Discussion

Present results indicate a small but significant deterioration in three outcome indicators of fetal growth and perinatal well-being between 1996 and 2017, particularly associated with a higher frequency of premature deliveries. Moreover, the correlation matrix did not show significant associations between sociodemographic variables with BW, BL, and GAD. Thus, GAD was the only factor associated with the observed negative changes in BW and BL. The latter results, in turn, are supported by contingency tables that compare the live births distribution categories of GAD with the corresponding categories of BW and BL (Tables 7 and 8).

Social variables of the present study indicate that early term and late pre-term deliveries happened in a higher proportion in groups of women who are employed, who live in urban areas, and who have a higher educational level in comparison with other categories (Tables 9–11). The higher frequency of GAD <39 weeks mostly reflects an excess of newborns in the early term 37–38 subcategory, a situation previously unreported in the Chilean population.

Two systematic reviews of the literature have found that pre-term incidence, among other perinatal outcomes, is negatively associated with different sociodemographic factors. One of them was done with studies from sub-Saharan Africa countries which showed that lower maternal education, maternal unemployment, and lower household wealth index were the socioeconomic status factors most commonly associated with adverse birth outcomes and infant undernutrition^{Reference Ndirangu, Newell, Bland and Thorne32}; however, one of the reported studies in that review showed that pre-term risk was increased with the presence of maternal education, especially at secondary and tertiary levels. The latter finding is similar to results presented here and also pointing to the need of further studies. The other systematic review of relevant articles, which were published worldwide from 1999 to 2007, revealed that socioeconomic disadvantage was also consistently associated with increased risk across socioeconomic measures, birth outcomes, and countries; many studies observed racial/ethnic differences in the effect of socioeconomic measures.^{Reference Delnord and Zeitlin23,Reference Blumenshine, Egerter and Barclay33}

Regarding the first two of the four demographic variables, it was found that maternal age and parity showed variations over time in their mean values. Maternal age increased its value, while parity decreased it. This change is in accordance with previous Chilean reports indicating the presence of delayed child bearing and a clear reduction in the number of children^{Reference Mardones34–Reference Fuentes, Sequeira and Tapia37}; the international literature has confirmed those observations all around the world.³⁸ Those two factors had the lowest influence on pre-term incidence in comparison with other outcomes in a recent meta-analysis of 14 cohort studies performed in four continents.^{Reference Kozuki, Lee and Silveira39} Multiple pregnancies increased their frequency over time but being a small minority, no further analysis was performed. The frequency of plural pregnancy, 1.55% in 1996, is slightly higher than the British figure for 1974, but with 2.32% in 2017 is nowadays doubling it.^{Reference Nylander, Barron and Thomson40} Besides, female sex did not vary over time and a test of the association of the two sexes with GAD showed significant but not relevant differences between them.

Data from the United States, Western Europe, and other high-income countries show that in the year 2010, the proportions of early term 37–38 weeks and late pre-term 34–36 weeks fluctuated between 15%–31% and 3%–6%, respectively.²⁰ The figures for these categories in Chile 2017 were 40.07% and 6.20%, respectively, thus, higher than the rates of developed countries, especially in cases of GAD 37–38 weeks. These differences deserve attention. First, because the proportion of infant deaths in these categories of pre-term in the United States and Europe is 22% and 26%, respectively.²⁰ Therefore, its reduction would help to decrease infant mortality in countries, such as Chile, that have a greater proportion of these births. Second, recent data suggest that infants born both at early term and late pre-term categories have a higher long-term risk of chronic diseases and early mortality (<45 years) than full-term babies.^{Reference Vogel, Chawanpaiboon and Moller21} Furthermore, those pre-term infants had less education, occupational level, and income than term infants in these studies.

Socioeconomic status is among those most frequently implicated as a contributor to the disparities in health and in the case of the United States could explain about 80% of premature mortality.^{Reference Shaver’s41,Reference Adler and Newman42} The elimination of health disparities requires an understanding of the specific impact and manner in which various factors influence differences in health among population groups. Two basic approaches to the study of the influence of SES on health are described by Kaplan: the compositional approach and the contextual approach. Compositional measures of socioeconomic status refer to characteristics of the individual, while contextual measures of socioeconomic status refer to characteristics of the individual’s environment.^{Reference Fraser and Abu-Saad43} In this paper, it was not possible to go beyond attributing well-documented variations in socioeconomic status, as measured by education or occupation to examining more proximal ways in which socioeconomic status influences health status and health outcomes. However, using the information from this study in a rather speculative manner, three ways are proposed below by which socioeconomic status may influence health. Those are maternal obesity, smoking and cesarean sections. After further studies, those suggestions may inform social policy and program design to effectively reduce health disparities.^{Reference Shaver’s41}

BW risk categories showed a low percentage variation: <3000 g increased by 3% and ≥4000 g decreased by almost 2%. As a recent work indicates, this rather small change might be explained by the antagonistic effects on BW of smoking versus pre-conceptional overweight.^{Reference Chattrapiban, Smit and Wijga44} Thus, BW alone would be an unreliable health indicator when both risk factors are simultaneously present. More than 30% of Chilean women are obese, and 30% reports smoking during pregnancy.^{45,Reference Mallol, Brandenburg and Madrid46}

Regarding obesity, it is well known that a higher than normal index of pre-pregnancy body weight/height increases several folds the risk of pre-term delivery and cesarean section.^{Reference Abenheim, Kinch and Morin47} Moreover, their babies are more likely to have BW ≥4000 g. Maternal underweight can also be an important factor associated with perinatal outcomes, especially BW <3000 g.^{Reference Mardones and Rosso48} Maternal nutrition plays a crucial role in influencing fetal growth and birth outcomes, and it is a modifiable risk factor of public health importance in the effort to prevent adverse birth outcomes, particularly among developing/low-income populations.^{Reference Fraser and Abu-Saad49} Obesity and underweight should be reduced, ideally before pregnancy, to prevent their negative effects. The focus on women’s pre-conception health and nutrition for long-term benefits has been recently stressed in an international initiative.^{Reference Hanson, Bardsley and De-Regil8} Weight gain should also be guided during pregnancy with a specific curve to improve fetal growth reducing live births with BW <3000 g and live births with BW ≥4000 g.^{Reference Mardones and Rosso48} Its diagnostic ability has been favorably compared in various studies.^{Reference Mardones, Rosso and Villarroel50–Reference Mardones, Rosso and Villarroel52} The use of this curve has been recommended for developing countries.^{Reference Gluckman, Hanson, Seng, Bardsley, Gluckman, Hanson, Seng and Bardsley7,Reference Hanson, Bardsley and De-Regil8}

Smoking during pregnancy has been associated with lower GAD and reduced fetal growth, therefore affecting BW and BL.^{Reference Zhou, Rosenthal and Sherman53} The tobacco epidemic has been addressed with individual and collective strategies, including increasing the prices of tobacco products by way of increased taxation.^{Reference Bambs, Álcantara, Larraín, Martín and Valenzuela54} In Chile, such a strategy led to a significant – but far from desirable – reduction in cigarette smoking in the female population, from 36.5% to 29.1% in less than a decade.^{Reference Bachelet and Lanas55} Obviously, complementary initiatives, such as comprehensive smoking cessation programs in primary health care, are needed.^{Reference Águila and Chamorro56}

National information indicates that 76% of deliveries in the private health system are cesarean sections; meanwhile, its rate is 40.5% in the public system.^{Reference Borrescio-Higa and Valdés57} As previously commented, social variables of the present study indicate that early term and late pre-term deliveries happened more frequently in groups of women who were employed, who lived in urban areas, and who had a higher educational level in comparison with other categories; those women have higher incomes and can afford medical care in private clinics.^{Reference Murray58}

The unnecessary use of labor induction and cesarean sections is a problem with ethical components. A recent study reported that between 2001 and 2014, the percentage of deliveries by cesarean section performed in Chile increased from 26% to 45%, while the average cesarean section rate in the Organization for Economic Cooperation and Development (OECD) countries increased from 20% to 27%.^{Reference Borrescio-Higa and Valdés57} The authors speculated that financial incentives for health personnel and private hospitals may be playing a role because WHO considers that between 10% and 15% of all deliveries will require a cesarean section.^{Reference Borrescio-Higa and Valdés57} Furthermore, the International Federation of Gynecologists and Obstetricians has declared its support for a greater role of midwives in normal deliveries, thus limiting medical intervention to complicated and high-risk cases.^{Reference Visser, Ayres-de-Campos and Shah59} A prestigious medical journal and the WHO have supported these motions.^60,61 Chile is in urgent need of educational programs aimed at the general population, health personnel and health administration to improve in this aspect. Midwives should play an important role. Their role in reducing maternal and infant mortality has been recognized.^{Reference Mardones-Restat and de Azevedo62–Reference Bossert and Leisewitz64} Over the last five decades, the increase in the number of schools of midwifery has made possible to increase the number of professionally assisted deliveries from around 60% in the mid-sixties to the current universal coverage.^{Reference Hevia63,65} They could be a key factor in reducing cesarean sections in both the public and private sectors.^{Reference Borrescio-Higa and Valdés57}