better identify possible problems at the institutional level in hospitals and thus help understand differences observed within the same country.