Children exposed to cocaine in utero are more likely to experience cognitive deficits for as long as two years after birth than are children not exposed to the drug.1 Among 415 infants born at an urban hospital between 1994 and 1996, those exposed to cocaine prenatally had significantly lower mental development scores both at age one and at age two. Moreover, prenatal exposure to cocaine was the only type of substance exposure that was significantly related to the children's cognitive status. At two years of age, cocaine-exposed children were significantly more likely than unexposed children to be classified as mentally retarded or as having a mild cognitive delay.
While past research has documented associations between prenatal cocaine exposure and a number of physical and behavioral problems during infancy, the few longitudinal studies that have been conducted have produced contradictory or inconclusive findings. In hopes of gaining a clearer understanding of the effects of cocaine exposure during pregnancy on young children's development, researchers recruited high-risk women who were delivering at a Cleveland hospital between 1994 and 1996 into a two-year prospective study. Initial assessments of both mothers and newborns were made immediately following delivery, and follow-up visits were conducted 6.5, 12 and 24 months afterward.
Women were classified as high-risk if they had obtained no prenatal care, they appeared intoxicated, they had received county social services in previous pregnancies, they admitted to hospital staff that they used drugs or staff judged them likely to be drug users. Urine samples were collected from all such women and were tested for the presence of drug metabolites. In addition, infants' first bowel movements were collected and tested for signs of prenatal drug exposure.
At enrollment, women were asked to recall the extent of their tobacco, marijuana, alcohol and cocaine use both in the month before they became pregnant and in each trimester. Researchers gathered information about mothers' educational level and socioeconomic status; they administered standard tests of vocabulary, intelligence and psychological distress to the mothers and (for infants who were not in their mothers' custody) other primary caregivers.
Infants were tested at each follow-up visit, using the Bayley Scales of Infant Development. The results were two measures, one of memory, language and problem-solving ability (the mental development index) and one of motor control and coordination (the psychomotor development index). In addition, at each visit, researchers updated information on caregivers' substance use and level of psychological distress.
Analyses were based on 415 women--218 who had tested positive for cocaine and 197 who had not. Roughly nine in 10 women made each visit, and all surviving children were brought for at least one follow-up visit.
Cocaine users and nonusers were generally nonwhite (81% and 79%, respectively), and most were of low socioeconomic status (98% each). Women who had used cocaine during pregnancy were significantly older than those who had not (30 vs. 26 years, on average), were significantly less likely to be married (7% vs. 17%), were more likely not to have graduated from high school (48% vs. 31%), averaged more lifetime births (3.6 vs. 2.7) and were more likely to have received no prenatal care (20% vs. 10%).
In addition, cocaine users were much more likely than nonusers to have used alcohol (86% vs. 65%), marijuana (50% vs. 13%) and tobacco (88% vs. 42%) during pregnancy. Finally, cocaine users scored worse than nonusers on vocabulary and intellectual development tests (although the latter difference was only marginally significant), and exhibited higher levels of psychological distress.
Measures of infant well-being also differed substantially between the two groups. Compared with infants born to nonusers, those born to cocaine users had a shorter gestation; were more likely to have been born before 37 weeks; weighed less, were shorter and had a smaller head circumference at birth; and were more likely to be low-birth-weight or small for gestational age. In addition, cocaine-exposed infants were much more likely than those not exposed to the drug to have been placed in the care of someone other than their mother (34-50% vs. 2-4% over the course of follow-up).
A multivariate analysis showed that prenatal cocaine exposure was the only substance-use variable that was significantly related to the children's cognitive status. Additionally, after the results were adjusted for the caregiver's intellectual development and the mother's parity and education, cocaine-exposed children had lower mental development scores than did unexposed children, both at 12 months (95.2 vs. 99.0) and at 24 months (82.7 vs. 88.7). Moreover, the researchers observed that while "scores for both groups decreased over time, children prenatally exposed to cocaine had scores that declined faster."
Controlling for the effects of birth outcomes (as indicated by Apgar scores, neonatal risk measures and mother's psychological distress at the time of delivery) had no effect on the overall relationship between cocaine exposure and child mental development. However, the researchers noted that when they added infant head circumference to the model, the strength of the relationship between cocaine exposure and mental development at 24 months decreased slightly, implying that "some of the negative effects of cocaine exposure on cognitive outcome were mediated through smaller head circumference at birth."
Cocaine-exposed children were significantly more likely to be classified as mentally retarded (i.e., scoring less than 70 on the index of mental development) at age 24 months than were those not exposed (14% vs. 7%). Similarly, mild cognitive delay (a score of less than 80) was more common for exposed children than for unexposed children (38% vs. 21%).
No significant associations were found between psychomotor development and prenatal cocaine exposure. In contrast, infants' age, sex and exposure to tobacco prenatally all had a significant impact on psychomotor development.
The researchers observe that unlike some previous research on prenatal cocaine exposure and its effect on cognitive development, their study was prospective, included a large sample, had high retention rates and controlled for a number of potentially confounding variables. The findings of an increased risk for cognitive impairment at age two among children exposed to cocaine prenatally, they conclude, suggests "the need for public health initiatives for substance abuse prevention and treatment of pregnant women."--M. Klitsch
1. Singer LT et al., Cognitive and motor outcomes of cocaine-exposed infants, Journal of the American Medical Association, 2002, 287(15):1952-1960.