Strengths and limitations
There are several strengths to this study. First, the prospective nature of this study reflects the direction of effects and enhances the validity of measurements. In addition, the large sample size provided a range of opportunities to adequately control for confounding factors. This enabled examination of the association between maternal anxiety and birthweight and gestation. That is, several confounders could be included at different levels, such as those operating prenatally, as well as those related to birthweight and gestation (e.g. birth complications). Contextual factors such as maternal age and education could also be included. Next, the unique sample included a large sibling population, which allowed adequate control for family variables such as maternal genetic effects. 11 A sibling comparison design using a large cohort study can test whether associations differ between children born after subsequent pregnancies to the same mother. A discordant sibling design could also rule out all environmental differences that may vary between families, which can’t be done using a population comparison design.14 However, although the sibling design is helpful to control for constant family factors, caution is required as effects could also be a result of unmeasured unshared environmental factors.15 The women participating with multiple pregnancies might also represent a selection bias: there is a possibility that the first pregnancy influences the second, as has been suggested by Sjölander et al.,16and Frisell et al.17 It should also be noted that important moderators that could influence maternal anxiety have not been examined (for example, genetic x environmental effects). In addition, maternal self-reporting was used to measure maternal anxiety. Social stigma about mental health problems, and especially a desire to appear healthy for the expected baby, could potentially lead to the under-reporting of anxiety symptoms. However, in large-scale cohort studies, the full range of symptoms can be found.