The cohort of 5250 infants consisted of 850 (16%) unimpaired, 1153 (22%) mildly impaired, and 1147 (22%) with NDI at 18–22 months CA and 2100 (40%) deaths. The maternal characteristics are shown in . Mothers of unimpaired, mildly impaired and NDI infants were more likely to have received antenatal steroids and be delivered by C-section than mothers of infants who died. Mothers of unimpaired and mildly impaired infants were more likely to have a higher level of education and private insurance than mothers of infants with NDI. In addition, mothers of unimpaired infants were more likely to be married and Caucasian than mothers of infants with NDI.
Maternal Characteristics of Infants with Known Outcomes at 18–22 months CA
Infant characteristics and morbidities are shown in . Both unimpaired and mildly impaired infants had greater birth weight and head circumference, were more likely to be female and have lower rates of the common neonatal morbidities compared to the NDI and died groups. Compared to infants with NDI, unimpaired infants also had greater birth length and higher gestational age, and both unimpaired and mildly impaired infants had fewer days to regain birth weight, less ventilation, and shorter hospitalization.
Infant Characteristics and Morbidities
Child outcomes are shown in . Based on our study design the unimpaired group had normal outcomes. The Bayley scores for the unimpaired group ranged from average to superior; 4% had MDI scores >115 and 5% had PDI scores > 115. Approximately 1% of children in the mild group had Bayley scores in the above-average range. Within the NDI group 5.6% had an MDI in the average range and 16.6% had a PDI in the average range. Moderate to severe CP was present in 18.4% of the NDI group although almost half were walking independently, 81% had normal swallowing and 72% were feeding independently. The number of impairments ranged from 1 to greater than 3. Of the mildly impaired infants, 53% were classified based on a single factor (10% PDI< 85, 33% MDI <85, 10% mild motor problems, and <1% other), 32% for 2 factors, and 15% for 3 or more factors. Of the infants with NDI, 56% were classified on a single factor (11% PDI<70, 41% for an MDI <70, 4% other) 28% for 2 factors, and 16% for 3 or more factors. The mild group was doing well functionally with 85% as independent walkers, 96% with normal swallowing and 92% feeding independently. The unimpaired and mild groups had better growth and were less likely to have growth parameters <10th % compared to the NDI group. The unimpaired group was least likely to have a head circumference <10% or symmetric growth restriction.
Child Outcomes at 18–22 Months CA
Results of the logistic regression models with clustered, forward stepwise regression are shown in . All variables entered into the regression models are shown. Variables retained in the final model are shown in bold; variables dropped from the model prior to consideration of the next cluster had a p>0.1. In model one the outcome modeled was survival to 18 months (versus death) for 5250 infants. The total R-square of 0.4175 indicates that a total of 41.8% of the variance was explained by the final model. The largest contributor to the model was the cluster of infant factors present at birth (female, birth weight, singleton, and SGA) which contributed 37% of the variance. Maternal social and demographic factors provided a minor contribution of 0.54% of the variance above and beyond the factors present at birth. Maternal interventions studied contributed an additional 4.2% of the variance.
Forward Stepwise Regression Models
Model 2 shows the regression which modeled the three level outcomes at 18 months of unimpaired survival, mildly impaired survival, and impaired survival or death, for the total birth cohort of 5250 infants. The same clusters of variables were entered as model 1. The largest contributor to the model was again the cluster of infant factors present at birth which contributed 25.3% of the variance. Maternal social and demographic factors provided a minor contribution of 1.9% of the variance; however this was almost 4 times greater than in the model for death. Only maternal environmental factors differed in effects between the unimpaired or mildly impaired survival versus NDI or death. White race and married differentiated only unimpaired survival from NDI or death whereas maternal age ≥ 20 years differentiated only mildly impaired survival from NDI or death. The model explained 29% of the variance.
Model three shows the regression that modeled the three level outcome of unimpaired survival, mildly impaired survival, versus NDI or death at 18 months for the subset of 3232 infants alive at discharge for whom death or developmental outcomes are known. A neonatal morbidities cluster and neonatal intervention cluster were added. Infant factors present at birth contributed 8.5% of the variance compared to 37% in Model 1. Maternal social and demographic factors provided an increased contribution of 3.6% of the variance which was substantially greater than in the model for death. Major neonatal morbidities and interventions contributed an additional 5.6% and 6.6% of the variance, respectively. Maternal pregnancy related interventions were no longer significant. Whereas the majority of differences were significant for both unimpaired and mild impairment versus NDI or death, white race and SGA differentiated only unimpaired survival versus NDI or death. The final model explained a total of 24.2% of the variance.
and show the distribution of outcomes of death, unimpaired, mild, and NDI status in increments of 100 grams birth weight and 1 week of gestation for the total birth cohort of 6090 infants and includes incomplete follow-up and lost to follow-up. There is a steady decrease in the death rate from 85.2% for infants ≤ 500 grams to 9.9% for infants 901–1000 grams and a steady increase in the percent unimpaired from 0.9 % (5 infants) at ≤ 500 grams to 24.3% (279 infants) at 901–1000 grams (13.9% for total cohort). The death rate decreases from 82.4% at ≤ 23 weeks to 17.3% at 26 weeks whereas the unimpaired rate increases from 1.2% at ≤ 23 weeks to 17.2% at 26 weeks and continues in the range of 20 to 25% at higher gestations. The NDI rate among the 6090 infants is 14%.
Outcomes in 100 gram increments for 6090 ELBW infants.
Outcomes in 1 week of gestation increments for 6090 ELBW infants.