Fetal Growth Restriction Lambs Research Articles

Cardiovascular decline in offspring during the perinatal period in an ovine model of fetal growth restriction.

Fetal growth restriction (FGR) increases the risk cardiovascular disease (CVD) in adulthood. Placental insufficiency and subsequent chronic fetal hypoxemia are causal factors for FGR, leading to a redistribution of blood flow that prioritizes vital organs. Subclinical signs of cardiovascular dysfunction are evident in growth-restricted neonates; however, the mechanisms programming for CVD in adulthood remain unknown. This study aimed to determine the potential mechanisms underlying structural and functional changes within the heart and essential (carotid) and nonessential (femoral) vascular beds in growth-restricted lambs. Placental insufficiency was surgically induced in ewes at 89 days gestational age (dGA, term = 148dGA). Three age groups were investigated: fetal (126dGA), newborn (24 h after preterm birth), and 4-wk-old lambs. In vivo and histological assessments of cardiovascular indices were undertaken. Resistance femoral artery function was assessed via in vitro wire myography and blockade of key vasoactive pathways including nitric oxide, prostanoids, and endothelium-dependent hyperpolarization. All lambs were normotensive throughout the first 4 wk of life. Overall, the FGR cohort had more globular hearts compared with controls (P = 0.0374). A progressive decline in endothelium-dependent vasodilation was demonstrated in FGR lambs compared with controls. Further investigation revealed that impairment of the prostanoid pathway may drive this reduction in vasodilatory capacity. Clinical indicators of CVD were not observed in our FGR lambs. However, subclinical signs of cardiovascular dysfunction were present in our FGR offspring. This study provides insight into potential mechanisms, such as the prostanoid pathway, that may warrant therapeutic interventions to improve cardiovascular development in growth-restricted newborns.NEW & NOTEWORTHY Our findings provide novel insight into the potential mechanisms that program for cardiovascular dysfunction in growth-restricted neonates as our growth-restricted lambs exhibited a progressive decline in endothelium-dependent vasodilation in the femoral artery between birth and 4 wk of age. Subsequent analyses indicated that this reduction in vasodilatory capacity is likely to be mediated by the prostanoid pathway and prostanoids could be a potential target for therapeutic interventions for fetal growth restriction (FGR).

Cardiovascular responses to mild perinatal asphyxia in growth-restricted preterm lambs.

Growth-restricted neonates have worse outcomes after perinatal asphyxia, with more severe metabolic acidosis than appropriately grown neonates. The cardiovascular physiology associated with fetal growth restriction (FGR) may alter their response to asphyxia. However, research on asphyxia in FGR is limited. Here we compared cardiovascular hemodynamics in preterm FGR and control lambs during mild perinatal asphyxia. We induced FGR in one twin at 89 days gestation (term 148 days), while the other served as a control. At 126 days gestation, lambs were instrumented to allow arterial blood pressure and regional blood flow recording, and then mild perinatal asphyxia was induced by umbilical cord clamping, and resuscitation followed neonatal guidelines. FGR lambs maintained carotid blood flow (CBF) for 7 min, while control lambs rapidly decreased CBF (P < 0.05). Fewer growth-restricted lambs needed chest compressions for return of spontaneous circulation (ROSC) (17 vs. 83%, P = 0.02). The extent of blood pressure overshoot after ROSC was similar, but it took longer for MAP to return to baseline in FGR lambs (18.83 ± 0.00 vs. 47.67 ± 0.00 min, P = 0.003). Growth-restricted lambs had higher CBF after ROSC (P < 0.05) and displayed CBF overshoot, unlike control lambs (P < 0.03). In conclusion, preterm growth-restricted lambs show resilience during perinatal asphyxia based on prolonged CBF maintenance and reduced need for chest compressions during resuscitation. However, CBF overshoot after ROSC may increase the risk of cerebrovascular injury in FGR.NEW & NOTEWORTHY Preterm growth-restricted lambs maintain carotid blood flow for longer than control lambs during asphyxia and have a lower requirement for chest compressions than control lambs during resuscitation. Preterm growth-restricted, but not control, lambs displayed an overshoot in carotid blood flow following return of spontaneous circulation.

Sexual dimorphic gene expression profile of perirenal adipose tissue in ovine fetuses with growth restriction.

Worldwide, fetal growth restriction (FGR) affects 7%-10% of pregnancies, or roughly 20.5 million infants, each year. FGR increases not only neonatal mortality and morbidity but also the risk of obesity in later life. Currently, the molecular mechanisms by which FGR "programs" an obese phenotype are not well understood. Studies demonstrate that FGR females are more prone to obesity compared to males; however, the molecular mechanisms that lead to the sexually dimorphic programming of FGR are not known. Thus, we hypothesized that FGR leads to the sexually dimorphic programming of preadipocytes and reduces their ability to differentiate into mature adipocytes. To test the hypothesis, we utilized a maternal hyperthermia-induced placental insufficiency to restrict fetal growth in sheep. We collected perirenal adipose tissue from near-term (∼140days gestation) male and female FGR and normal-weight fetal lambs (N = 4 to 5 in each group), examined the preadipocytes' differentiation potential, and identified differential mRNA transcript expression in perirenal adipose tissue. Male FGR fetuses have a lower cellular density (nuclei number/unit area) compared to control male fetuses. However, no difference was observed in female FGR fetuses compared to control female fetuses. In addition, the ability of preadipocytes to differentiate into mature adipocytes with fat accumulation was impaired in male FGR fetuses, but this was not observed in female FGR fetuses. Finally, we examined the genes and pathways involved in the sexually dimorphic programming of obesity by FGR. On enrichment of differentially expressed genes in males compared to females, the Thermogenesis KEGG Pathway was downregulated, and the Metabolic and Steroid Biosynthesis KEGG pathways were upregulated. On enrichment of differentially expressed genes in male FGR compared to male control, the Steroid Biosynthesis KEGG Pathway was downregulated, and the PPAR Signaling KEGG pathway was upregulated. No pathways were altered in females in response to growth restriction in perirenal adipose tissue. Thus, the present study demonstrates a sexually dimorphic program in response to growth restriction in sheep fetal perirenal adipose tissue.

Is Umbilical Cord Blood Therapy an Effective Treatment for Early Lung Injury in Growth Restriction?

Fetal growth restriction (FGR) and prematurity are often co-morbidities, and both are risk factors for lung disease. Despite advances in early delivery combined with supportive ventilation, rates of ventilation-induced lung injury (VILI) remain high. There are currently no protective treatments or interventions available that target lung morbidities associated with FGR preterm infants. Stem cell therapy, such as umbilical cord blood (UCB) cell administration, demonstrates an ability to attenuate inflammation and injury associated with VILI in preterm appropriately grown animals. However, no studies have looked at the effects of stem cell therapy in growth restricted newborns. We aimed to determine if UCB treatment could attenuate acute inflammation in the first 24 h of ventilation, comparing effects in lambs born preterm following FGR with those born preterm but appropriately grown (AG). Placental insufficiency (FGR) was induced by single umbilical artery ligation in twin-bearing ewes at 88 days gestation, with twins used as control (appropriately grown, AG). Lambs were delivered preterm at ~126 days gestation (term is 150 days) and randomized to either immediate euthanasia (unventilated controls, AGUVC and FGRUVC) or commenced on 24 h of gentle supportive ventilation (AGV and FGRV) with additional cohorts receiving UCB treatment at 1 h (AGCELLS, FGRCELLS). Lungs were collected at post-mortem for histological and biochemical examination. Ventilation caused lung injury in AG lambs, as indicated by decreased septal crests and elastin density, as well as increased inflammation. Lung injury in AG lambs was attenuated with UCB therapy. Ventilated FGR lambs also sustained lung injury, albeit with different indices compared to AG lambs; in FGR, ventilation reduced septal crest density, reduced alpha smooth muscle actin density and reduced cell proliferation. UCB treatment in ventilated FGR lambs further decreased septal crest density and increased collagen deposition, however, it increased angiogenesis as evidenced by increased vascular endothelial growth factor (VEGF) expression and vessel density. This is the first time that a cell therapy has been investigated in the lungs of growth restricted animals. We show that the uterine environment can alter the response to both secondary stress (ventilation) and therapy (UCB). This study highlights the need for further research on the potential impact of novel therapies on a growth restricted offspring.

Neurovascular effects of umbilical cord blood-derived stem cells in growth-restricted newborn lambs

BackgroundNeonatal ventilation exacerbates brain injury in lambs with fetal growth restriction (FGR), characterized by neuroinflammation and reduced blood-brain barrier integrity, which is normally maintained by the neurovascular unit. We examined whether umbilical cord blood stem cell (UCBC) treatment stabilized the neurovascular unit and reduced brain injury in preterm ventilated FGR lambs.MethodsSurgery was performed in twin-bearing pregnant ewes at 88 days’ gestation to induce FGR in one fetus. At 127 days, FGR and appropriate for gestational age (AGA) lambs were delivered, carotid artery flow probes and umbilical lines inserted, lambs intubated and commenced on gentle ventilation. Allogeneic ovine UCBCs (25 × 106 cells/kg) were administered intravenously to lambs at 1 h of life. Lambs were ventilated for 24 h and then euthanized.ResultsFGR (n = 6) and FGR+UCBC (n = 6) lambs were growth restricted compared to AGA (n = 6) and AGA+UCBC (n = 6) lambs (combined weight, FGR 2.3 ± 0.4 vs. AGA 3.0 ± 0.3 kg; p = 0.0002). UCBC therapy did not alter mean arterial blood pressure or carotid blood flow but decreased cerebrovascular resistance in FGR+UCBC lambs. Circulating TNF-α cytokine levels were lower in FGR+UCBC vs. FGR lambs (p < 0.05). Brain histopathology showed decreased neuroinflammation and oxidative stress, increased endothelial cell proliferation, pericyte stability, and greater integrity of the neurovascular unit in FGR+UCBC vs. FGR lambs.ConclusionsUmbilical cord blood stem cell therapy mitigates perinatal brain injury due to FGR and ventilation, and the neuroprotective benefits may be mediated by stabilization of the neurovascular unit.

Fetal Growth Restriction Alters Cerebellar Development in Fetal and Neonatal Sheep.

Fetal growth restriction (FGR) complicates 5–10% of pregnancies and is associated with increased risks of perinatal morbidity and mortality. The development of cerebellar neuropathology in utero, in response to chronic fetal hypoxia, and over the period of high risk for preterm birth, has not been previously studied. Therefore, the objective of this study was to examine the effects of FGR induced by placental insufficiency on cerebellar development at three timepoints in ovine fetal and neonatal development: (1) 115 days gestational age (d GA), (2) 124 d GA, and (3) 1-day-old postnatal age. We induced FGR via single umbilical artery ligation (SUAL) at ~105 d GA in fetal sheep, term is ~147 d GA. Animals were sacrificed at 115 d GA, 124 d GA, and 1-day-old postnatal age; fetuses and lambs were weighed and the cerebellum collected for histopathology. FGR lambs demonstrated neuropathology within the cerebellum after birth, with a significant, ~18% decrease in the number of granule cell bodies (NeuN+ immunoreactivity) within the internal granular layer (IGL) and an ~80% reduction in neuronal extension and branching (MAP+ immunoreactivity) within the molecular layer (ML). Oxidative stress (8-OHdG+ immunoreactivity) was significantly higher in FGR lambs within the ML and the white matter (WM) compared to control lambs. The structural integrity of neurons was already aberrant in the FGR cerebellum at 115 d GA, and by 124 d GA, inflammatory cells (Iba-1+ immunoreactivity) were significantly upregulated and the blood-brain barrier (BBB) was compromised (Pearls, albumin, and GFAP+ immunoreactivity). We confirm that cerebellar injuries develop antenatally in FGR, and therefore, interventions to prevent long-term motor and coordination deficits should be implemented either antenatally or perinatally, thereby targeting neuroinflammatory and oxidative stress pathways.

Neuropathology as a consequence of neonatal ventilation in premature growth-restricted lambs.

Fetal growth restriction (FGR) and prematurity are associated with high risk of brain injury and long-term neurological deficits. FGR infants born preterm are commonly exposed to mechanical ventilation, but it is not known whether ventilation differentially induces brain pathology in FGR infants compared with appropriate for gestational age (AGA) infants. We investigated markers of neuropathology in moderate- to late-preterm FGR lambs, compared with AGA lambs, delivered by caesarean birth and ventilated under standard neonatal conditions for 24 h. FGR was induced by single umbilical artery ligation in fetal sheep at 88-day gestation (term, 150 days). At 125-day gestation, FGR and AGA lambs were delivered, dried, intubated, and commenced on noninjurious ventilation, with surfactant administration at 10 min. A group of unventilated FGR and AGA lambs at the same gestation was also examined. Over 24 h, circulating pH, Po2, and lactate levels were similar between groups. Ventilated FGR lambs had lower cerebral blood flow compared with AGA lambs ( P = 0.01). The brain of ventilated FGR lambs showed neuropathology compared with unventilated FGR, and unventilated and ventilated AGA lambs, with increased apoptosis (caspase-3), blood-brain barrier dysfunction (albumin extravasation), activated microglia (Iba-1), and increased expression of cellular oxidative stress (4-hydroxynonenal). The neuropathologies seen in the ventilated FGR brain were most pronounced in the periventricular and subcortical white matter but also evident in the subventricular zone, cortical gray matter, and hippocampus. Ventilation of preterm FGR lambs increased brain injury compared with AGA preterm lambs and unventilated FGR lambs, mediated via increased vascular permeability, neuroinflammation and oxidative stress.

Postnatal effects of intrauterine treatment of the growth-restricted ovine fetus with intra-amniotic insulin-like growth factor-1.

Fetal growth restriction increases the risk of fetal and neonatal mortality and morbidity, and contributes to increased risk of chronic disease later in life. Intra-amniotic insulin-like growth factor-1 (IGF1) treatment of the growth-restricted ovine fetus improves fetal growth, but postnatal effects are unknown. Here we report that intra-amniotic IGF1 treatment of the growth-restricted ovine fetus alters size at birth and mechanisms of early postnatal growth in a sex-specific manner. We also show that maternal plasma C-type natriuretic peptide (CNP) products are related to fetal oxygenation and size at birth, and hence may be useful for non-invasive monitoring of fetal growth restriction. Intrauterine IGF1 treatment in late gestation is a potentially clinically relevant intervention that may ameliorate the postnatal complications of fetal growth restriction. Placental insufficiency-mediated fetal growth restriction (FGR) is associated with altered postnatal growth and metabolism, which are, in turn, associated with increased risk of adult disease. Intra-amniotic insulin-like growth factor-1 (IGF1) treatment of ovine FGR increases growth rate in late gestation, but the effects on postnatal growth and metabolism are unknown. We investigated the effects of intra-amniotic IGF1 administration to ovine fetuses with uteroplacental embolisation-induced FGR on phenotypical and physiological characteristics in the 2 weeks after birth. We measured early postnatal growth velocity, amino-terminal propeptide of C-type natriuretic peptide (NTproCNP), body composition, tissue-specific mRNA expression, and milk intake in singleton lambs treated weekly with 360μg intra-amniotic IGF1 (FGRI; n=13 females, 19 males) or saline (FGRS; n=18 females, 12 males) during gestation, and in controls (CON; n=15 females, 22 males). There was a strong positive correlation between maternal NTproCNP and fetal oxygenation, and size at birth in FGR lambs. FGR lambs were ∼20% lighter at birth and demonstrated accelerated postnatal growth velocity. IGF1 treatment did not alter perinatal mortality, partially abrogated the reduction in newborn size in females, but not males, and reduced accelerated growth in both sexes. IGF1-mediated upregulation of somatotrophic genes in males during the early postnatal period could suggest that treatment effects are associated with delayed axis maturation, whilst treatment outcomes in females may rely on the reprogramming of nutrient-dependent mechanisms of growth. These data suggest that the growth-restricted fetus is responsive to intra-amniotic intervention with IGF1, and that sex-specific somatotrophic effects persist in the early postnatal period.

Effects of antenatal melatonin therapy on lung structure in growth-restricted newborn lambs.

Oxidative stress arising from suboptimal placental function contributes to a multitude of pathologies in infants compromised by fetal growth restriction (FGR). FGR infants are at high risk for respiratory dysfunction after birth and poor long-term lung function. Our objective was to investigate the contribution of oxidative stress to adverse lung development and the effects of melatonin administration, a powerful antioxidant, on lung structure in FGR lambs. Placental insufficiency and FGR was surgically induced in 13 fetal sheep at ∼105 days of gestation by ligation of a single umbilical artery. Maternal intravenous melatonin infusion was commenced in seven of the ewes 4 h after surgery and continued until birth. Lambs delivered normally at term and lungs were collected 24 h after birth for histological assessment of lung structure and injury and compared with appropriately grown control lambs (n = 8). FGR fetuses were hypoxic and had lower glucose during gestation compared with controls. Melatonin administration prevented chronic hypoxia. Within the lung, FGR caused reduced secondary septal crest density and altered elastin deposition compared with controls. Melatonin administration had no effect on the changes to lung structure induced by FGR. We conclude that chronic FGR disrupts septation of the developing alveoli, which is not altered by melatonin administration. These findings suggest that oxidative stress is not the mechanism driving altered lung structure in FGR neonates. Melatonin administration did not prevent disrupted airway development but also had no apparent adverse effects on fetal lung development.NEW & NOTEWORTHY Fetal growth restriction (FGR) results in poor respiratory outcomes, which may be caused by oxidation in utero. We investigated the contribution of oxidative stress to adverse lung development and the effects of melatonin administration, a powerful antioxidant, on lung structure in FGR lambs. FGR disrupted septation of the developing alveoli, which is not altered by melatonin administration. Oxidative stress may not be the mechanism driving altered lung structure in FGR neonates.

Does growth restriction increase the vulnerability to acute ventilation-induced brain injury in newborn lambs? Implications for future health and disease.

Fetal growth restriction (FGR) and preterm birth are frequent co-morbidities, both are independent risks for brain injury. However, few studies have examined the mechanisms by which preterm FGR increases the risk of adverse neurological outcomes. We aimed to determine the effects of prematurity and mechanical ventilation (VENT) on the brain of FGR and appropriately grown (AG, control) lambs. We hypothesized that FGR preterm lambs are more vulnerable to ventilation-induced acute brain injury. FGR was surgically induced in fetal sheep (0.7 gestation) by ligation of a single umbilical artery. After 4 weeks, preterm lambs were euthanized at delivery or delivered and ventilated for 2 h before euthanasia. Brains and cerebrospinal fluid (CSF) were collected for analysis of molecular and structural indices of early brain injury. FGRVENT lambs had increased oxidative cell damage and brain injury marker S100B levels compared with all other groups. Mechanical ventilation increased inflammatory marker IL-8 within the brain of FGRVENT and AGVENT lambs. Abnormalities in the neurovascular unit and increased blood-brain barrier permeability were observed in FGRVENT lambs, as well as an altered density of vascular tight junctions markers. FGR and AG preterm lambs have different responses to acute injurious mechanical ventilation, changes which appear to have been developmentally programmed in utero.

Effects of Antenatal Melatonin Treatment on the Cerebral Vasculature in an Ovine Model of Fetal Growth Restriction

Chronic moderate hypoxia, such as occurs in fetal growth restriction (FGR) during gestation, compromises the blood-brain barrier (BBB) and results in structural abnormalities of the cerebral vasculature. We have previously determined the neuroprotective and antioxidant effects of maternal administration of melatonin (MLT) on growth-restricted newborn lambs. The potential of maternal MLT therapy for the treatment of cerebrovascular dysfunction-associated developmental hypoxia has also been demonstrated in newborn lambs. We assessed whether MLT had an effect on the previously reported structural and cerebral vascular abnormalities in chronically hypoxic FGR lambs. Single umbilical-artery ligation surgery was performed in fetuses at approximately 105 days of gestation (term: 147 days) to induce placental insufficiency and FGR, and treatment with either saline or an MLT infusion (0.1 mg/kg) was started 4 h after surgery. Ewes delivered naturally at term and lambs were euthanased 24 h later. We found a significant reduction in the number of laminin-positive blood vessels within the subcortical and periventricular white matter (SCWM and PVWM) and the subventricular zone (SVZ) in FGR (p < 0.0005) and FGR + MLT brains (p < 0.0005 vs. controls), with no difference found between FGR and FGR + MLT animals. This was associated with a significant decrease in VEGF immunoreactivity in FGR and FGR + MLT brains versus controls (p < 0.0005; SCWM and PVWM) and in the SVZ in FGR brains versus controls (p < 0.005) and also with significantly lower levels of proliferating blood vessels versus controls (p < 0.0005). Glucose transporter-1 immunoreactivity (vascular endothelium) was decreased in FGR versus control lambs (p < 0.0005) in SCWM, PVWM, and the SVZ; it was significantly increased in FGR + MLT lambs compared with FGR lambs in SCWM and PVWM (p < 0.005) and even more markedly in the SVZ (p < 0.0005). FGR brains showed a 72% reduction in pericyte coverage versus control lambs and 68% versus FGR + MLT in PVWM. In SCWM, we found a 77 and 73% reduction compared with control and FGR + MLT lambs, respectively, while in the SVZ, we observed a 68% reduction versus controls and a 70% reduction in FGR versus FGR + MLT lambs. Astrocyte end-feet coverage in the SCWM showed a significant 24% reduction in FGR versus control levels, a 42% decrease within the PVWM, and a 35% decrease within the SVZ versus controls. MLT normalized astrocyte attachment to blood vessels, with no difference seen between controls and FGR + MLT animals in any of the brain regions examined. We also observed a decrease in albumin extravasation and microhemorrhage in controls and FGR + MLT brains versus FGR lambs. Our results demonstrate that umbilicoplacental insufficiency is associated with FGR-produced vascular changes in the white matter and SVZ of FGR newborn brains and that maternal MLT prevented disruption of the BBB by protecting perivascular cells essential for the maintenance of vascular homeostasis and stability.

Altered cardiovascular function at birth in growth-restricted preterm lambs.

Cardiovascular dysfunction at birth may underlie poor outcomes after fetal growth restriction (FGR) in neonates. We compared the cardiovascular transition between FGR and appropriately grown (AG) preterm lambs and examined possible mechanisms underlying any cardiovascular dysfunction in FGR lambs. FGR was induced in ewes bearing twins at 0.7 gestation; the twin was used as an internal control (AG). At 0.8 gestation, lambs were delivered and either euthanized with their arteries isolated for in vitro wire myography, or ventilated for 2 h. At 60 min, inhaled nitric oxide (iNO) was administered in a subgroup for 30 min. Molecular assessment of the nitric oxide (NO) pathway within lung tissue was conducted. FGR lambs had lower left ventricular output and cerebral blood flow (CBF) and higher systemic vascular resistance compared with AG lambs. INO administration to FGR lambs rapidly improved cardiovascular and systemic hemodynamics but resulted in decreased CBF in AG lambs. Isolated arteries from FGR lambs showed impaired sensitivity to NO donors, but enhanced vasodilation to Sildenafil and Sodium nitroprusside, and altered expression of components of the NO pathway. Cardiovascular dysfunction at birth may underlie the increased morbidity and mortality observed in preterm FGR newborns. Impaired NO signaling likely underlies the abnormal vascular reactivity.

Fetal growth restriction has long-term effects on postnatal lung structure in sheep.

We have previously shown that fetal growth restriction (FGR) during late gestation in sheep affects lung development in the near-term fetus and at 8 wk after birth. In the present study, our aim was to determine the effects of FGR on the structure of the lungs at 2 y after birth; our hypothesis was that changes observed at 8 wk after birth would persist until maturity. FGR was induced in sheep by umbilicoplacental embolization, which was maintained from 120 d until delivery at term (approximately 147 d); birth weights of FGR lambs were 41% lower than in controls. At 2 y after birth, body and lung weights were not different, but there were 28% fewer alveoli and alveoli were significantly larger than in controls; hence there was a 10% reduction in the internal surface area relative to lung volume in FGR sheep compared with controls. The lungs of FGR sheep, compared with controls, had thicker interalveolar septa as a result of increased extracellular matrix deposition; the alveolar blood-air barrier was also thicker, largely because of an 82% increase in basement membrane thickness. These changes are qualitatively similar to those observed at 8 wk. Our data show that structural alterations in the lungs induced by FGR that were apparent at 8 wk were still evident at 2 y after birth, indicating that FGR may result in permanent changes in the structure of the lungs of the offspring and may affect respiratory health and lung aging later in life.

Effects of fetal growth restriction on lung development before and after birth: a morphometric analysis.

Our aim was to determine the effects of fetal growth restriction (FGR) during late gestation on the structure of the lungs in the fetus near term and at 8 weeks after birth. The studies were performed using two groups of pregnant sheep and their offspring. In both groups, FGR was induced by umbilico-placental embolisation (UPE); for fetal studies, UPE was performed from 120 days of gestation until 140 days (term, approximately 146 days), when fetuses were killed for tissue analysis. For postnatal studies, UPE continued from 120 days until delivery at term; postnatal lambs were killed at 8 weeks after birth for tissue analysis. UPE led to a thicker pulmonary blood-air barrier at 140 days of gestation and this difference, which was due to a thickened basement membrane, was still present at 8 weeks after birth. At 8 weeks, we also observed a smaller number of alveoli per respiratory unit, thicker interalveolar septa, and a greater volume density of lung tissue in FGR lambs compared to controls. These changes would be expected to impair gas exchange and alter the mechanical properties of the lungs. Our data show that structural alterations in the lungs induced by placental insufficiency were more evident at 8 weeks of postnatal age than near term, indicating that the effects of FGR on the lung may become more serious with age and may affect respiratory health later in life.