Prenatal exposure to drugs has long been thought to have disastrous consequences for child development, but a pioneering study at the University of Maryland suggests that the effect may be much milder than feared.

A team led by Richard Barth, Dean of Social Work at the University of Maryland, has tracked the developmental trajectories of adopted children who were exposed to substance abuse in their mother’s womb.

The new findings originate from a recent analysis of the data from his team’s California Long-Range Adoption Study which has been following the progress of 1,000 children since their adoption in the late 1980s.

Two, four, eight and 14 years after adoption, new parents were asked to complete questionnaires on topics including the characteristics of the adopted child, birth parents and adoptive family, as well as parents’ ratings of academic performance, behavioral functioning and parenting issues.

To investigate the long-term trajectories of children exposed to cocaine, marijuana or heroin during pregnancy, Barth selected a sample of exposed and non-exposed children for whom there was complete data for at least three of the four rounds of collection.

This produced a sample of 275 children for analysis in relation to the two central questions: the effects of prenatal exposure to substance abuse on children’s behavior, and the likely influence of other factors, such as age at adoption and foster care status.

For the first eight years following adoption very few differences were detected between the behavior of the exposed and non-exposed children. Except to say that both groups reported higher incidence of ADHD type behaviors that were likely to interfere with school performance.

At the 14-year follow up, differences between the two groups were more noticeable. Substance-exposed children demonstrated slightly more behavioral problems than non-exposed counterparts, prompting the suggestion that exposure has a subtle effect more likely to manifest itself during the turbulence of adolescence.

However, the overall rate of change in the behavior of exposed and non-exposed children did not differ significantly at any time during the 14 year follow period. This means that although overall levels of behavioral problems were higher among the exposed group, their trajectories were much the same as for the non-exposed group.

Thus, contrary to popular belief, substance exposure alone was not responsible for triggering a cascade of negative circumstances in later development.

What of the other influences on post-adoption behavior that the Maryland team were interested in? They found that outcomes at the 14-year mark for children adopted after the age of three were poorer than for those adopted younger. This result is comparable to other research in the child development field.

One counter-intuitive and potentially challenging finding related to foster care status. Children adopted from foster care had better behavioral outcomes than those adopted privately. Barth and his team theorize that this might be down to differences in adoptive parents’ expectations. For example, adopters of children from foster care might be prepared to expect more significant behavior problems and therefore to be better able to cope. The perception that their adopted children were behaving better than they expected may have affected how they rated behaviors in the questionnaire.

Despite this positive finding, older children adopted from foster care fared worse than both their younger counterparts and those adopted privately. These children appeared to be more susceptible to behavior problems later in life.

Some limitations in the study are acknowledged. There was a high dropout rate (22.6% of the original wave 1 sample by the time of the wave 4 collection). The infrequency of data collection was also considered to be a flaw.

These limitations aside, Barth’s is one of the first longitudinal studies to track the outcomes of children exposed to substance abuse during pregnancy and the findings have several implications for policy and practice.

They indicate that prevention and early intervention services do have a place for children who have experienced prenatal exposure to substance abuse, particularly to mitigate any problems in adolescence. They also suggest that both exposed and non-exposed children should be adopted at earlier ages.

• Summary of Crea T, Barth R, Guo S, Brooks D (2008) "Behavioral Outcomes for Substance-Exposed Adopted Children: Fourteen Years Postadoption", American Journal of Orthopsychiatry, 78, 1, 11-19.