Using data from a unique birth cohort in Québec, we explored the influence of poverty, its timing, and its accumulation on the number of mother-reported asthma-like attacks and the mother’s perception of her child’s health from birth to 41 months. In summary, results suggest that different dynamics of poverty contribute to the observed negative health outcomes among children living in poverty: Concurrent poverty is associated with more mother-reported asthma-like attacks and with a higher risk of being perceived in poorer health status. One-period-lagged poverty is associated with more mother-reported asthma-like attacks and this remained consistent after controlling for concurrent poverty. The number of mother-reported asthma-like attacks is significantly higher among children in the chronic poverty class compared to those in the never-poor class, particularly at 17 and 29 months. Perceived health status at 5 months was more likely to be less than very good among chronically poor children compared to never-poor children. For both outcomes, the magnitude of the difference between the never poor and the chronically poor children tend to decline over time. Below, we first discuss children’s experience of poverty and their experience of ill-health across time before discussing results on the longitudinal poverty-health relations.
More than one quarter of the children (27.6%) were born into households with insufficient income. The proportion of poor children decreased to about 20% at 41 months of age. At that age, 15% had experienced transient poverty and 17% had spent their entire first 3.5 years of life in a household with insufficient income. Strictly considered, the LICO provides a means of assessing relative rather than absolute poverty. It provides a means of identifying “those who are substantially worse off than the average”
(p 56). The LICO is currently the most widely accepted measure of deprivation in Canada, and is often cited in research and public policy discussions. The rates of poverty among young children observed here were similar to those for households with children under 18 years of age reported by the Canadian Council on Social Development
The results showed that on average, experience of ill health among QLSCD children varied across time. Taking into account children’s sex and birth order, maternal age, education and immigration status, day-care utilization, single parenthood, and birth order variability, the number of mother-reported asthma-like attacks decreased over time. The decrease in the mean number of mother-reported asthma-like attacks is consistent with the normal history of asthma symptoms in that the majority of symptoms likely develop in the preschool years and decrease later towards remission. Sears
 reviewed studies suggesting that the highest incidence of asthma occurs during early childhood, followed by a decrease in later childhood. A longitudinal study conducted in Tucson showed that 59.2% of children who developed wheezing before their 3rd birthday were symptom-free when followed up at 6 years of age
. Our results suggest that decrease in symptom frequency may be observed at earlier ages.
The likelihood of being perceived in less than very good health by the mother tended to increase across time, but this increase was statistically significant only at 17 months. We are unaware of any study that has longitudinally analyzed how the mother’s perception of her child’s health evolves as the child ages. Studies among adults show that self-rated health declines slowly across time
[24, 27, 49], but we have no reason to expect similar patterns among young children. However, research on inequalities in child health shows that socioeconomic differences in health risks significantly narrow in adolescence, suggesting that we would expect young children’s health to improve as they approach adolescence
Turning to the poverty-health relationship, our study confirmed with different analytic strategies the deleterious impact of poverty on aspects of child physical health. In particular, the study showed that living in a household with insufficient income, compared to the absence of poverty, results in more frequent mother-reported asthma-like attacks and a higher likelihood of being perceived in less than very good health. The added value of our study resides in showing that the timing of exposure to poverty potentially affects children’s health in terms of these two health indicators during early childhood.
Contemporaneous poverty predicted worse overall health status as measured by maternal perception of health being less than very good. These results support the hypothesis of “immediate” effects which is suggested in the literature as a possible explanation of the pathways from poverty to ill-health
Irrespective of other covariates, insufficient income reported during the previous survey round was significantly associated with the number of mother-reported asthma-like attacks. This influence remained statistically significant even after controlling for concurrent poverty status. As shown in Table
4, the effect of previous-round poverty on the number of mother-reported asthma-like attacks suggests an effect in addition to an immediate effect (i.e. critical period) but could also suggest a cumulative effect of the previous time period. These findings are consistent with those of previous studies which link early childhood poverty to aspects of poor health in adolescence
 and adulthood
[25, 27, 54], consistent with the hypothesis that poverty can have a delayed or cumulative effect for this outcome.
Evidence of poor health as a function of cumulative exposure to poverty was observed for mother-reported asthma in our study. Membership in the chronic poverty latent class was associated with a higher mean number of mother-reported asthma-like attacks per annum. Although our data may lack statistical power to clearly show differences in mother-reported asthma-like attacks related to poverty in the baseline round despite an OR of 2.44, an increased risk of mother-reported asthma-like attacks from baseline to 29 months was indeed observed among children in the chronically poor group in comparison to a statistically significant decrease in the non-poor group. Previous studies have reported conflicting results on the relationship between poverty and asthma among children
. Some studies have reported a higher prevalence of asthma among less privileged children compared to privileged children
[56, 57] while others have reported no association between poverty and childhood asthma
[58, 59]. The use of different measures of poverty from one study to another may explain in part this incongruence. Interestingly, the effects of poverty on the severity of asthma attacks seem less controversial. Poverty seems to be associated with more severe cases of asthma attacks
[56, 57, 60]. In our study, chronic exposure to poverty was associated with maternal perception of less than very good child health, but the effect of poverty was evident during the first 17 months of life.
Care was taken to mitigate the effect of measurement errors associated with an income-driven poverty measure on the accuracy of our estimates. Use of instrumental variables is an accepted means of correcting for this type of measurement error. We verified that the household income rank satisfied the main conditions of representing a good instrumental variable for the relationship between poverty and health in childhood. Multilevel procedures for repeated measures accounted for autocorrelation while latent class analysis helped in isolating chronic poverty from transient poverty so that potential residual measurement errors in the former could be captured. However, it is possible that the transient poverty class represents such a heterogeneous group of children that the meaning of any patterns would be difficult to interpret. The lack of a significant association between transient poverty and the two health indicators under study may therefore reflect this measurement shortcoming.
Given this limitation and the fact that we had only three to four measurement occasions depending on the indicator, we were unable to adequately verify the four hypotheses underlying the poverty-health relationship in early childhood. Nevertheless, this limitation does not jeopardize the conclusion that the timing of poverty and chronic poverty matter to children’s physical health in the first few years of life and that the patterns of these effects may differ depending on the outcome. Another limitation of the study is that we did not correct for the QLSCD's design effect, which increases the risk of falsely rejecting the null hypothesis at 5%. However, in keeping with methodological suggestions, we used a conservative alpha level of 1%
. At this significance level, the effects of previous round poverty status and cumulative poverty on number of mother-reported asthma-like attacks was still statistically significant as was the association between concurrent poverty and maternal perception of the child’s health status. Finally, the measures are based on parents’ reporting which may include subjectivity and/or memory errors. In particular, one may question the validity of maternal perception of the child’s health. However, we showed in a previous study that when children were 17 months old, the mother’s perception of the child’s health was significantly associated with the presence of acute and chronic health problems and specifically with the occurrence of mother-reported asthma-like attacks
. This suggests that maternal perception of the child’s health in the QLSCD is a measure with acceptable validity.
Sample attrition is common in longitudinal studies like the QLSCD. With 92% of the initial sample still participating at the third annual round, the low attrition of the QLSCD is quite exceptional. Nevertheless, children from poor households are over-represented among children lost to follow-up. It is thus possible that we are underestimating the magnitude of poverty effects when detected and we may be unable to detect any effects when in fact they do exist.