Second, this is the first study to rely primarily on a bio-marker of postnatal ETS exposure in examining this association, thus reducing recall bias.
Second, due to a lack of adequate markers of postnatal POP exposure, such as those used in a previous PCB study (Walkowiak et al. 2001), our observation of associations between postnatal exposure via breastfeeding and sexually dimorphic behavior in children is still of only "limited" evidence.
Furthermore, the association was independent of region of birth, a probable marker for postnatal sun exposure correlated with long term residence.
In an observational study, it is not possible to determine whether associations are causal, and there is continuing debate regarding the possibility that early-life factors, such as breastfeeding duration, are acting as markers of the postnatal environment (35, 36).
Along with the widespread use of the Edinburgh Postnatal Depression Scale (EPDS), depression has become the marker for postnatal maladjustment.
Prenatal exposure to ambient ultraviolet radiation during the first trimester was probably not just a marker for postnatal exposure because the association had temporal specificity and was not evident for exposure at the time of birth.
AGD, a postnatal marker of testosterone action during the fetal male masculinization window, is reduced by in utero phthalate exposure in the rat (Macleod et al. 2010).
An inverse relationship existed between decreased DPOAE amplitude (a marker of diminished cochlear function) and postnatal serum PCB concentrations, with the strongest association observed for cumulative levels of PCBs.
Prolonged and late postnatal steroids treatment in very low birth weight infants may pose an increased risk for the development of severe retinopathy of prematurity; however, use of postnatal steroids may also be a marker for severity of illness.
Primary outcomes will be continuous and categorical markers of prenatal and postnatal somatic growth.
