Introduction
Affecting around 30–40% of the world’s population, allergies are the most common and earliest-onset non-communicable diseases [
1]. With a remarkable and rapid increase in prevalence especially in industrialized countries, their prevention and treatment have become a public health priority [
2].
Despite increasing interest, the pathogenesis of allergic disease remains unclear. Even though the importance of a hereditary component is well established, there is considerable evidence that the effect of family history partly reflects shared environmental risk factors [
3]. Epidemiological studies emphasize the importance of early environmental factors [
4]. In utero adaption to the maternal environment by means of epigenetic modifications are considered to have evolved as effective survival strategies preparing the fetus for postnatal life. However, fetal adaptation comes with the risk of a mismatch between the predicted and the actual encountered environment and is assumed to contribute to the development of allergic disease [
5,
6].
This study aims to investigate the effect of several maternal and perinatal factors and their association with allergic sensitization in a pediatric population.
Discussion
Prior work has established the importance of perinatal and maternal factors in the development of allergic disease. Godfrey et al. for example reported that disproportionate fetal growth was associated with raised IgE concentrations in adult life [
4]. However, these studies mostly focused on anthropometric measurements, which are potentially confounded by gestational age and infant sex. In this study, we tested the extent to which maternal factors and routinely measured perinatal factors could be associated with the development of allergic sensitization in children. Aside from strengthening the association of previously suggested risk factors of allergic sensitization, one novel perinatal factor was found to have an effect on childhood atopy, namely assisted conception.
While few previous studies have investigated the effect of assisted conception and/or maternal subfertility on allergic disease, our study is the first to report an increased risk of allergic sensitization in offspring after assisted conceptions. In accordance with our findings, some prior studies have reported an association between assisted conceptions/maternal subfertility and asthma [
12‐
14]. Noteworthy, however, is that preceding studies used questionnaires, diagnosis codes, or prescription of antihistamines to identify atopic patients. While antihistamines can be used for treatment of other diseases, and diagnosis codes and questionnaires don’t guarantee underlying allergic status, the use of SPTs is a major strength of this study.
Assisted conception was found to be mildly correlated to maternal age. Though slightly correlated both variables remained in the binary logistic regression model as predictors after backward elimination, thus indicating that they have independent effects on allergic sensitization status during childhood. We further established that the assumptions of the binary logistic regression were not violated by the co-existence of these two variables in the model (Additional file
1: Table S4). Furthermore, we show that this finding is not due to time-dependent changes of the availability of assisted conception (Additional file
1: Figure S2, Tables S6, S7). Therefore, we assume that our analysis identified a true effect of assisted conception on pediatric allergic sensitization status. Though this has to be evaluated carefully in future studies, we hypothesize that such an effect might be explained by immunological shifts in the placenta during pregnancy.
Notable is that this study fails to identify differences in anthropometric measurements between sensitized children and the general population, after adjusting for gestational age and sex. This is not surprising, since a comprehensive review [
15] pointed out, that methodological differences between previous studies have made the relationship between anthropometric measurements and allergic sensitization ambiguous. However, our study shows that both gestational age and infant sex, were significantly associated with allergic sensitization. While the association of male sex with increasing risk for allergic disease during early childhood is established [
16], our study provides additional support to the fairly new concept that prematurity may decrease the risk of allergic sensitization in childhood.
Even though findings are heterogeneous, evidence that prematurity might be associated with a lower prevalence of allergic sensitization is accumulating. Two large Swedish register studies [
17,
18] report that prematurity is associated with a lower prevalence of allergic rhinitis, in agreement with our findings. The relationship between gestational age and prevalence of allergic sensitization might be explained by hormone dependent shift in the placental immune environment. It can be hypothesized that a lower gestational age is associated with lower explosibility for progesterone which in turn leads to a less Th2 predominant environment in utero [
19]. Alternatively, it might reflect the effect of underlying risk factors of premature delivery.
In parallel with some previous research, we find that advanced maternal age (> 35 years) is a risk factor for allergic sensitization in childhood. As proposed by Dowhower Karpa et al. [
20], increasing maternal age might expose offspring to a greater risk for atopic disease due to age-dependent changes in the gut’s microbiome, in particular the decrease of Lactobacilli and the increase in Clostridia have been linked to increased risk for atopy in the offspring.
Our results also strengthen the widely established association of increasing parity/birth order as a protective factor. Previous studies have suggested that increasing parity leads to in utero programming, which results in a more anti-inflammatory intrauterine environment, reflected by lower cord blood IgE levels at birth [
21].
In our study, we found that mothers of sensitized children were less frequently overweight and smoked less. Therefore, our regression model suggests that maternal pre-pregnancy overweight and maternal smoking might be associated with a lower prevalence of allergic disease (Fig.
2). In regards to maternal BMI, some preceding studies suggested that the relationship of maternal BMI to allergic disease was U-shaped [
22], implying that maternal underweight as well as maternal overweight were associated with a higher risk for atopy. Since most previous studies identifying maternal overweight as a risk factor, were conducted in the northern hemisphere, it is possible that differences in diet, or gut microbiome, could explain that maternal overweight was associated with a lower prevalence of allergic sensitization in this study. Furthermore, some previous studies [
23] suggested that maternal smoking might be associated with a lower prevalence of allergic sensitization. Though the majority of scientific reports indicate that smoking and obesity are associated with a increased prevalence of allergic sensitization in offspring [
24,
25]. A possible explanation for this studies deviating results might therefore be that, since a family history of allergic disease is a strong predictive factor for allergic sensitization [
26], mothers of sensitized children are likely to opt for a healthier lifestyle motivated by their own allergic disease.
Interestingly, positive SPT were more common in offspring of mothers born in Asia. This supports previous findings made by the HealthNuts study group [
27] and could suggest that maternal continent of birth affects the development of atopy.
We compared children with confirmed allergic sensitization with a population including approximately 20% sensitized children [
8]. While this aggravated identification of risk factors, it also ensured that the risk factors identified differed strongly between groups and hence should be repeatable in other populations. Furthermore, the use of z-scores is a strength of this study, as they were calculated based on a geographically and contemporaneously matched population. Another major strength of this study is the use of SPT to quantify allergic sensitization. However, the retrospective design did not allow controlling for allergic family history, maternal education or socioeconomic status, which in previous reports have been associated with allergic disease [
16,
26]. Furthermore, this was a monocentric study and while the population size in this study is comparable to many other studies in this field, larger multicenter cohorts are needed to confirm these findings. It should also be mentioned again that children in the presented population underwent SPT due to clinical indications and hence were not screened with standard allergen panels. While clinical indication for SPT allows us to identify a group of children with symptomatic allergic sensitization, the absence of standard panels complicates comparison between groups. Furthermore, some previous studies [
11], as well as the comparison between included and excluded children presented here (Table
1), indicate that sensitization pattern of children might be age dependent, though this dimension was not considered in the present study it should be considered in future studies of pediatric populations.
This study has identified associations between perinatal and maternal factors and allergic sensitization. In part our results confirm some previous reports but add to the current literature by using an objective measure of allergic sensitization and applying a multiple regression backward elimination approach. Interestingly, we show an increased risk of atopy after assisted conception and in offspring of mothers born in Asia, neither of which have been described in a more general context of allergic sensitization. These findings support the notion that early periconceptional and other environmental factors are likely to predispose towards atopy and warrant further mechanistic and epidemiological studies, aiming to develop primary preventive strategies for childhood allergies.
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