Previous studies have attempted to disentangle the etiology of the relationship between MSDP and smoking as well as externalizing behaviors in the offspring (see Knopik, 2009
for a review). While, using a subset of these data, Knopik et al (2009)
recently showed univariate associations between ADHD problems and MSDP, a preponderance of studies, including ours, support the hypothesis that for externalizing behaviors, such as ADHD and CD, transmission of risk may be attributable to a latent predisposition to non-normative behaviors (Silberg et al., 2003
; Maughan et al., 2004
; Knopik et al., 2005
; Knopik et al., 2006
; D'Onofrio et al., 2008
) – in other words, women with a predisposition to non-normative/problem behaviors tend to also smoke during their pregnancy and their offspring are at increased risk for externalizing psychopathology via a direct transmission of genetic risk from mother (and father) to offspring. A recent study found no differences in rates of externalizing behaviors between siblings discordant for MSDP also alluding to the possible role of family environment that may jointly increase likelihood of MSDP as well as later externalizing behaviors in the offspring (D'Onofrio et al., 2008
) – the potential role of parental behaviors or general permissive parenting may be explored in this context.
The literature surrounding the impact of MSDP on birth weight and other birth complications is quite robust. We validate those findings here - offspring whose mothers reported MSDP, on average, weighed 215.5 grams (7.6 ounces) less at birth when compared to offspring whose mothers may have been regular smokers but did not report MSDP. Animal studies demonstrate the effects of nicotine on fetal and placental development – a common hypothesis here posits that nicotine exerts anorexigenic effects on both mother and fetus (Perkins et al., 1994
; Ernst et al., 2001
) exacerbated by hypoxia related to respiratory difficulties in the fetus (Abel, 1980
; Byrd and Howard, 1995
; Cnattingius, 2004
). Some studies have also considered the possible mediating role of lower maternal weight gain, lower maternal BMI and premature birth – however, the effect of MSDP on low birth weight appears to be independent and direct.
After adjusting for maternal and paternal covariates, low scholastic achievement, via self-reported school grade was not associated with MSDP, suggesting that other familial factors that correlate with both MSDP and offspring scholastic achievement may be at play. While early studies (Butler and Goldstein, 1973
; Fogelman, 1980
; Fogelman and Manor, 1988
)demonstrated strong associations between MSDP and scholastic achievement, recently, Lambe et al., (2006)
have reported that MSDP during the first but not the second pregnancy is associated with low scholastic achievement in the second offspring. Additionally, consistent with our finding, another study (Braun et al., 2009
) reported that maternal education attenuated the relationship between MSDP and intellectual abilities, further reinforcing the role of correlated familial factors.
The findings of the present study should be viewed with some limitations in mind. First, the parent studies were designed to study outcomes in the offspring of twin pairs - while this can be particularly useful in disentangling the role of genetic factors from environmental influences, we were limited in our ability to do this due to (a) reduced power and (b) because the fathers, but not the mothers, were twins. Thus, a formal test of genetic influences of MSDP on offspring outcome is not possible (Eaves et al., 2005
). Additionally, the fathers were members of the VETS – this, as well as strategies for contacting mothers and offspring, may influence the generalizability of our findings. Second, reports of MSDP were retrospective and cross-sectional and may have been influenced by recall bias or other personal factors. While studies show high reliability in self-reported smoking during pregnancy (Heath et al., 2003
; Reich et al., 2003
; Pickett et al., 2009
), recent increases in dissemination of information on adverse consequences of MSDP on offspring may have discouraged some mothers from disclosing it. Third, maternal gestational weight was not available on all participants and hence could not be used as a control. Fourth, maternal nicotine dependence was assessed using the H.S.I and DSM-based nicotine dependence measures were not available. Finally, as the mother often reported on the offspring and herself (e.g. ADHD), rater bias may have influenced our findings to some extent.
MSDP increases neonatal intensive care unit admissions by 20%(Adams et al., 2002
). There continues to be a clear need for health care professionals to counsel expectant mothers (and where possible, fathers/spouses/partners of expectant mothers) regarding the adverse consequences of smoking during their pregnancy and offer behavioral counseling or safe pharmacological aids. Not only does MSDP, directly and indirectly, influence the well-being of the offspring (from fetal stages through young adulthood) but also has key health consequences for the expecting mother. As articulated in the core objectives of Healthy People 2010 (Healthy People 2010, 2000
), reducing risk behaviors and increasing education surrounding prenatal health, will be vital in attaining better quality of life for mothers, their offspring and consequently, families in general.