The unconditional growth model was significant indicating that mental (t = 105.2, p < 0.001) and psychomotor (t = 108.3, p < 0.001) performance (raw scores) improved across the first postnatal year (see ). As expected, scaled scores calculated with reference to developmental norms, did not increase (p > 0.03). shows the correlation between mental and psychomotor development scaled scores across the three assessment time points. As illustrated, modest stability is seen in BSID scores with a stronger association among temporally adjacent measures.
Intercorrelations among MDI and PDI scaled scores across the first postnatal year
The unconditional growth model was significant indicating that, as expected, salivary cortisol levels increased across gestation (t = 13.6, p < 0.001). Salivary cortisol levels increased from 0.27 at 15 gestational weeks to 0.61 at 37 gestational weeks. shows the correlation between cortisol measures across pregnancy. Moderate correlations were seen between temporally adjacent assessments.
Intercorrelations among prenatal maternal cortisol samples
Maternal Psychological State
Maternal perceived stress (means ranged from 20.1 to 21.6), depression (means ranged from 13.7 to 15.1) and state anxiety (means ranged from 17.7 to 18.6) did not significantly change over the course of pregnancy (p's ranged from 0.06 to 0.30). Pregnancy specific anxiety changed significantly across gestation (t = 3.4, p < 0.01). Levels were highest early in pregnancy and decreased through mid gestation. Mean pregnancy specific anxiety scores ranged from 17.3 to 18.7.
Each of the four psychosocial measures was stable over pregnancy. Within each measure intercorrelations ranged from 0.51 to 0.75 during pregnancy. Not only were scores on the psychosocial measures stable during the prenatal period, but as expected, concurrent measures of state anxiety, perceived stress and depression were intercorrelated (average r's = 0.66, p's < .001). The associations between these measures and pregnancy specific anxiety were lower, but also significant (average r's = 0.41, p's < .001).
During the postnatal period, levels of state anxiety (means ranged from 16.8 to17.5), perceived stress (means ranged from 20.1 to 21.1) and depression (means ranged from 12.8 to 12.9) were relatively stable (r's ranged from 0.45 to 0.68) and concurrent measures were intercorrelated (average r's = 0.67). Mean levels of parenting stress ranged from 64.2 to 66.6 and parenting stress was relatively stable over the first postnatal year (r's ranged from 0.48 to 0.67).
Prenatal Maternal Cortisol and Maternal Psychological state
Data indicate that maternal cortisol was not significantly associated with maternal anxiety, depression or pregnancy specific anxiety at any of the five prenatal assessment time points (all r's < 0.18). Prenatal maternal perceived stress was not associated with maternal cortisol at any of the prenatal assessment time points (all r's < 0.13) with the exception of the 36+ weeks’ GA assessment. At this time point maternal cortisol was negatively associated with perceived stress levels [r(119) = −.21, p < .05].
Maternal Prenatal Stress Measures and Trajectories of Infant Development
We first examined whether the rate of infant development over the first postnatal year was associated with prenatal cortisol at each assessment interval, 15 weeks, 19 weeks, 25 weeks, 31 weeks and 37 weeks. For these analyses raw scores (rather than scaled scores) were used so that developmental changes could be evaluated. Data indicate that the level of maternal cortisol both early and late in gestation predicted infant mental development. Lower maternal cortisol at the first prenatal assessment (15 weeks) predicted accelerated infant mental development (steeper slope of raw scores) across the first postnatal year (t = −2.5, p < 0.01) resulting in enhanced cognitive functioning at 12 months of age (t = 2.4, p < 0.05). Furthermore, higher maternal cortisol late in gestation (37 weeks) additionally predicted accelerated infant development (a steeper slope of raw MDI scores) over the first postnatal year (t = 1.9, p < 0.05) resulting in a tendency for higher mental development raw scores at 12 months (t = 1.6, p = 0.10). Maternal prenatal cortisol at 19, 25 and 31 week assessments did not predict the trajectory of infant development assessed using mental development raw scores. No measure of maternal cortisol at anty time period predicted psychomotor development raw scores. None of the measures of maternal psychological distress (perceived stress, state anxiety, pregnancy specific anxiety and depression) were significantly associated with infant development using raw mental or psychomotor scores. These data suggest that the pattern of maternal cortisol during gestation predicts the trajectory of infant mental development over the first postnatal year resulting in significant differences in performance only at 12 months.
Patterns of Maternal Cortisol Associated with Infant Development
A second set of analyses were performed to identify the pattern of maternal cortisol that predicted optimal performance at 12 months. For these analyses the scaled scores were used to allow for comparison to developmental norms. These models offer the additional advantage of assessing the slope or trajectory of maternal cortisol across gestation. As in the previous analyses, cortisol and BSID scores were modeled as continuous variables. As shown in , infants scoring higher on the 12 month MDI were exposed to (i) lower maternal cortisol before 18 weeks gestational age (GA) (t's ranged from −2.1 to −2.7, p's < 0.05), (ii) an accelerated increase in maternal cortisol across gestation (t = 3.7, p < 0.001), and (iii) higher maternal cortisol after 30 weeks GA (t's ranged from 2.0 to 2.9, p's < 0.05). Interestingly, this profile of maternal cortisol additionally predicted whether or not 12 month old infants received a score that would be categorized as delayed using standardized cut offs on the BSID (p's < 0.05). Because the intercepts that were the strongest predictors of a low MDI score were high maternal cortisol at 13 weeks GA and low maternal cortisol at 38 weeks GA, a stepwise regression model was implemented to determine whether early and late cortisol jointly or independently predicted MDI. The stepwise model indicated that, after entering covariates into the model, early and late cortisol independently predicted infant MDI (13 week GA intercept: ΔR2 = 0.05, Beta = −0.23, t = −2.5, p < 0.01; 38 week GA intercept: ΔR2 = 0.03, Beta = 0.17, t = 2.0, p < 0.05). Together early and late maternal cortisol accounted for 8% of the variance in infant cognitive performance, as assessed with the MDI. At 13 weeks of gestation a 0.1 μg/dl increase in cortisol was associated with a 4 point decrease in MDI. At 38 weeks of gestation a 0.1 μg/dl increase in cortisol was associated with a 2 point increase in MDI. PDI was not significantly associated with maternal cortisol levels (p's > 0.3).
Infant performance on the MDI at 12 months of age is associated with the trajectory of maternal cortisol across gestation. The top and bottom SD for MDI are modeled with HLM.
To determine whether there was a profile of prenatal maternal cortisol that predicted infant cognitive development, the slope of maternal cortisol across gestation was calculated for each woman. When the slope of maternal cortisol across pregnancy was added to the model it was demonstrated that cortisol slope was a stronger predictor of infant MDI (ΔR2 = 0.06, Beta = 0.24, t = 2.8, p < 0.01) than either early or late cortisol levels. Furthermore, after cortisol slope was entered into the model the effects of maternal cortisol early and late in pregnancy (intercepts at 13 or 38 weeks GA) were no longer significant (p's > 0.3). These data suggest that it is the profile of maternal cortisol across gestation that best predicts infant development.
Maternal Psychosocial Stress and Infant Development
A second set of HLM models were computed to examine associations between psychosocial indicators of maternal stress and infant development. Among all the measures of maternal distress (perceived stress, state anxiety, pregnancy specific anxiety and depression) only pregnancy specific anxiety was significantly associated with infant development. Infant performance on the MDI was associated with both pregnancy specific anxiety early in gestation and overall rate of change (shape of the curve) of pregnancy specific anxiety across gestation (t = 2.4, p < 0.05). As shown in , infants scoring low on the MDI had mothers with the profile of (i) high pregnancy specific anxiety before 16 weeks GA (t's ranged from −2.6 to −1.9, p's < 0.05) and (ii) a steeper decline in pregnancy specific anxiety through mid gestation (t's for rate of acceleration or instantaneous slope ranged from 2.1 to 2.9, p's < 0.05), such that no differences in level of pregnancy specific anxiety were detected beyond 16 weeks of gestation. Maternal pregnancy specific anxiety did not significantly predict whether infants were categorized as delayed using BSID criteria (p's > 0.17). Thirteen weeks GA was the time point at which level of pregnancy specific anxiety most strongly predicted infant cognitive development. At 13 gestational weeks a 5 point increase in pregnancy specific anxiety was associated with a 2 point decrease on the MDI. A stepwise regression was implemented to determine whether levels of pregnancy specific anxiety at 13 weeks GA or the rate of change of pregnancy specific anxiety over gestation best predicted infant scores on the MDI. After adjusting for covariates, overall rate of change of pregnancy specific anxiety was a stronger predictor of MDI than the 13 week GA intercept (ΔR2 = 0.05, Beta = −0.21, t = −2.4, p < .05) and accounted for the variance associated with the 13 week GA intercept (p = 0.24).
Elevated pregnancy specific anxiety early in gestation is associated with poorer performance on the MDI at 12 months of age. The top and bottom SD for MDI are modeled with HLM.
Influence of Maternal Psychological and Endocrine Factors on Infant Development
A hierarchical regression model was implemented to determine whether psychosocial (pregnancy specific anxiety) and endocrine (cortisol) measures independently or jointly predicted portions of the variance in infant cognitive development. Furthermore, after adjusting for covariates, maternal cortisol measures (slope and intercepts, ΔR2 = 0.07, p < .01) and maternal pregnancy specific anxiety indicators (slope and intercept, ΔR2 = 0.05, p < .05) independently predicted MDI. Together these two measures accounted for 12% of the variance in infant performance on the MDI at 12 months of age.