Disaster-related Prenatal Maternal Stress Research Articles

Atypical brain structure and function in young adults exposed to disaster-related prenatal maternal stress: Project Ice Storm.

Studies have shown that prenatal maternal stress (PNMS) affects brain structure and function in childhood. However, less research has examined whether PNMS effects on brain structure and function extend to young adulthood. We recruited women who were pregnant during or within 3 months following the 1998 Quebec ice storm, assessed their PNMS, and prospectively followed-up their children. T1-weighted magnetic resonance imaging (MRI) and resting-state functional MRI were obtained from 19-year-old young adults with (n = 39) and without (n = 65) prenatal exposure to the ice storm. We examined between-group differences in gray matter volume (GMV), surface area (SA), and cortical thickness (CT). We used the brain regions showing between-group GMV differences as seeds to compare between-group functional connectivity. Within the Ice Storm group, we examined (1) associations between PNMS and the atypical GMV, SA, CT, and functional connectivity, and (2) moderation by timing of exposure. Primarily, we found that, compared to Controls, the Ice Storm youth had larger GMV and higher functional connectivity of the anterior cingulate cortex, the precuneus, the left occipital pole, and the right hippocampus; they also had larger CT, but not SA, of the left occipital pole. Within the Ice Storm group, maternal subjective distress during preconception and mid-to-late pregnancy was associated with atypical left occipital pole CT. These results suggest the long-lasting impact of disaster-related PNMS on child brain structure and functional connectivity. Our study also indicates timing-specific effects of the subjective aspect of PNMS on occipital thickness.

Neural correlates of disaster-related prenatal maternal stress in young adults from Project Ice Storm: Focus on amygdala, hippocampus, and prefrontal cortex.

Studies have shown that prenatal maternal stress alters volumes of the amygdala and hippocampus, and alters functional connectivity between the amygdala and prefrontal cortex. However, it remains unclear whether prenatal maternal stress (PNMS) affects volumes and functional connectivity of these structures at their subdivision levels. T1-weighted MRI and resting-state functional MRI were obtained from 19-year-old young adult offspring with (n = 39, 18 male) and without (n = 65, 30 male) exposure to PNMS deriving from the 1998 ice storm. Volumes of amygdala nuclei, hippocampal subfields and prefrontal subregions were computed, and seed-to-seed functional connectivity analyses were conducted. Compared to controls, young adult offspring exposed to disaster-related PNMS had larger volumes of bilateral whole amygdala, driven by the lateral, basal, central, medial, cortical, accessory basal nuclei, and corticoamygdaloid transition; larger volumes of bilateral whole hippocampus, driven by the CA1, HATA, molecular layer, fissure, tail, CA3, CA4, and DG; and larger volume of the prefrontal cortex, driven by the left superior frontal. Inversely, young adult offspring exposed to disaster-related PNMS had lower functional connectivity between the whole amygdala and the prefrontal cortex (driven by bilateral frontal poles, the left superior frontal and left caudal middle frontal); and lower functional connectivity between the hippocampal tail and the prefrontal cortex (driven by the left lateral orbitofrontal). These results suggest the possibility that effects of disaster-related PNMS on structure and function of subdivisions of offspring amygdala, hippocampus and prefrontal cortex could persist into young adulthood.

20.1 Maternal Mental Health and Infant Temperament After Exposure to Hurricane Maria

The study aims to identify the exposure to disaster-related prenatal maternal stress (PNMS) experienced by pregnant women in Puerto Rico (PR) during and after Hurricane Maria and to evaluate their relationship with child temperament.

The impact of prenatal maternal stress due to potentially traumatic events on child temperament: A systematic review.

The objective of the current study was to complete a systematic review of the relationship between prenatal maternal stress due to potentially traumatic events (PTEs) and child temperament. Eligible studies through June 2020 were identified utilizing a search strategy in PubMed and PsycInfo. Included studies examined associations between prenatal maternal stress due to PTE and child temperament. Two independent coders extracted study characteristics and three coders assessed study quality. Of the 1969 identified studies, 20 met full inclusion criteria. Studies were classified on two dimensions: (1) disaster-related stress and (2) intimate partner violence during pregnancy. For disaster-related prenatal maternal stress, 75% (nine out of 12) of published reports found associations with increased child negative affectivity, 50% (five out of 10) also noted associations with lower effortful control/regulation, and 38% (three out of eight) found associations with lower positive affectivity. When considering prenatal intimate partner violence stress, 80% (four out of five) of published reports found associations with higher child negative affectivity, 67% (four out of six) found associations with lower effortful control/regulation, and 33% (one out of three) found associations with lower positive affectivity. Prenatal maternal stress due to PTEs may impact the offspring's temperament, especially negative affectivity. Mitigating the effects of maternal stress in pregnancy is needed in order to prevent adverse outcomes on the infant's socioemotional development.

Effect of Natural Disaster-Related Prenatal Maternal Stress on Child Development and Health: A Meta-Analytic Review

The evidence supporting the idea that natural disaster-related prenatal maternal stress (PNMS) influences the child’s development has been accumulating for several years. We conducted a meta-analytical review to quantify this effect on different spheres of child development: birth outcomes, cognitive, motor, physical, socio-emotional, and behavioral development. We systematically searched the literature for articles on this topic (2756 articles retrieved and 37 articles included in the systematic review), extracted the relevant data to calculate the effect sizes, and then performed a meta-analysis for each category of outcomes (30 articles included across the meta-analyses) and meta-regressions to determine the effect of some factors of interest on the association between PNMS and child development: type of PNMS (objective, psychological, cognitive, diet), type of natural disaster (ice storm, flood/cyclone), type of report (maternal, third-party observer, medical), timing of exposure (preconception exposure included or not) and child age at assessment (under 10 or 10 years and older). We found that PNMS significantly influences all spheres of child development. Higher PNMS levels were associated with longer gestational age, larger newborns, and higher BMI and adiposity levels, as well as worse cognitive, motor, socio-emotional, and behavioral outcomes.

Childhood Anxiety: Prenatal Maternal Stress and Parenting in the QF2011 Cohort.

In this study we examine whether specific 'anxiety-maintaining' parenting behaviors (i.e., overinvolvement and/or negativity) exacerbate the effects of disaster-related prenatal maternal stress (PNMS) on school-age anxiety symptoms. Women (N = 230), pregnant at the time of the 2011 Queensland Floods, reported on their experience of flood-related PNMS (objective hardship, cognitive appraisal, subjective distress). At 4-years, mother-child dyads were coded for maternal overinvolvement and negativity during a challenging task; at 6-years mothers reported on their children's anxiety symptoms and their own mood, N = 83. Results showed no associations between PNMS and 6-year anxiety, nor did parenting moderate these effects. Poorer maternal concurrent mood was associated with greater anxiety symptoms at 6years (β = 0.52). Findings suggest maternal concurrent mood, but not exposure to disaster-related PNMS nor 'anxiety-maintaining' parenting behaviors at preschool age, is related to school-age anxiety symptoms.

Disaster-related prenatal maternal stress, and childhood HPA-axis regulation and anxiety: The QF2011 Queensland Flood Study

BackgroundThe fetal programming hypothesis suggests that prenatal maternal stress (PNMS) influences aspects of fetal development, such as the Hypothalamic Pituitary Adrenal (HPA) axis, enhancing susceptibility to emotional problems. No study (to our knowledge) has investigated this pathway considering development of preschool anxiety symptoms.Using data from the Queensland Flood study (QF2011), our objective was to determine whether toddler HPA-axis functioning mediated the association between aspects of flood-related PNMS and child anxiety symptoms at 4-years, and whether relationships were moderated by the timing of the stressor in utero or by the child’s sex. MethodsWomen, pregnant during the 2011 Queensland floods (N = 230), were recruited soon afterwards and completed questionnaires regarding their objective hardship (e.g., loss of personal property), subjective distress (post-traumatic-like symptoms) and cognitive appraisal of the disaster. At 16 months, indexes of the child’s diurnal cortisol rhythm (awakening response, total daily output, diurnal slope [N = 80]), and stress reactivity (N = 111), were obtained. At 4-years, N = 117 mothers reported on their own mood and their children’s anxiety symptoms; of these, N = 80 also had valid child cortisol reactivity data, and N = 64 had diurnal cortisol rhythm data. ResultsA greater cortisol awakening response at 16 months mediated the relationship between subjective PNMS and anxiety symptoms at 4-years. Greater toddler daily cortisol secretion predicted more anxiety symptoms, independent of PNMS. The laboratory stressor did not elicit a cortisol response. PNMS effects were not dependent upon child sex nor on gestational timing of flood exposure. ConclusionsIndexes of diurnal cortisol in toddlerhood may represent vulnerability for anxiety symptoms in preschoolers, both independent of, and following, exposure to disaster-related prenatal maternal subjective distress.

The Children of Superstorm Sandy: Sex Moderates Link Between Electrodermal Reactivity and Prenatal Storm Exposure

The observed increase in frequency and magnitude of natural disasters around the world mandates careful understanding of their impact on vulnerable populations. Research has shown links between natural disaster-related prenatal maternal stress (PNMS) and offspring deficits in emotion regulation and stress responsivity. However, the role of sex differences in these outcomes remains poorly understood.

Disaster-related prenatal maternal stress predicts HPA reactivity and psychopathology in adolescent offspring: Project Ice Storm

BackgroundPrenatal stress has been associated with adverse outcomes in offspring, including elevated risk of psychopathology. Fetal programming of the hypothalamic-pituitary-adrenal (HPA) axis has been posited as a biological mechanism underlying such consequences. The present study aimed to examine whether dysregulation of the offspring HPA axis mediates the relationship between prenatal stress exposure and adolescent psychopathology. MethodsFive months after the Quebec ice storm of 1998, women who had been pregnant at the time of the storm completed questionnaires about their objective hardship and subjective distress from the disaster. A total of 45 of their children, exposed to the ice storm in utero, participated at 13 years of age. Adolescents completed the Trier Social Stress Test while providing salivary samples to measure circulating cortisol levels. Maternal report of adolescent behaviors was assessed with the Child Behavior Checklist. ResultsResults from the study found that greater objective hardship was associated with elevated offspring cortisol reactivity at 13 years of age. Furthermore, greater subjective distress was associated with greater externalizing behaviors. While lower cortisol reactivity predicted greater externalizing behaviors, it did not mediate the association between maternal objective hardship or subjective distress and offspring externalizing or internalizing behaviors. ConclusionsFindings suggest that objective hardship in pregnancy has long-term implications for offspring HPA axis functioning, which is also associated with externalizing behaviors. While dysregulation of the offspring HPA axis did not mediate the association between prenatal stress and offspring psychopathological symptoms, further research is warranted to investigate programming of alternative biological systems.

Toddler Temperament Mediates the Effect of Prenatal Maternal Stress on Childhood Anxiety Symptomatology: The QF2011 Queensland Flood Study.

It is not known whether alterations to temperamental characteristics associated with prenatal maternal stress (PNMS) exposure account for the development of childhood anxiety symptomatology (internalizing behaviors and anxiety symptoms). The QF2011 Queensland flood study examined whether (1) toddler temperamental characteristics explained the association between PNMS exposure and childhood anxiety symptomatology; and (2) whether effects were dependent upon child sex or the timing of gestational exposure to PNMS. We investigated the effects of various aspects of flood-related stress in pregnancy (objective hardship, cognitive appraisal, subjective distress) on maternal report of 16-month toddler temperament (attentional control, shy-inhibition, negative reactivity), 4-year maternal-reported childhood anxiety symptomatology (internalizing and anxiety symptoms; N = 104), and teacher reports of internalizing behaviors (N = 77). Severity of maternal objective hardship during pregnancy and shy-inhibited behaviors were uniquely associated with 4-year child anxiety symptoms. Mediation analyses found that higher levels of 16-month negative reactivity accounted, in part, for the relationship between increased maternal objective flood-related hardship and greater internalizing behaviors (maternal but not teacher report). Neither child sex nor gestational timing of exposure moderated the hypothesized mediations. Our findings highlight several pathways through which varying aspects of disaster-related PNMS may influence early childhood anxiety symptomatology.

A cross-lagged panel analysis of children's sleep, attention, and mood in a prenatally stressed cohort: The QF2011 Queensland flood study

BackgroundIt is well recognized that childhood sleep, attention and mood problems increase risk for multiple adverse outcomes across the life-span; therefore, understanding factors, such as prenatal maternal stress, that underlie these types of childhood problems is critical for developing interventions that may optimize longer-term functioning. Our goal was to determine the association between disaster-related stress in pregnancy and young children's sleep, attention, and anxious/depressed symptoms. MethodsSoon after a major flood in Australia in 2011, we assessed various aspects of disaster-related prenatal maternal stress (PNMS) in women who had been pregnant at the time. Mothers rated several domains of their children's development with the Child Behavior Checklist (CBCL) at ages 2½ (n = 134) and 4 years (n = 118). ResultsThe primary finding was that more severe objective flood-related hardship in pregnancy predicted higher sleep problem scores at 2½ years, and that a negative maternal cognitive appraisal of the flood predicted lower attention problem scores at 2½ years. A cross-lagged panel analysis examined the association between children's sleep, attention, and anxious/depressed symptoms within and across ages. Results showed that these problems were likely to co-occur at each age, and that they were stable from 2½ to 4 years. Additionally, anxious/depressed scores at age 2½ predicted sleep problem scores at 4 years, all else being equal. LimitationsLimitations of the study include a relatively small sample size and the children's outcome data relied on maternal report using the CBCL, rather than independent observation of the children's functioning, which may have introduced reporter bias. ConclusionsThese findings highlight the importance of early intervention for these childhood problems to optimize long-term mental health, particularly under conditions of prenatal stress.

Larger Amygdala Volume Mediates the Association Between Prenatal Maternal Stress and Higher Levels of Externalizing Behaviors: Sex Specific Effects in Project Ice Storm.

Introduction: The amygdala is a brain structure involved in emotional regulation. Studies have shown that larger amygdala volumes are associated with behavioral disorders. Prenatal maternal depression is associated with structural changes in the amygdala, which in turn, is predictive of an increase in behavioral problems. Girls may be particularly vulnerable. However, it is not known whether disaster-related prenatal maternal stress (PNMS), or which aspect of the maternal stress experience (i.e., objective hardship, subjective distress, and cognitive appraisal), influences amygdala volumes. Nor is it known whether amygdala volumes mediate the effect of PNMS on behavioral problems in girls and boys.Aims: To assess whether aspects of PNMS are associated with amygdala volume, to determine whether timing of exposure moderates the effect, and to test whether amygdala volume mediates the association between PNMS and internalizing and externalizing problems in 11½ year old children exposed in utero, to varying levels of disaster-related PNMS.Methods: Bilateral amygdala volumes (AGV) and total brain volume (TBV) were acquired using magnetic resonance imaging, from 35 boys and 33 girls whose mothers were pregnant during the January 1998 Quebec Ice Storm. The mothers' disaster-related stress was assessed in June 1998. Child internalizing and externalizing problems were assessed at 11½ years using the Child Behavior Checklist (CBCL). Hierarchical regression analyses and mediation analyses were conducted on boys and girls separately, controlling for perinatal and postnatal factors.Results: In boys, subjective distress was associated with larger right AGV/TBV when mothers where exposed during late pregnancy, which in turn explained higher levels of externalizing behavior. However, when adjusting for postnatal factors, the effect was no longer significant. In girls, later gestational exposure to the ice storm was associated with larger AGV/TBV, but here, higher levels of objective PNMS were associated with more externalizing problems, which was, in part, mediated by larger AGV/TBV. No effects were detected on internalizing behaviors.Conclusion: These results suggest that the effects of PNMS on amygdala development and externalizing symptoms, as assessed in boys and girls in early adolescence, can be influenced by the timing of the stress in pregnancy, and the particular aspect of the mother's stress experience.

3288 Prenatal maternal exposure to disaster-related stress and effortful control in early childhood

OBJECTIVES/SPECIFIC AIMS: Our overall objectives are to determine (i) whether natural disaster-related prenatal maternal stress (PNMS) alters infants’ effortful control (EC) at two years of age, and (ii) if the timing of exposure moderates its effects on toddlers EC. METHODS/STUDY POPULATION: We propose a longitudinal study with 50 mother-toddler dyads. Natural disaster-related PNMS would be measured at 12-15 and 18-24 months of age and will include: objective exposure and maternal distress. EC will be measured with a questionnaire and a Laboratory Temperament Assessment Battery at two years of age. RESULTS/ANTICIPATED RESULTS: We anticipate that children exposed to Hurricane-related PNMS would present low EC levels compared to those with low prenatal exposure. DISCUSSION/SIGNIFICANCE OF IMPACT: These results are expected to provide evidence for further promoting early intervention and ameliorating negative effects of PNMS on child outcomes.

The 5-HTTLPR polymorphism of the serotonin transporter gene and child's sex moderate the relationship between disaster-related prenatal maternal stress and autism spectrum disorder traits: The QF2011 Queensland flood study.

The 5-HTTLPR polymorphism of the serotonin transporter has been shown to play a role in autism spectrum disorders (ASD). Moreover, disaster-related prenatal maternal stress (PNMS) has also been shown to be associated with ASD. However, no study to date has examined whether these two factors, either individually or in combination, are predictive of ASD traits in the same sample. We hypothesized that children, particularly boys, with the LL genotype exposed to high levels of disaster-related PNMS would exhibit higher levels of ASD traits compared to boys with the LS or SS genotypes and girls regardless of genotype. Genotype and ASD levels obtained using the Australian normed Autism Spectrum Rating Scales - Short Form were available for 105 30-month-old children exposed to varying levels of PNMS following the 2011 Queensland Flood. For boys, higher ASD traits were associated with the 5-HTTLPR LL genotype in combination with either a negative maternal appraisal of the flood, or high levels of maternal composite subjective stress, PSTD-like or peritraumatic dissociation symptoms. For girls, maternal peritraumatic dissociation levels in combination with the 5-HTTLPR LS or SS genotype were associated with higher ASD traits. The present findings are the first to demonstrate that children's genotype moderates effects of disaster-related PNMS on ASD traits, with different pattern according to child sex.

The role of prenatal maternal stress in the development of childhood anxiety symptomatology: The QF2011 Queensland Flood Study.

It is possible that findings suggesting a link between prenatal maternal stress (PNMS) and anxiety symptoms in offspring are confounded by postnatal and/or shared mother-child heritability effects. Following exposure to a natural disaster, the Queensland Flood Study investigated the unique and additive effects of various types of disaster-related PNMS (objective hardship, cognitive appraisal, and subjective distress) on childhood anxiety symptomatology (internalizing and/or anxiety symptom measures). Timing of flood exposure during pregnancy and child sex were examined as potential moderators. After controlling for maternal psychosocial factors, greater objective hardship as a result of the floods was significantly associated with greater anxiety symptoms (N = 114) and marginally associated with greater internalizing behaviors (N = 115). Earlier timing of the flood in pregnancy was associated with greater anxiety symptoms. No such associations were found between any PNMS measure and teacher-rated child internalizing behaviors (N = 90). Sex and timing did not moderate associations. Our findings suggest that, in isolation, increased maternal hardship due to exposure to an independent stressor, during pregnancy, may have a programming effect on childhood anxiety symptoms.

Natural disaster-related prenatal maternal stress is associated with alterations in placental glucocorticoid system: The QF2011 Queensland Flood Study

We investigated the effects of a natural disaster (a sudden flood) as a source of prenatal maternal stress (PNMS) on the placental glucocorticoid system and glucose transporters. Whether the gestational age at the time of the flood moderated these effects was also evaluated. Placental samples were collected from participants in the 2011 Queensland Flood Study (QF2011) who were pregnant in the first or second trimester at the onset of the flood. Detailed questionnaire results for objective hardship and composite subjective distress were obtained to assess stress levels. Subjective distress was significantly associated with a reduction in placental NR3C1-β mRNA levels for males only (β = −0.491, p = 0.005). In female placentas, objective hardship was marginally linked with lower SLC2A1 mRNA levels while subjective distress was a marginally significant predictor of higher placental SLC2A4 mRNA levels. Gestational age at the time of the flood was a significant moderator of the effect of subjective distress on placental mRNA levels for NR3C1-α (p = 0.046) and HSD11B1 (p = 0.049) in male placentas: if the flood occurred in mid-pregnancy, lower subjective distress predicted higher HSD11B1 while higher subjective distress predicted lower NR3C1-α placental mRNA level. While results did not show any PNMS effects on placental HSD11B2 mRNA and protein levels, and activity, we showed a reduction in placental NR3C1-β mRNA level in male placentas. Our results show evidence of distinct placental glucocorticoid and glucose systems adaptations to PNMS as a function of fetal sex and gestational timing of exposure, with high subjective PNMS in mid-pregnancy associated with lower levels of expression of glucocorticoid-promoting gene in males, leaving the fetus less protected against maternal stress. The exact mechanism by which natural disaster-related PNMS acts on the placenta and the impact on fetal programming requires further investigation.

Prenatal maternal stress is associated with toddler cognitive functioning: The Iowa Flood Study

• Disaster-related prenatal maternal stress was related to lower cognitive abilities in 30-month-old toddlers. • Disaster-related prenatal maternal stress was associated with better language abilities in 30-month-old boys. • While the mechanisms remain unknown, disaster-related prenatal maternal stress impacts early cognitive development in toddlers.

The Effects of Prenatal Maternal Stress on Early Temperament: The 2011 Queensland Flood Study.

This study examined the effects of disaster-related prenatal maternal stress on infant temperament and whether the sex of the infant or the timing of the stressor in pregnancy would moderate the effects. Mothers' objective experiences of a sudden-onset flood in Queensland, Australia, their subjective emotional reactions, and cognitive appraisal of the event were assessed. At 6 months postpartum, 121 mothers reported their infant's temperament on the 5 dimensions of the Short Temperament Scale for Infants. When controlling for postnatal maternal factors, subjective prenatal maternal stress and cognitive appraisal of the disaster were associated with easier aspects of infant temperament. However, several interesting interactions emerged showing negative effects of the flood. With higher levels of objective hardship in pregnancy, boys (but not girls) received more irritable temperament ratings. When the flood occurred early in pregnancy, higher levels of objective hardship predicted more arrhythmic infant temperament. Finally, mothers whose emotional response to the flood exceeded the hardship they endured reported significantly more active-reactive infants. Prenatal maternal stress from a natural disaster predicted more difficult temperament ratings that were moderated by infant sex, timing of the flood in gestation, and mother's emotional response to the disaster.

A potential psychological mechanism linking disaster-related prenatal maternal stress with child cognitive and motor development at 16 months: The QF2011 Queensland Flood Study.

Fetal exposure to prenatal maternal stress can have lifelong consequences, with different types of maternal stress associated with different areas of child development. Fewer studies have focused on motor skills, even though they are strongly predictive of later development across a range of domains. Research on mechanisms of transmission has identified biological cascades of stress reactions, yet links between psychological stress reactions are rarely studied. This study investigates the relationship between different aspects of disaster-related prenatal maternal stress and child cognitive and motor development, and proposes a cascade of stress reactions as a potential mechanism of transmission. Mothers in the Queensland Flood Study (QF2011) exposed to a major flood during pregnancy completed questionnaires assessing flood exposure, symptoms of peritraumatic distress, dissociation, and posttraumatic stress (PTSD), and cognitive appraisal of the overall flood consequences. At 16 months post-partum, children's (N = 145) cognitive and motor development was assessed using the Bayley-III. Flood exposure predicted child cognitive development and maternal PTSD symptoms and negative cognitive appraisal were significantly negatively related to child motor development, with all relationships moderated by timing of exposure. Together, a cascade of stress reactions linked maternal flood exposure to poorer fine motor development. These findings suggest that the way stress reactions operate together is as important as the way they operate in isolation, and identifies a potential psychological mechanism of transmission for the effects of prenatal stress. Results have implications for conceptualizing prenatal stress research and optimizing child development in the wake of natural disasters. (PsycINFO Database Record

Prenatal maternal stress shapes children's theory of mind: the QF2011 Queensland Flood Study.

Research shows that stress in pregnancy has powerful and enduring effects on many facets of child development, including increases in behavior problems and neurodevelopmental disorders. Theory of mind is an important aspect of child development that is predictive of successful social functioning and is impaired in children with autism. A number of factors related to individual differences in theory of mind have been identified, but whether theory of mind development is shaped by prenatal events has not yet been examined. In this study we utilized a sudden onset flood that occurred in Queensland, Australia in 2011 to examine whether disaster-related prenatal maternal stress predicts child theory of mind and whether sex of the child or timing of the stressor in pregnancy moderates these effects. Higher levels of flood-related maternal subjective stress, but not objective hardship, predicted worse theory of mind at 30 months (n=130). Further, maternal cognitive appraisal of the flood moderated the effects of stress in pregnancy on girls' theory of mind performance but not boys'. These results illuminate how stress in pregnancy can shape child development and the findings are discussed in relation to biological mechanisms in pregnancy and stress theory.