Prenatal alcohol exposure and development at preschool age: main results of a French study

Fine motor skills in a population of children in remote Australia with high levels of prenatal alcohol exposure and Fetal Alcohol Spectrum Disorder

BACKGROUND

Many children in the remote Fitzroy Valley region of Western Australia have prenatal alcohol exposure (PAE). Individuals with PAE can have neurodevelopmental impairments and be diagnosed with one of several types of Fetal Alcohol Spectrum Disorder (FASD). Fine motor skills can be impaired by PAE, but no studies have developed a comprehensive profile of fine motor skills in a population-based cohort of children with FASD. We aimed to develop a comprehensive profile of fine motor skills in a cohort of Western Australian children; determine whether these differed in children with PAE or FASD; and establish the prevalence of impairment.

METHODS

Children (n = 108, 7 to 9 years) were participants in a population-prevalence study of FASD in Western Australia. Fine motor skills were assessed using the Bruininks-Oseretsky Test of Motor Proficiency, which provided a Fine Motor Composite score, and evaluated Fine Manual Control (Fine Motor Precision; Fine Motor Integration) and Manual Coordination (Manual Dexterity; Upper-Limb Coordination). Descriptive statistics were reported for the overall cohort; and comparisons made between children with and without PAE and/or FASD. The prevalence of severe (≤ 2nd percentile) and moderate (≤16th percentile) impairments was determined.

RESULTS

Overall, Fine Motor Composite scores were 'average' (M = 48.6 ± 7.4), as were Manual Coordination (M = 55.7 ± 7.9) and Fine Manual Control scores (M = 42.5 ± 6.2). Children with FASD had significantly lower Fine Motor Composite (M = 45.2 ± 7.7 p = 0.046) and Manual Coordination scores (M = 51.8 ± 7.3, p = 0.027) than children without PAE (Fine Motor Composite M = 49.8 ± 7.2; Manual Coordination M = 57.0 ± 7.7). Few children had severe impairment, but rates of moderate impairment were very high.

CONCLUSIONS

Different types of fine motor skills should be evaluated in children with PAE or FASD. The high prevalence of fine motor impairment in our cohort, even in children without PAE, highlights the need for therapeutic intervention for many children in remote communities.

Maternal alcohol use during pregnancy and offspring trajectories of height and weight: A prospective cohort study

BACKGROUND

Previous studies have examined associations between alcohol use in pregnancy and offspring birth size but evidence on whether associations persist during childhood is limited.

METHODS

We examined the association between maternal drinking during pregnancy and trajectories of offspring weight and height from 0 to 10 years in 7597 mother-child pairs in the Avon Longitudinal Study of Parents and Children. To strengthen the inference, we compared the maternal alcohol-offspring growth association with the partner alcohol-offspring growth association, to partially control for unmeasured confounding. We also performed sensitivity analyses restricting our analysis to women of white ethnicity and participants with three or more growth measures.

RESULTS

Maternal occasional or light daily drinking during pregnancy was not associated with reduced birth weight, birth length or offspring growth trajectories up to age 10 years. The infants of heavy drinking mothers were born 0.78cm shorter (95% CI -1.34, -0.22) and 0.22kg lighter (95% CI -0.34, -0.09) than infants of pregnancy abstainers but by age 10, offspring of heavy drinking mothers were of comparable height (mean difference 0.59cm, 95% CI -0.93, 2.11) and weight (mean difference 0.41kg, 95% CI -0.70, 1.52). These associations were not observed for heavy partner drinking and were not altered in sensitivity analyses.

CONCLUSION

Maternal occasional or light daily drinking is not associated with birth weight, birth length or postnatal growth, but residual confounding may persist. Maternal heavy drinking may have an intrauterine association with reduced birth weight and length but this association is overcome during childhood.

Fine motor skills in children with prenatal alcohol exposure or fetal alcohol spectrum disorder

OBJECTIVE

Prenatal alcohol exposure (PAE) can cause fetal alcohol spectrum disorders (FASD) and associated neurodevelopmental impairments. It is uncertain which types of fine motor skills are most likely to be affected after PAE or which assessment tools are most appropriate to use in FASD diagnostic assessments. This systematic review examined which types of fine motor skills are impaired in children with PAE or FASD; which fine motor assessments are appropriate for FASD diagnosis; and whether fine motor impairments are evident at both "low" and "high" PAE levels.

METHODS

A systematic review of relevant databases was undertaken using key terms. Relevant studies were extracted using a standardized form, and methodological quality was rated using a critical appraisal tool.

RESULTS

Twenty-four studies met inclusion criteria. Complex fine motor skills, such as visual-motor integration, were more frequently impaired than basic fine motor skills, such as grip strength. Assessment tools that specifically assessed fine motor skills more consistently identified impairments than those which assessed fine motor skills as part of a generalized neurodevelopmental assessment. Fine motor impairments were associated with "moderate" to "high" PAE levels. Few studies reported fine motor skills of children with "low" PAE levels, so the effect of lower PAE levels on fine motor skills remains uncertain.

CONCLUSIONS

Comprehensive assessment of a range of fine motor skills in children with PAE is important to ensure an accurate FASD diagnosis and develop appropriate therapeutic interventions for children with PAE-related fine motor impairments.

Executive functioning deficits in preschool children with Fetal Alcohol Spectrum Disorders

Executive function (EF) deficit is a hallmark of Fetal Alcohol Spectrum Disorders (FASD), but the vast majority of available evidence comes from school-age children and adolescents. Very little is known about EF during the critical developmental period prior to 6 years of age in FASD. We evaluated EF in 39 children with FASD (3.0-5.5 years) and a comparison group of 50 age-matched, nonexposed controls. Measures included the EF Scale for Early Childhood and a Delay of Gratification task. Compared to age-matched controls, preschool children with FASD had impairments on the EF Scale and showed more impulsivity on the Delay of Gratification task. To confirm the EF Scale finding, FASD group performance was compared to a separate normative dataset (N = 1,400). Those with FASD performed below normal (M = -0.57, SD = 0.92). Within the FASD group, IQ was correlated with the EF Scale (partial r = .60, p = .001) and Delay of Gratification (partial r = .58, p = .005). EF Scale performance did not differ significantly across levels of FASD severity (fetal alcohol syndrome [FAS], partial FAS, or alcohol-related neurobehavioral disorder [ARND]). However, compared to normative data, those with FAS had the largest deficits (M = -0.91 SD from the mean, SE = 0.23), followed by partial FAS (M = -0.66 SD from the mean, SE = 0.26), then ARND (M = -0.36 SD from the mean, SE = 0.20). These novel data show that EF deficits manifest well before the age of 6 years in children with FASD, that they occur across the spectrum, and that EF may be most impaired in children with more severe forms of FASD and/or lower IQs.

Gross motor deficits in children prenatally exposed to alcohol: a meta-analysis

BACKGROUND AND OBJECTIVES

Gross motor (GM) deficits are often reported in children with prenatal alcohol exposure (PAE), but their prevalence and the domains affected are not clear. The objective of this review was to characterize GM impairment in children with a diagnosis of fetal alcohol spectrum disorder (FASD) or "moderate" to "heavy" maternal alcohol intake.

METHODS

A systematic review with meta-analysis was conducted. Medline, Embase, Allied and Complementary Medicine Database, Cumulative Index to Nursing and Allied Health Literature, PsycINFO, PEDro, and Google Scholar databases were searched. Published observational studies including children aged 0 to ≤18 years with (1) an FASD diagnosis or moderate to heavy PAE, or a mother with confirmed alcohol dependency or binge drinking during pregnancy, and (2) GM outcomes obtained by using a standardized assessment tool. Data were extracted regarding participants, exposure, diagnosis, and outcomes by using a standardized protocol. Methodological quality was evaluated by using Strengthening the Reporting of Observational Studies in Epidemiology guidelines.

RESULTS

The search recovered 2881 articles of which 14 met the systematic review inclusion criteria. The subjects' mean age ranged from 3 days to 13 years. Study limitations included failure to report cutoffs for impairment, nonstandardized reporting of PAE, and small sample sizes. The meta-analysis pooled results (n = 10) revealed a significant association between a diagnosis of FASD or moderate to heavy PAE and GM impairment (odds ratio: 2.9; 95% confidence interval: 2.1-4.0). GM deficits were found in balance, coordination, and ball skills. There was insufficient data to determine prevalence.

CONCLUSIONS

The significant results suggest evaluation of GM proficiency should be a standard component of multidisciplinary FASD diagnostic services.

Drinking or not drinking in pregnancy: the multiplicity of confounding influences

AIMS

Studies investigating associations between prenatal exposure to low-moderate doses of alcohol and mental health development in childhood are inconsistent. The aim of the present study was to compare women who drink and who do not drink alcohol in pregnancy on a number of potential confounding variables, and to investigate whether any latent variables could be identified among these.

METHODS

Data were obtained from the Danish National Birth Cohort.

EXPOSURE

cumulated alcohol intake in full pregnancy (n = 63,464). The women were subdivided into intake groups 0, >0-10, >10-30, >30-90 and >90 units of alcohol in full pregnancy. Hereafter, the abstainers were subdivided into an all-time and a pregnancy-abstaining group, and the high intakers (>90) were subdivided into a high (>90-180) and a very high (>180) intake group.

OUTCOME

self-reported and register-based information on socio-demographic and lifestyle factors, and latent variables from an exploratory factor analysis.

RESULTS

Significant differences were observed between the intake groups on virtually all parameters. Significant differences were observed between the abstaining groups and the high-intake groups. The exploratory factor analyses identified a number of latent variables between the potential confounding variables.

CONCLUSION

Differences on confounding factors may in part explain the lack of consistency in the literature investigating prenatal exposure to low-moderate doses of alcohol and mental health development. It is cautiously concluded that the failure to control for these factors introduces residual and/or unmeasured confounding into the analyses, and thus masks the potential (small) effect of being exposed to low doses of alcohol in pregnancy. It is recommended that future studies control for factor scores rather than for the observed variables as is practice today.

Fetal alcohol syndrome

Alcohol is a physical and behavioural teratogen. Fetal alcohol syndrome (FAS) is a common yet under-recognized condition resulting from maternal consumption of alcohol during pregnancy. While preventable, FAS is also disabling.Although FAS is found in all socioeconomic groups in Canada, it has been observed at high prevalence in select First Nations and Inuit communities in Canada.This statement addresses FAS prevention, diagnosis, early identification and management for health care professionals.Prevention of FAS must occur at two levels. Primary prevention involves eliminating FAS through classroom or community education, and encouraging women to avoid consuming alcohol before conception and throughout pregnancy. Secondary prevention involves identifying women who are drinking while pregnant and reducing their consumption. This statement describes a variety of screening strategies including Tolerance-Annoyance, Cut Down, Eye Opener (T-ACE). Medical practitioners should recommend abstinence starting with the first prenatal visit. Prompt referral for alcohol treatment is recommended for pregnant individuals who are unable to stop drinking alcohol.This statement describes the diagnosis of FAS, partial or atypical FAS, alcohol-related birth defects and alcohol-related neurodevelopmental disorder. With a history of in-utero alcohol exposure, a diagnosis of FAS should be considered with current or previous growth deficiency, select facial abnormalities involving the upper lip and eyes, and neurodevelopmental abnormalities. These features are best quantified with the use of a four-digit diagnostic method.Strategies for early identification of possible alcohol-related abnormalities are outlined.Intervention focuses on optimizing development, managing behavioural difficulties and providing appropriate school programming. Of prime importance is earliest possible childhood intervention to prevent secondary disabilities that may result from delay while awaiting a definitive diagnosis of FAS.

Inter-hemispheric functional connectivity disruption in children with prenatal alcohol exposure

BACKGROUND

MRI studies, including recent diffusion tensor imaging (DTI) studies, have shown corpus callosum abnormalities in children prenatally exposed to alcohol, especially in the posterior regions. These abnormalities appear across the range of fetal alcohol spectrum disorders (FASD). Several studies have demonstrated cognitive correlates of callosal abnormalities in FASD including deficits in visual-motor skill, verbal learning, and executive functioning. The goal of this study was to determine whether inter-hemispheric structural connectivity abnormalities in FASD are associated with disrupted inter-hemispheric functional connectivity and disrupted cognition.

METHODS

Twenty-one children with FASD and 23 matched controls underwent a 6-minute resting-state functional MRI scan as well as anatomical imaging and DTI. Using a semi-automated method, we parsed the corpus callosum and delineated 7 inter-hemispheric white matter tracts with DTI tractography. Cortical regions of interest (ROIs) at the distal ends of these tracts were identified. Right-left correlations in resting fMRI signal were computed for these sets of ROIs, and group comparisons were made. Correlations with facial dysmorphology, cognition, and DTI measures were computed.

RESULTS

A significant group difference in inter-hemispheric functional connectivity was seen in a posterior set of ROIs, the para-central region. Children with FASD had functional connectivity that was 12% lower than in controls in this region. Subgroup analyses were not possible owing to small sample size, but the data suggest that there were effects across the FASD spectrum. No significant association with facial dysmorphology was found. Para-central functional connectivity was significantly correlated with DTI mean diffusivity, a measure of microstructural integrity, in posterior callosal tracts in controls but not in FASD. Significant correlations were seen between these structural and functional measures, and Wechsler perceptual reasoning ability.

CONCLUSIONS

Inter-hemispheric functional connectivity disturbances were observed in children with FASD relative to controls. The disruption was measured in medial parietal regions (para-central) that are connected by posterior callosal fiber projections. We have previously shown microstructural abnormalities in these same posterior callosal regions, and the current study suggests a possible relationship between the two. These measures have clinical relevance as they are associated with cognitive functioning.

Neonatal binge alcohol exposure produces dose dependent deficits in interstimulus interval discrimination eyeblink conditioning in juvenile rats

Alcohol consumption in neonatal rats produces cerebellar damage and is widely used to model 3rd-trimester human fetal alcohol exposure. Neonatal "binge-like" exposure to high doses of alcohol (5 g/kg/day or more) impairs acquisition of eyeblink classical conditioning (EBC), a cerebellar-dependent Pavlovian motor learning task. We have recently found impairments in interstimulus interval (ISI) discrimination--a complex task variant of EBC--in adult rats following postnatal day (PD) 4-9 alcohol exposure at doses of 3, 4, and 5 g/kg/day. Because robust developmental differences in conditioned response (CR) generation and CR latency measures are present between untreated juveniles and adults in this task, we sought to extend alcohol findings to juvenile rats (PD30). Five neonatal treatment groups were used: (1) undisturbed controls, (2) sham intubation controls, (3) 3 g/kg/day of alcohol (blood alcohol concentration {BAC}=139.9 mg/dl), (4) 4 g/kg/day of alcohol (BAC=237.3 mg/dl), or (5) 5 g/kg/day of alcohol (BAC=301.8 mg/dl). Intubations occurred over PD4-9. ISI discrimination training in juveniles (PD30-33) revealed dose-dependent CR deficits in all three alcohol-exposed groups relative to controls. Contrary to expected outcomes, CR latency measures were not significantly affected as a function of neonatal treatment. Comparison of these findings with our recent study in adults suggests that alcohol-induced impairments in ISI discrimination EBC may be greater in adults relative to juveniles. The present findings provide further evidence that ISI discrimination may provide greater sensitivity to functional deficits resulting from moderate levels of neonatal alcohol exposure relative to single-cue EBC paradigms.

Sensory processing, school performance, and adaptive behavior of young school-age children with fetal alcohol spectrum disorders

This study described sensory processing behaviors and sensory-motor abilities in children with fetal alcohol spectrum disorders (FASD) and explored their relationship to home and school function. A clinic-referred sample of 25 children with FASD, ages 5 to 8 years, was compared with 26 children with typical development, balanced for age, gender, and race/ethnicity, on standardized tests examining sensory processing, sensory-motor performance, school performance, and adaptive behavior. Children with FASD scored significantly more poorly on sensory processing, sensory-motor, adaptive, and academic achievement measures, and demonstrated more problem behaviors at home and school. Correlations were significant between measures of sensory processing and sensory-motor performance, adaptive behavior, and some aspects of academic performance. Sensory processing and related foundational sensory-motor impairments should be considered when determining the developmental needs of children with FASD. These impairments may co-occur with and contribute, at least in part, to decreased adaptive and school function.

Computational models of ethanol-induced neurodevelopmental toxicity across species: Implications for risk assessment

Computational, systems-based approaches can provide a quantitative construct for evaluating risk in the context of mechanistic data. Previously, we developed computational models for the rat, mouse, rhesus monkey, and human, describing the acquisition of adult neuron number in the neocortex during the key neurodevelopmental processes of neurogenesis and synaptogenesis. Here we apply mechanistic data from the rat describing ethanol-induced toxicity in the developing neocortex to evaluate the utility of these models for analyzing neurodevelopmental toxicity across species. Our model can explain long-term neocortical neuronal loss in the rodent model after in utero exposure to ethanol based on inhibition of proliferation during neurogenesis. Our human model predicts a significant neuronal deficit after daily peak BECs reaching 10-20 mg/dl, which is the approximate BEC reached after drinking one standard drink within one hour. In contrast, peak daily BECs of 100 mg/dl are necessary to predict similar deficits in the rat. Our model prediction of increased sensitivity of primate species to ethanol-induced inhibition of proliferation is based on application of in vivo experimental data from primates showing a prolonged rapid growth period in the primate versus rodent neuronal progenitor population. To place our predictions into a broader context, we evaluate the evidence for functional low-dose effects across rats, monkeys, and humans. Results from this critical evaluation suggest subtle effects are evident at doses causing peak BECs of approximately 20 mg/dl daily, corroborating our model predictions. Our example highlights the utility of a systems-based modeling approach in risk assessment.

Defining the behavioral phenotype in children with fetal alcohol spectrum disorders: A review

While there exists a large body of literature on cognitive functions in children with prenatal alcohol exposure, it remains undetermined if these children exhibit a unique profile of cognitive-behavioral functioning or a behavioral phenotype. Researchers have consistently found that intellectual functioning, as assessed by IQ tests, of children with prenatal alcohol exposure is deficient. There is increasing evidence that prenatal alcohol exposure is associated with slow information processing and attentional problems, in particular inattentiveness. Studies examining specific cognitive abilities such as language, visual perception, and memory in alcohol-affected children have shown performance decrements associated with increased task complexity. Children with prenatal alcohol exposure have also been found to exhibit significant deficits in daily functional skills or adaptive behavior, with deficits in socialization becoming pronounced during adolescence. The above findings point to the conclusion that a generalized deficit in complex information processing constitutes the central cognitive-behavioral characteristic of children with prenatal alcohol exposure. Researchers have consistently documented that specific brain regions are more affected by alcohol than other regions. The problem of mapping focal anomalies of the brain with a generalized pattern of deficits can be solved by taking developmental processes into consideration.

Neurobehavioral Characteristics of Children with Fetal Alcohol Spectrum Disorders in Communities from Italy: Preliminary Results

BACKGROUND

There has been considerable effort expended on defining neurobehavioral characteristics of children with fetal alcohol spectrum disorders (FASD). Children with FASD display a range of cognitive deficits and behavioral problems. In this article, we report on the neurobehavioral characteristics of children with FASD in selected communities in Italy. It was expected that both inattentive and hyperactive/impulsive characteristics would discriminate children with FASD from controls and that the groups would also differ on intellectual functioning, language comprehension, and academic skills.

METHODS

Eighty-two children, 22 diagnosed with FASD and 60 control children, participated in this study. The children were administered tests of nonverbal reasoning, language comprehension, academic achievement, and behavior.

RESULTS

On tests of nonverbal reasoning and language comprehension, the FASD group earned lower scores than did controls. Moreover, on a test of academic achievement the FASD group scored lower. When comparing these 2 groups on disruptive behavioral symptomatology, similar results were obtained, the FASD group showing greater attentional difficulties and hyperactivity/impulsivity behaviors and more overall behavioral problems. Stepwise logistic regression analysis showed that a model containing inattention and error scores on the language comprehension task correctly classified 85% of the participants. Compared with the control group, a significantly greater proportion of children with FASD met the Diagnostic and Statistical Manual of Mental Disorders-fourth edition (DSM-IV) criteria of ADD, inattentive type, as reported by teachers. In contrast, hyperactive symptoms among children with FASD were comparable with the control group. Teachers rated children with FASD as having more inattentive behaviors and as performing lower in academic skills than controls. The association between reported hyperactivity symptoms and achievement scores was nonsignificant for both language and math scores, suggesting that it is not the hyperactivity causing problems, but the child's inattention.

CONCLUSIONS

This research indicates that a nonclinic-referred sample of Italian children with FASD display a profile of neurobehavioral functioning consistent with that reported by other researchers. Furthermore, the neurobehavioral characteristic most identified with children diagnosed with FASD was inattention followed by hyperactivity.

Moderate Prenatal Alcohol Exposure and Cognitive Status of Children at Age 10

BACKGROUND

The effects of prenatal alcohol exposure (PAE) on measures of intelligence have been well documented in children with fetal alcohol syndrome. However, deficits in general intellectual ability in children with low to moderate PAE are less well understood. The objective of this study was to assess the association between moderate PAE and cognitive ability in children at age 10 controlling for other prenatal and birth factors, maternal and child psychosocial factors, and environmental characteristics.

METHODS

Data were collected as part of the Maternal Health Practices and Child Development Project, a prospective study of prenatal substance use with 636 mother-child pairs. Women were assessed during each trimester of pregnancy and with their children at birth; 8 and 18 months; and 3, 6, and 10 years. Each phase included an evaluation of growth, development, cognitive, and psychological functioning. At age 10, cognitive ability was assessed using the composite score and verbal, abstract/visual, quantitative, and short-term memory area scores of the Stanford-Binet Intelligence Test, fourth edition. Maternal intellectual ability, maternal prenatal and current drug use, maternal and child psychosocial characteristics, demographics, and home environment were included in the analysis.

RESULTS

A significant relation was found between alcohol exposure during the first and second trimesters and the composite score of the Stanford-Binet for African American children at age 10. Significant relations were also found for the verbal, abstract/visual, and quantitative subscales. Additional predictors of IQ at age 10 included mother's IQ, home environment, and child's report of depression.

CONCLUSIONS

There is a significant association between PAE and cognitive ability at age 10 among African American offspring. There was no relation between PAE and scores on the Stanford-Binet scales among the Caucasian offspring.

Pulque intake during pregnancy and lactation in rural Mexico: alcohol and child growth from 1 to 57 months

OBJECTIVE

To examine maternal intake of a mildly alcoholic beverage (pulque) during pregnancy and lactation, and its potential effect on postpartum child growth and attained size.

DESIGN

A prospective cohort study that followed mothers (during pregnancy and lactation) and their offspring (from birth to approximately 57 months of age).

SETTING

Six villages in rural, central Mexico.

SUBJECTS

Subjects are 58 mother-child pairs. Pulque intake was measured as part of a dietary assessment that was conducted for 2days/month during pregnancy and early lactation.

RESULTS

Most mothers consumed pulque during pregnancy (69.0%) and lactation (72.4%). Among pulque drinkers, the average ethanol intake was 125.1 g/week during pregnancy and 113.8 g/week during lactation. Greater pulque intake during lactation, independent of intake during pregnancy, was associated with slower weight and linear growth from 1 to 57 months, and smaller attained size at 57 months. Low-to-moderate pulque intake during pregnancy, in comparison to either nonconsumption or heavy intake, was also associated with greater stature at 57 months.

CONCLUSIONS

Pulque intake during lactation may have adversely influenced postnatal growth in this population. Public health interventions are urgently needed in Mexico to reduce heavy intake of pulque by pregnant and lactating women, and to replace intake with foods that provide the vitamins and minerals present in the traditional alcoholic beverage.

Attenuated growth of breast-fed children exposed to increased concentrations of methylmercury and polychlorinated biphenyls

Breast-feeding has been linked to slowed postnatal growth. Although the basis for this "weanling's dilemma" is unclear, environmental contaminants in human milk may be of relevance. We studied a Faroese birth cohort of 182 singleton children, born at term in 1994-95. Concentrations of mercury in cord blood and of polychlorinated biphenyls in maternal milk were measured, and duration of breast-feeding was recorded. At 18 months, children who had been exclusively breast-fed for at least 6 months weighed 0.59 kg less [95% confidence interval (CI) = 0.03, 1.16 kg] and were 1.50 cm [95% CI = 0.52, 2.47 cm] shorter than those not breast-fed. However, calculated transfer of contaminants from human milk fully explained the attenuated growth. Irrespective of duration of breast-feeding, a doubling of the mercury concentration in cord blood was associated with a decrease in weight at 18 months by 0.19 kg (95% CI = 0.03, 0.35 kg) and in height by 0.26 cm (95% CI = -0.02, 0.55 cm). Weight and height at 42 months showed the same tendencies, but the main effect occurred before 18 months of age. Thus, in communities with increased contaminant exposures, risks associated with lactational transfer of toxicants to the infant must be considered when judging the benefits of prolonged breast-feeding.