Birth weight confounders

Neonatal Cocaine freely crosses the placental barrier, and prenatal exposure to cocaine alters neurobehavioral development in rat pups (Sobrian et al. 1990). The effects on humans exposed prenatally to cocaine is a complicated matter, because so many other concurrent factors contribute to development. Common confounds are prenatal care and maternal polydrug use. Prenatal cocaine use is associated with reduced gestational age, birth weight, body length, and head circumference (Richardson et al. 1999). In children exposed prenatally to cocaine, some studies have shown behavioral differences evident at 1 to 3 years of age (Richardson et al. 1993 Richardson 1998). Associations are also made with impulsivity and attention deficits at age 6 (Leech et al. 1999). 

Hg/dL and over are associated with adverse effects on sperm or semen and that BLLs of 20 pg/dL and over are associated with delayed conception time. Decreases in sperm count, density, and concentration were seen in men who had mean BLLs of 15-68 pg/dL. The NTP also concluded that there was sufficient evidence that maternal BLLs under 5 pg/dL are associated with reduced fetal growth or lower birth weight. There is limited evidence that maternal BLLs under 10 pg/dL are associated with preterm birth and spontaneous abortion. Prospective studies reviewed by the NTP provided limited evidence that prenatal exposure to BLLs under 10 pg/dL is associated with reduced postnatal growth in children. The NTP recognized that its conclusions about prenatal lead exposure were confounded by possible continuing postnatal exposure to lead (associated with BLLs under 10 pg/dL) that is also associated with reduced postnatal growth in children. 

Variables that might serve as confounders of the prenatal PbBibirth weight relationship were chosen, priori, on the basis of their theoretical and/or known empirical relationship with blood lead and/or birth weight (see Table 2). 

Figure 3 The estimated relationship between maternal blood lead concentration and birth weight after controlling for significant covariates and confounders. The significant (p < 0.008) blood lead by maternal age interaction is show n = 202...

Figure 5 The dose-effect relationship between maternal blood lead concentration and infant birth weight after controlling for significant covariates and confounders.

The illustrative data are those for cord PbB as it relates to gestational age, birth weight, length, and head circumference. As shown in Table 3, the adjustment for Hct% attenuated the effect size for each measure in models without control of confounding variables. In models with covariate control, Hct% correction made little difference, probably because there was little variance that could be attenuated. In the absence of a definitive basis of choice for the perinatal period, these analyses are reported as empirical findings that may be related to the methodology employed in these studies. 

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