Low birth weight, caused by preterm birth or intrauterine growth restriction (IUGR) or both, is known to be associated with increased rates of cardiovascular disease and non–insulin-dependent diabetes in adult life.1–3 The fetal-origins hypothesis proposes that these diseases originate through adaptations of the fetus when it is undernourished. These adaptations may be cardiovascular, metabolic, or endocrine, and they may change the structure and function of the body permanently.4–11
The mechanisms involved in determining a high vascular risk in fetuses with IUGR are not yet understood clearly, particularly whether adverse events occurring early in intrauterine life result in any vessel abnormalities. Skilton et al12 describe the ultrasonography-based measurement of abdominal aortic intima media thickness in children as a feasible, accurate, and sensitive marker of atherosclerosis risk.13–18 In fact, ultrasonography is able to assess early vascular changes that may be linked to atherosclerosis. Infants who had IUGR have a thicker aorta, suggesting that prenatal events (eg, impaired fetal growth) might be associated with structural changes in the main vessels.12–14 In fact, a postmortem study performed on infants showed that the abdominal aorta was the first site to be involved in the progression of atherosclerosis.19 Early endothelial dysfunction, impairment of arterial vasodilatory function, and aortic intima media thickness may play an important role in premature stiffening of the aortic vessels, which predisposes these individuals to hypertension, stroke, nephropathies, and metabolic syndrome.18 However, during the fetal and infant periods, the natural course of aortic intima media thickness still remains unclear.
The aim of this study was to measure by ultrasonography aortic intima media thickness and diameter in fetuses with IUGR and in appropriate for gestational age (AGA) fetuses and aortic intima media thickness in the children after a mean follow-up of 18 months to test the hypothesis of a predisposition to vascular dysfunction in IUGR.
MATERIALS AND METHODS
The study was performed in the University Hospital of Padua from January 2006 to August 2008. Written informed consent was obtained from each woman before enrollment, and the project was approved by the University Hospital Committee for Research on Human Subjects. Two women were part of a multicenter study that did not interfere with the protocol of the present study.20
The protocol was designed to study fetuses that had IUGR and those that were AGA. Participants were selected during routine ultrasonographic fetal biometry and Doppler velocimetry in the third trimester of pregnancy. Fetuses were classified as having IUGR if the estimated fetal weight was below the 10th percentile and umbilical artery pulsatility index was more than 2 standard deviations; they were classified as AGA if the estimated fetal weight was between the 10th and 90th percentiles and Doppler velocimetry was normal.
Inclusion criteria at enrollment were singleton pregnancy, gestational age determined from known last menstrual period, and ultrasound dating before 20 weeks of gestation. Exclusion criteria were twin pregnancy; major congenital anomalies; pregnancies complicated by maternal history of cardiovascular disease or endocrine disorders such as diabetes, hypercholesterolemia, preeclampsia, thyroid, and adrenal problems; and clinical chorioamnionitis. Women who consumed alcohol, nicotine, or any medication such as ritodrine and corticosteroids (except for fetal lung maturation) were excluded.
Data concerning women, pregnancies, and deliveries were recorded according to the routine practice of the Department of Obstetrics and Gynecology.
All women underwent ultrasound and Doppler examinations during pregnancy (at least three times in the case of IUGR). In each fetus with IUGR and each AGA fetus, estimated fetal weight and antenatal testing routinely performed were available. Aortic intima media thickness and diameter measurements and aortic blood-flow velocity were determined for each fetus with IUGR and each AGA fetus at a mean gestational age of 32 weeks (range 30 to 34 weeks).
All parameters were measured by high-resolution ultrasound scan using an ultrasound machine equipped with a 3.5- to 5-MHz linear array transducer (Antares, Siemens Medical Solutions, Mountain View, CA). Intima media thickness and diameter were measured in a coronal or sagittal view of the fetus at the dorsal arterial wall of the most distal 15 mm of the abdominal aorta sampled below the renal arteries and above the iliac arteries as previously described12–14; gain settings were used to optimize image quality. Abdominal aortic intima media thickness was defined as the distance between the leading edge of the blood–intima interface and the leading edge of the media-adventitia interface on the far wall of the vessel.14 This vessel was selected because it is reported to be the first involved in the atherosclerotic process, in particular the dorsal arterial wall, which is the most lesion-prone site seen in autopsy samples.19 Three measurements were taken, and the arithmetic mean aortic intima media thickness was considered for the study (Fig. 1). Aortic diameter was measured at the same level of aortic intima media thickness, from inner wall to the wall edges. All images were taken at end-diastole of the cardiac cycle to minimize the variability. End-diastole was determined as the maximal expansion of the vessel using the cine-loop capability of the ultrasound machine once the images of the entire cardiac cycle were frozen. Blood-flow velocities of the aortic arch were measured using color Doppler velocimetry in the initial portion of the aorta distal to the aortic isthmus. The vessel was visualized in a longitudinal view of the fetus. The transducer was tilted to obtain an angle of insonation as close to 0° as possible and always less than 30°; the high-pass filter was reduced to the minimum. Each measurement was taken during fetal apnea after three consecutive, similar waveforms were obtained. Pulsatility index and time-averaged velocities (defined as the area under the velocity spectral envelope) then were measured using the machine software. The same aortic intima media thickness measurements were performed in the children in a supine position at a mean postnatal age of 18 months.
Fetal examination required no more than 20 minutes, with about 40 minutes required for the children. All the ultrasound studies in fetuses and children were performed by a single, skilled practitioner (E.C.).
Before starting the main research, the intraobserver and interobserver agreement were evaluated in the measurement of aortic intima media thickness. Aortic intima media thickness was measured in 22 AGA fetuses and in 22 children at 18 months. Two practitioners (E.C., V.Z.), blinded among themselves, measured the aortic intima media thickness three times on the same fetus and child after a 2-minute waiting period measured with a stopwatch. Intraobserver and interobserver correlation coefficients were 0.876 and 0.856, respectively.
Statistical analysis was performed using the SPSS 17 software package (SPSS Inc., Chicago, IL). Data are presented as median and range. Abdominal aortic intima media thickness, diameter, and velocity of aortic blood flow were compared between groups using the Mann-Whitney test. Relationships between aortic intima media thickness at 32 weeks of gestation and at 18 months of age and other parameters were studied using the Spearman test. A P<.05 was considered statistically significant.
Seventy white women from the Veneto region (Italy) met the inclusion criteria. Among them, 38 had fetuses with IUGR and 32 had AGA fetuses. Anthropometrical and clinical characteristics of the study population are shown in Table 1.
Median maternal age at delivery was 32.1 years (range 19.5–44.7) in the IUGR group and 31.6 years (range 21.2–42) in the AGA group. Twelve women (32%) in the IUGR group delivered vaginally, and 26 (68%) delivered by cesarean. In the AGA group, there were 17 (53%) vaginal deliveries and 15 (47%) cesarean deliveries (12 repeat cesarean deliveries and three on maternal request). Median gestational age at delivery for fetuses with IUGR was 33 weeks (range 30–35), and median birth weight was 1,830 g (range 1,470–2,310). Median gestational age in the AGA group was 36.7 weeks (range 33–41), and median birth weight was 3,200 g (range 2,740–3,620). There were no differences in maternal age, parity, and mode of delivery between the groups.
Median aortic intima media thickness for fetuses at 32 weeks of gestation was significantly different between fetuses with IUGR (1.9 mm, range 1.35–2.37 mm) and AGA fetuses (1.15 mm, range 0.95–1.43 mm, P<.001) (Fig. 2). Moreover, a significant negative correlation was found between aortic intima media thickness and estimated fetal weight at 32 weeks in fetuses with IUGR (P<.003, r=−0.58), indicating a trend of increasing aortic thickness with severity of IUGR not found in AGA fetuses (P=.3, r=−0.2).
The median diameter of the abdominal aorta at 32 weeks of gestation was significantly different between the two groups of fetuses (4.5 mm [range 3.66–5.4] compared with 3.6 mm [range 2.86–4.24], P<.001). A significant difference was found in median aortic blood-flow velocity (42.5 cm/s [range 11.9–89.7] and 23.3 cm/s [range 8.7–31.9], P<.001).
At the time of follow-up, the parents of 25 children who had had IUGR and 25 children who had been AGA agreed to the study. Parents of 10 children who had had IUGR and seven who had been AGA declined. Three children who had had IUGR were moving and crying during the aortic intima media thickness measurements, and follow-up was stopped after 40 minutes. The median body weight of the children who had had IUGR was 12,300 g (range 11,100–13,500), and the median body weight of those who had been AGA was 12,200 g (range 11,200–13,400). Therefore, no significant differences in anthropometric features in the study population were found (Table 2). Median aortic intima media thickness at a postnatal age of 18 months (range 17–21 months) was 2.4 mm (range 1.5–3.1 mm) in the IUGR group and 1.03 mm (range 0.88–1.24 mm) in AGA group. Therefore, aortic intima media thickness was significantly increased in children who had had IUGR compared with those who had been AGA (P<.001) (Fig. 3). Moreover, a positive linear correlation was observed between the prenatal aortic intima media thickness values and postnatal aortic intima media thickness (P<.019, r=0.48) in the children who had had IUGR, which was not found in those who had been AGA. At follow-up, there was no significant difference between median diameters of the abdominal diameter in the children who had had IUGR compared with those who had been AGA (6.8 mm [range 4.2–9] compared with 7.5 mm [range 5.2–9], P=.21).
The present study shows that aortic wall thickness is significantly increased in fetuses and children with IUGR compared with AGA fetuses and children.
Moreover, aortic intima media thickness measurements in fetuses with IUGR are inversely related to estimated fetal weight, showing that low birth weight and Doppler abnormalities may be correlated with an altered vascular structure causing possible endothelial damage. This is consistent with the finding that atherosclerosis begins to develop first in the intima of the aorta.19
This study shows that the assessment of aortic intima media thickness in fetuses and children with IUGR and AGA fetuses and children was feasible and reproducible. There were no cases in which we were unable to obtain the measurement. The intraobserver and interobserver variability demonstrated the effectiveness, simplicity, and reproducibility of the technique. This study demonstrates a noninvasive ultrasound method to evaluate preclinical atherosclerosis in preterm fetuses and children.12,16,17
Moreover, this study provides a reliable method to evaluate a marker of atherosclerosis in a longitudinal fashion before and after birth. The study was carried out in the same population, providing the advantage of having enrolled fetuses whose blood flow dynamics were known in utero as well as after delivery. In fact, the fetuses with IUGR in the study were those with an umbilical artery pulsatility index more than 2 standard deviations compared with fetuses with appropriate growth and normal blood flow dynamics.
Furthermore, these studies classified children as having IUGR without taking into account the cardiovascular status of the fetus before birth, with particular attention on the Doppler evaluation. We know of no report of children who had IUGR or who were small for gestational age that differentiates these two entities resulting in the initial cardiovascular changes in utero; no underlying cause has been demonstrated previously for the resulting cardiovascular disease later in life.
Moreover, the present study shows that the velocity of blood flow through the aortic arch was higher in fetuses with IUGR than in AGA fetuses. We suggest that this is related to a higher cardiac afterload due to increased resistance in the umbilical artery and placental insufficiency.
Nevertheless, previous studies have shown that, in children who had IUGR, aortic intima media thickness was greater in those with the lowest birth weight, suggesting that atherogenesis and an increased arterial stiffness may be a potential mechanism mediating the mentioned epidemiological link between impaired fetal growth and cardiovascular disease in adulthood, similar to major environmental risk factors such as cigarette smoking and hypertension.12–15 Unlike these studies, which focus on older age groups, our method allows for prospective examination of the effects of impaired fetal growth, without the effect of postnatal confounders, by measuring aortic intima media thickness. We now may evaluate this known marker of endothelial dysfunction and potential arterial health directly in utero from the second to third trimester of gestation and in children.
Our results are in agreement with previous studies that correlate low birth weight with endothelial damage that may influence arterial function and the overall incidence of cardiovascular events.17,18 These results suggest that, in addition to other pathogenic mechanisms, an increased arterial thickness probably is present already in fetuses with IUGR from intrauterine life and this, together with other factors such as smoking, fat consumption, and inactivity, could play a role in programming adult disease as previously suggested.12–14
Limitations of the present study are the small sample size, studying the same ethnic group, and data that did not account for differences in gender. Although these tests require advanced ultrasound equipment and experience in ultrasound measurements, which are not performed routinely, we believe that such measurements provide a valuable assessment from an early age as to the potential development of cardiovascular disease in adulthood.
In conclusion, we found that, in fetuses and children with IUGR, aortic intima media thickness is increased compared with AGA fetuses and children, suggesting that IUGR may represent an in utero marker of potential atherosclerosis development.
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© 2009 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.
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