shows the demographic characteristics of the cohort stratified by exposure groups. Women had a mean age of 30 years at inclusion. Students were younger than the other groups, that is, 74.3% were below 30 years of age. About half (52.1%) of the women had not given birth previously. Among the students, 66.6% were primiparous. On average, 45.8% of the women had used paracetamol, most prevalent in the mixed exposure group (52.3%) and least prevalent in the farmer group (39.6%). A majority (79.4%) of the women were non-smokers during pregnancy. The LMW group had the highest tobacco use with 17.4% smoking 10+ cigarettes/day. A similar pattern was seen at follow-up at 7 years of age, but in general, fewer mothers smoked at follow-up, with a decrease from one in four to one in six. The lowest prevalence of atopy (15.2%) was seen in the HMW group, whereas the prevalence was highest among students (23.8%).
Table 1 Demographics, categorised according to mothers’ exposure during pregnancy into seven categories: high-molecular weight (HMW), low-molecular weight/irritants (LMW), mixed (a mixture of HMW and LMW), farmers, unclassifiable, students and references (more ...)
Online supplementary table E1
presents the characteristics among participants and non-participants. The distributions of exposure groups were quite similar between participants and non-participants, but with a slightly lower number of LMW exposed and students among the participating mothers compared with the non-participating mothers. More of the participating mothers were non-smokers during pregnancy, 79% versus 73%; they were slightly older (30 vs 29 years), better educated (academic level, 34% vs 25%), belonged to a higher socioeconomic group (employees at the highest level, 25% vs 18%), and more were expecting their first child compared with the non-participants, 52% vs 49%. Maternal atopic status was similar in the two groups. At birth, children of participating mothers, on average, weighed 30 g more and the number of SGA children was 9% in participants compared with 10.7% among non-participants.
shows the prevalence of asthma stratified for atopic status defined as ever AD. The overall prevalence of asthma was 16%; 23% for AD and 14% for non-AD children. Children from the LMW and the HMW-exposure groups had the highest prevalence of asthma, both in total (18–19%), and for the AD group (27–28%) and non-AD group (16–17%). Farmers’ children had the lowest prevalence of asthma, which was not significantly different from that of the other exposure groups. In online supplementary table E2
, the prevalence of asthma was further stratified by sex. The prevalence of asthma was higher among boys (19%) compared with girls (13%), and for both sexes the overall prevalence of asthma was higher for AD compared with non-AD children. Children from the LMW and HMW exposure groups had the highest prevalence of asthma in boys; for girls, this was only true in respect of the LMW exposure group.
Unadjusted prevalence of asthma stratified for atopy defined as ever atopic dermatitis (AD) in the 7-year-old children (n=41 724)
shows the crude and adjusted ORs for the associations between mothers’ occupational exposure during pregnancy and asthma in the child. In model 2, levels of education and SES were added to the a priori chosen confounders. For HMW agents, ORs were non-significantly increased in the range 1.26–1.44 in both the crude and the adjusted analyses, especially for AD children. Children of mothers exposed to LMW agents during pregnancy had an increased OR for asthma in the crude analysis for both AD (OR 1.38 (95% CI 1.14 to 1.67)) and non-AD children (OR 1.26 (1.13 to 1.39). The same tendency was seen in model 1 with OR 1.17 (0.95 to 1.44) for AD and 1.10 (0.98 to 1.22) for non-AD children. The ORs were further decreased in model 2 with OR 1.10 (0.88 to 1.37) for AD and 1.03 (0.91 to 1.15) for non-AD children.
Table 3 Unadjusted and adjusted logistic regression models showing the association between mothers’ occupational exposure during pregnancy and asthma in the child at age 7 years stratified by atopic status of the child defined as ever atopic dermatitis (more ...)
For the other exposure groups, no significant association between maternal exposure and asthma was observed in the children. In the crude analysis, a borderline significant association between being in student/mixed groups and asthma was seen with OR 1.05–1.08, but without confirmation in model 1 or 2.
The analysis in was repeated taking the 7-year-old children's current parental ETS into account. This did not change the results (data not shown). The same was true when paternal atopic status was included.
Even though educational level and SES were considered as potential confounders, they were correlated to our exposure variable, partly because the DISCO-code was included in the definition of SES. We therefore decided to use model 1 in the ensuing analysis. We did not find any significant interactions between exposure and ever AD when the interaction exposure*ever AD was included in the model.
Model 1 was repeated after stratification for sex; please see online supplementary table E3
. In general, similar results were found for boys and girls, that is, children of mothers exposed to LMW agents during pregnancy had a borderline increased OR for asthma in both girls (1.14 (0.98 to 1.32)) and boys (1.08 (0.95 to 1.23)). For girls, the association was significant for AD children, 1.43 (1.09 to 1.88), whereas for boys this was true for non-AD children, 1.23 (1.04 to 1.46).
In online supplementary table E4
, ORs for covariates in model 1 are displayed with asthma as the outcome of interest. The mother's age was inversely associated to asthma in the child. The children of women <25 years of age had an increased OR for asthma, 1.40 (1.27 to 1.55), compared with 25-year-old to 29-year-old women (reference). The risk was further decreased for 30–34 years, OR 0.85 (0.79 to 0.91) and 35+ year-old women, OR 0.77 (0.70 to 0.84). Maternal pre-pregnancy BMI was positively associated with asthma with ORs, 1.17 (1.09 to 1.26) and 1.35 (1.21 to 1.49) for BMI 25–29 and above 30, respectively, compared with BMI 20–24 (reference). Children with maternal atopic disposition had increased OR for asthma (1.69 (1.59 to 1.80)). The use of paracetamol and antibiotics during pregnancy was associated with asthma in the children, OR 1.08 (1.02 to 1.14) and 1.17 (1.09 to 1.26), respectively. Smoking during pregnancy was positively associated with asthma in a dose-dependent manner with OR 1.20 (1.10 to 1.31) for 1–10 cigarettes/day), and OR 1.42 (1.30 to 1.55) for >10 cigarettes/day compared with non-smokers (reference). Parity was positively associated with asthma, with OR 1.14 (1.07 to 1.21) for 1–2 pregnancies and OR 1.19 (0.97 to 1.46) for three or more pregnancies compared with no earlier pregnancy (reference), whereas pet ownership was not associated with childhood asthma, OR 0.97 (0.92 to 1.03).
Being a boy and being born SGA were both positively associated with asthma with ORs 1.57 (1.48 to 1.66) and 1.17 (1.07 to 1.29), respectively.
The analysis in online supplementary table E2
was repeated after stratification for atopic status in the children, and almost identical results were seen for AD and non-AD children (data not shown).
shows the adjusted logistic regression analysis on the association between mothers’ prenatal and postnatal exposure and asthma in the children. For LMW agents, both prenatal and postnatal exposure were significantly associated with asthma in the children, ORs 1.10 (1.00 to 1.22) and 1.15 (1.05 to 1.28), respectively. After mutual adjustments, only postnatal exposure (OR 1.13 (0.99 to 1.29)) and the combined effects of prenatal and postnatal exposure (OR 1.34 (1.19 to 1.51)) were associated with asthma in the child. For the other exposure groups, no significant associations were seen for either prenatal or postnatal exposure.
Figure 1 Adjusted logistic regression analysis on the association between mothers’ prenatal and postnatal exposure and asthma in 7-year-old children. Reference: reference workers. Pre, prenatal exposure; post, postnatal exposure; post just pre, postnatal (more ...)
The association between maternal-specific occupations (included in the LMW exposure group) and asthma in the children is displayed in . In the adjusted analysis, hairdressing was borderline associated with asthma in the child, OR 1.29 (0.99 to 1.69), without any significant difference between AD and non-AD children. For AD children, cleaning was borderline associated with asthma with OR 1.56 (0.98 to 2.47). A similar pattern was observed for non-AD children and mother's work in the chemical industry, OR 1.54 (0.97 to 2.45).
Crude and adjusted logistic regression analyses on associations between mothers’ LMW jobs and asthma in 7-year-old children
The above analysis was repeated after stratification for sex; please see online supplementary table E5. AD boys of mothers working as cleaners during pregnancy (2.29 (1.24 to 4.26) and non-AD girls of women working in the chemical industry (OR 2.15 (1.12 to 4.09) had increased OR for asthma.