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International Scholarly Research Notices / 2014 / Article

Research Article | Open Access

Volume 2014 |Article ID 890534 | 6 pages |

Correlation between First and Second Trimester Uterine Artery Doppler Velocimetry and Placental Bed Histopathology

Academic Editor: Gendie Lash
Received29 Mar 2014
Accepted19 May 2014
Published13 Jul 2014


Aim. To evaluate the relationship between uterine artery Doppler indices and placental bed histopathology independent of clinical outcome. Materials and Methods. Uterine artery measurements were performed to 510 pregnant women who had come for routine antenatal care in 11–14th and 20–24th weeks. Placental bed biopsies from 141 cases were taken during cesarean section. Physiological changes and abnormal placental histology findings were investigated and compared with Doppler findings. Results. 116 biopsies were accepted as adequate biopsy and included in the study. Physiological changes were seen in 100 biopsies. Statistically significant higher PI and RI values in second trimester and higher notch rate in both trimesters were detected in the abnormal placental histology group (). Conclusion. Strong relationship between uterine artery Doppler indices and preeclampsia or intrauterine growth retardation has been shown in previous studies. In our study, we concluded that there is significant relationship between Doppler findings and placental bed histopathology independent of clinical course.

1. Introduction

Spiral arteries change into loose vessels for providing increased blood flow to fetus during pregnancy. These changes in uteroplacental vessels were identified by Brosens et al. and named “physiological changes” in 1967 [1]. Spiral artery remodelling begins before cellular interaction with trophoblasts. The first changes in spiral arteries involve endothelial vacuolation, arterial dilation, and smooth muscle disorganization. Similar structural alterations were seen in ectopic pregnancies and in the decidua parietalis of intrauterine pregnancies [2]. Decomposition of arterial smooth muscle and fibrinoid material accumulation are seen when trophoblasts begin to appear near arteries [3]. Trophoblasts settle in this fibrinoid structure and take place of smooth muscle cells [4]. Trophoblasts change their adhesion molecules before invading the spiral arteries [5]. Oxygen tension determines whether cytotrophoblasts proliferate or invade, thereby regulating placental growth and cellular architecture. At early gestation, trophoblasts form plugs in the maternal spiral arteries occluding the flow of maternal blood to the intervillous space [68]. Thus, trophoblasts have proliferative characteristics under low oxygen pressure [9].

Trophoblasts replace maternal endothelium as far as the inner third of the myometrium. Placental bed biopsies of pregnancies that had been complicated with preeclampsia or intrauterine growth retardation (IUGR) showed inadequate trophoblastic invasion on myometrial component of spiral arteries [10]. Studies showed that defective invasion is not only seen on myometrial segment, but also decidual [11]. In addition, subintimal thickening, fibrinoid necrosis, acute thrombosis, perivascular nuclear cell infiltration in spiral arteries, and trophoblastic giant cells in stroma are found in preeclampsia and IUGR cases [12, 13]. Defective placentation is an important predisposing factor and endothelial injury is associated with signs of these diseases [1417].

First and second trimester uterine artery Doppler assessments have high predictive value for clinical outcome (prediction of preeclampsia and IUGR) [18]. Studies have shown that, in pregnancies which are complicated with preeclampsia and/or IUGR, fetal and uterine artery Doppler indices are related to placental bed biopsy histopathology [19, 20]. In these studies, Doppler measurements were performed after preeclampsia and/or IUGR clinic had been emerged and biopsies were taken during cesarean section.

In studies which have correlated Doppler velocimetry and clinical outcomes, wide range of specificity and sensitivity values were found [18]. High resistance findings in Doppler velocimetry are signs of defective placentation, but clinical disease has multifactorial pathophysiology. The aim of the present study is to correlate the histopathology of the placental bed and first and second trimester Doppler velocimetry of uterine arteries.

2. Material and Method

2.1. Study Design

A prospective study of women who attended for antenatal care at 11–14th and 20–24th weeks of gestation was constructed from a cohort of 510 women with singleton pregnancies who was followed up in Istanbul University Cerrahpasa Medical Faculty Hospital, Perinatology Department, Turkey, between January 2010 and July 2011. 270 cases delivered in our clinic and 141 cases delivered by cesarean section and were included in the study. Cases who were complicated with essential hypertension, diabetes mellitus, and collagen vascular diseases were excluded because these diseases’ pathological findings are classified separately. Approval for the study was given by Istanbul Clinical Research Ethical Committee and informed consent was obtained.

2.2. Doppler Ultrasonography and Sample Collection

In 11–14th and 20–24th weeks of gestation during routine antenatal screening bilateral uterine artery Doppler measurements were performed. All measurements were performed with the mothers in a semirecumbent position. Colour-flow imaging was used to visualize the ascending branch of the uterine arteries. Pulsed Doppler velocimetry was performed with a sample volume of 5 mm. Two measurements were made from each artery and mean value was calculated.

During cesarean section after removal of placenta and membranes, biopsies were taken from placental bed side by palpation with Kevorkian punch biopsy tool. 3-4 μm paraffin sections were obtained from placental bed biopsy specimens. After staining with hematoxylin-eosin, cytokeratin 7, and periodic acid schiff, specimens were examined under light microscopy. All placental bed biopsies were examined by the same pathologist who was blinded to all clinical data.

Biopsies are accepted as histologically enough if they(1)are placental bed biopsy (contain trophoblast),(2)contain spiral artery,(3)contain myometrial component.

Biopsy specimens were classified as “normal histology” if the histological features of the normal human placental bed (endovascular trophoblast invasion in decidual and myometrial segment, normal spiral artery appearance) were documented or “abnormal placental histology” if there were inadequate response to placentation, acute atherosis, increased extravillous trophoblasts, and thrombosis or luminal obliteration of spiral arteries.

2.3. Clinical Definitions

Intrauterine growth restriction was diagnosed when fetal abdominal circumference was more than two standard deviations below the mean for gestational age and also confirmed by the serial assessment of the fetal growth parameters. Preeclampsia was defined as the onset of hypertension (blood pressure of 140/90 mmHg or greater, 6 or more hours apart) and consistent proteinuria (>300 mg/day) during the latter half of the pregnancy and both remitting remotely after delivery. Oligohydramnios was diagnosed when amniotic fluid index was less than 5 cm.

2.4. Statistics

Data analysis was made by a personal computer with SPSS 16.0 statistical program. Demographic and clinical characteristics of the study groups were compared using Pearson’s chi-square test. Student’s -test or Mann-Whitney -test was used for comparison of two groups where appropriate. A value was considered significant.

3. Results

Indications for cesarean delivery were prior cesarean section (76/141), cephalopelvic disproportion (36/141), breech presentation (17/141), and fetal distress during labor (12/141). Successful biopsy rate was 82,2% (116/141). Patients with insufficient biopsy were excluded from the study. There were no pathological findings in 100 of the biopsies and they constituted the normal histology group. 16 biopsies were considered as abnormal placental histology. Defective physiological change was seen in all abnormal biopsies. Apart from this, in three biopsies, there were acute atherosis, in another one there was increased extravillous trophoblasts, and in another one there were both acute atherosis and increased extravillous trophoblasts (Figures 1, 2, and 3).

Demographic and obstetric characteristics of cases enrolled in the study are shown in Table 1. Birth weight values were statistically significantly different (). There were no significant differences between two groups in other parameters.

Normal placental histology group ( )Abnormal placental histology group ( ) value

Age27,4 ± 4,828,8 ± 50,461
Parity1 (0–6)0 (0–3)0,499
Birth weight3243 (±417)2788 (±822)0,015
Birth week38 (35–41)38 (32–40)0,419

Values are expressed as median (range) or mean (standard deviation).

Clinical disease was developed in six pregnancies. Four of them had preeclampsia; two of them had IUGR. In the normal histology group one fetus was diagnosed as IUGR. But the baby had additional anomalies (posterior urethral valve, prune belly syndrome) and died after birth. There were no preeclampsia cases in the normal histology group. Other IUGR case and preeclampsia cases were in the abnormal placental histology group. Notch was detected in these cases in either trimester. Each of these cases PI and RI values was >95th percentile.

The mean values and statistical evaluation of first and second trimester uterine artery pulsatility index (PI) and resistance index (RI) measurements are given in Table 2. Uterine artery RI and PI values were significantly different between two groups in second trimester. Bilateral notch ratio was significantly different between two groups in each trimester. Bilateral notch was found in all abnormal cases. Uterine artery PI and RI mean values were 1,42 and 0,67 in abnormal placental histology group. For normal histology group, they were 12, 1,02, and 0,56% in order ().

Normal placental histology group ( )Abnormal placental histology group ( ) value

1.trimester PI 1,64 ± 0,45 1,91 ± 0,52 0,158
1.trimester RI 0,70 ± 0,08 0,73 ± 0,1 0,462
1.trimester notch %45%100<0,001
2.trimester PI1,02 ± 0,311,42 ± 0,21<0,001
2.trimester RI0,56 ± 0,080,67 ± 0,05<0,001
2.trimester notch%12%100<0,001

Values are expressed as mean ± standard deviation.

4. Discussion

Different methods for placental bed sampling were described in previous studies [21, 22]. Punch biopsy forceps, sharpened over forceps, and scissor were used for sampling in different studies. In these studies, criterion for sufficient biopsy was detection of trophoblasts or spiral arteries in the specimen. Gerretsen et al. showed that a single large biopsy had a higher rate of success compared to multiple biopsies [10]. It is not necessary to take biopsy deeper than few millimeters in the myometrium. In one study, it was shown that the number of interstitial trophoblasts was decreased dramatically deeper than 3 millimeters in the myometrium [23]. In previous studies, rates of successful biopsies were between 44 and 70% [2224]. In our study, placental biopsies were taken during cesarean section after delivery and manual removal of placenta; one biopsy was taken in the central portion of the placental bed with  mm Kevorkian punch biopsy forceps. We achieved 82.2% success rate in this study. High success rate in this study may be due to use of standardized method by the one operator. We followed up 510 pregnancies in antenatal period, but 270 women were admitted to our clinic for delivery. In Istanbul, patients usually prefer private hospitals for vaginal delivery and they admit to university hospitals if cesarean delivery is mandatory. High cesarean ratio in our hospital could be explained by this condition.

Defective physiological change of spiral arteries is manifested by increased resistance and pulsatility index of uterine artery and branches. In preeclampsia and IUGR cases after the clinical disease is emerged there is correlation between Doppler studies and pathological findings in the placental bed biopsies [20, 25]. In IUGR cases with high pulsatility index and/or notching in Doppler assessment, there is significantly higher ratio of pathological findings in placental bed biopsies compared to the group that shows normal Doppler findings [20]. Also birth weight is associated with placental bed vascular pathologies [11]. But in our study there was no significant difference among two groups for birth weights.

Voigt and Becker showed that pathological uterine artery Doppler indices were correlated with defective placentation with 90% sensitivity and 95% specificity [13]. In a study, Lin et al. accepted S/D > 2,5 as pathological Doppler finding and they concluded that with 92,3% PPV abnormal uterine artery Doppler indices were correlated with defective trophoblast invasion in myometrium [26]. But in these studies Doppler assessments were made after preeclampsia had been diagnosed at term pregnancy. In a meta-analysis, Cnossen et al. concluded that use of PI and notching together is the best predictor for preeclampsia and IUGR [18]. In another meta-analysis, Kleinrouweler et al. found that the combination of Doppler ultrasound parameters PI or RI and bilateral notching significantly improves the prediction of preeclampsia based on the patient characteristics BMI and systolic blood pressure [27]. In the present study, bilateral notching was found in 45/100 of the normal histology group in first trimester and 12/100 of the normal histology group in second trimester. The ratio was 16/16 of the abnormal placental histology group (). In the normal histology group, mean value of PI was 1,02 ± 0,31 and mean value of RI was 0,56 ± 0,08 at second trimester. In the abnormal placental histology group, means were 1,42 ± 0,21 and 0,67 ± 0,05, respectively. There was statistically significant difference between two groups in terms of RI and PI measurements (). We concluded that there was statistically significant correlation between second trimester Doppler assessment and pathological findings in the placental bed biopsy. Among these parameters the most reliable one was detection of bilateral notch.

In two studies, uterine artery RI was evaluated in women attending clinics for termination of pregnancy in the first trimester. Their results indicated that decidual natural killer cells isolated from pregnancies with a high RI, which are at a higher risk of preeclampsia, were less able to chemoattract trophoblasts and less able to induce extravillous trophoblast outgrowth from the villi, failed to induce vascular apoptosis, and secreted fewer proapoptotic factors. Using uterine artery Doppler RI screening before cell isolation has, therefore, identified that decidual natural killer cells from high RI pregnancies have altered interactions with trophoblasts and impaired vessel remodelling, which may lead to poor placentation [28, 29]. Whitley et al. showed that first trimester extravillous trophoblasts from pregnancies with high uterine artery resistance are more sensitive to apoptotic stimuli [30]. It was shown that high resistance uterine artery Doppler indices are related to defective endovascular trophoblast invasion in the first trimester [31]. In our study, in first trimester, mean value of PI was 1,64 ± 0,45 and mean value of RI was 0,70 ± 0,08 in the normal group. Means were 1,91 ± 0,52 and 0,73 ± 0,1 in the abnormal placental histology group. There was no statistically significant difference between two groups.

In this study, sensitivity and specificity were higher than in previous studies which correlated Doppler results with clinical outcome. Pathological Doppler results indicate high resistance in uterine artery due to defective placentation. But clinical disease is multifactorial so, in some cases with pathological Doppler findings, preeclampsia and/or IUGR are not developed. Also samples belong to very small portion of whole placental bed. Therefore, biopsies might not reflect overall vascular histology of the placental bed [32]. Eleven clinically normal pregnancies in the abnormal placental histology group may be explained with diffusiveness of the pathology in the placental bed.

The fact that all biopsies were examined by the same pathologist with a well-defined standardized protocol greatly increases the internal validity of the results. Potential limitations must be acknowledged. First, the placenta is a large organ, and the accurate assessment of placental bed pathology is difficult with single biopsy. Second, the study groups were small in number. We could evaluate 116 cases from a cohort of 270 women. 129 women delivered vaginally and another 25 cases’ bed biopsies were accepted as “insufficient biopsy.”

5. Conclusion

There are differences in sensitivity among studies which correlated Doppler velocimetry and preeclampsia. Because there are other factors that are effective in the emergence of clinical outcome. In this study, we found that there was statistically significant correlation between second trimester Doppler indices and placental bed biopsy findings independent of clinical course.


IUGR:Intrauterine growth retardation
AUC:Area under curve
PI:Pulsatility index
RI:Resistance index
BMI:Body mass index.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.


This research was supported by Istanbul University Department of Scientific Research Projects.


  1. I. Brosens, W.B. Robertson, and H.G. Dixon, “The physiological response of the vessels of the placental bed to normal pregnancy,” The Journal of Pathology and Bacteriology, vol. 93, no. 2, pp. 569–579, 1967. View at: Publisher Site | Google Scholar
  2. C. M. Craven, T. Morgan, and K. Ward, “Decidual spiral artery remodelling begins before cellular interaction with cytotrophoblasts,” Placenta, vol. 19, no. 4, pp. 241–252, 1998. View at: Publisher Site | Google Scholar
  3. H. Hees, W. Moll, K. H. Wrobel, and I. Hees, “Pregnancy-induced structural changes and trophoblastic invasion in the segmental mesometrial arteries of the guinea pig (Cavia porcellus L.),” Placenta, vol. 8, no. 6, pp. 609–626, 1987. View at: Google Scholar
  4. E. M. Ramsey and J. W. S. Harris, “Comparison of uteroplacental vasculature and circulation in the rhesus monkey and man,” in Contributions to Embryology, vol. 38, pp. 59–70, Carnegie Institution of Washington, 1996. View at: Google Scholar
  5. Y. Zhou, C. H. Damsky, K. Chiu, J. M. Roberts, and S. J. Fisher, “Preeclampsia is associated with abnormal expression of adhesion molecules by invasive cytotrophoblasts,” Journal of Clinical Investigation, vol. 91, no. 3, pp. 950–960, 1993. View at: Google Scholar
  6. J. Hustin and J. P. Schaaps, “Echocardiographic and anatomic studies of the maternotrophoblastic border during the first trimester of pregnancy,” The American Journal of Obstetrics and Gynecology, vol. 157, no. 1, pp. 162–168, 1987. View at: Google Scholar
  7. R. Jaffe and J. R. Woods Jr., “Color Doppler imaging and in vivo assessment of the anatomy and physiology of the early uteroplacental circulation,” Fertility and Sterility, vol. 60, no. 2, pp. 293–297, 1993. View at: Google Scholar
  8. E. Jauniaux, J. Hempstock, N. Greenwold, and G. J. Burton, “Trophoblastic oxidative stress in relation to temporal and regional differences in maternal placental blood flow in normal and abnormal early pregnancies,” The American Journal of Pathology, vol. 162, no. 1, pp. 115–125, 2003. View at: Google Scholar
  9. O. Genbacev, Y. Zhou, J. W. Ludlow, and S. J. Fisher, “Regulation of human placental development by oxygen tension,” Science, vol. 277, no. 5332, pp. 1669–1672, 1997. View at: Publisher Site | Google Scholar
  10. G. Gerretsen, H. J. Huisjes, and J. D. Elema, “Morphological changes of the spiral arteries in the placental bed in relation to pre-eclampsia and fetal growth retardation,” The British Journal of Obstetrics and Gynaecology, vol. 88, no. 9, pp. 876–881, 1981. View at: Google Scholar
  11. T. Y. Khong, F. de Wolf, W. B. Robertson, and I. Brosens, “Inadequate maternal vascular response to placentation in pregnancies complicated by pre-eclampsia and by small-for-gestational age infants,” The British Journal of Obstetrics and Gynaecology, vol. 93, no. 10, pp. 1049–1059, 1986. View at: Google Scholar
  12. C. M. Salafia, V. K. Minior, J. C. Pezzullo, E. J. Popek, T. S. Rosenkrantz, and A. M. Vintzileos, “Intrauterine growth restriction in infants of less than thirty-two weeks' gestation: associated placental pathologic features,” The American Journal of Obstetrics and Gynecology, vol. 173, no. 4, pp. 1049–1057, 1995. View at: Publisher Site | Google Scholar
  13. H. J. Voigt and V. Becker, “Uteroplacental insufficiencycomparisonof uteroplacental blood flow velocimetry and histomorphology of placental bed,” Journal of Maternal-Fetal Investigation, vol. 2, pp. 251–255, 1992. View at: Google Scholar
  14. C. W. G. Redman and I. L. Sargent, “Pre-eclampsia, the placenta and the maternal systemic inflammatory response—a review,” Placenta, vol. 24, pp. S21–S27, 2003. View at: Publisher Site | Google Scholar
  15. J. M. Roberts and K. Y. Lain, “Recent insights into the pathogenesis of pre-eclampsia,” Placenta, vol. 23, no. 5, pp. 359–372, 2002. View at: Publisher Site | Google Scholar
  16. J. M. Roberts and C. A. Hubel, “Is oxidative stress the link in the two-stage model of pre-eclampsia?” The Lancet, vol. 354, no. 9181, pp. 788–789, 1999. View at: Publisher Site | Google Scholar
  17. Y. Wang and J. S. Alexander, “Placental pathophysiology in preeclampsia,” Pathophysiology, vol. 6, no. 4, pp. 261–270, 2000. View at: Publisher Site | Google Scholar
  18. J. S. Cnossen, R. K. Morris, G. Ter Riet et al., “Use of uterine artery Doppler ultrasonography to predict pre-eclampsia and intrauterine growth restriction: a systematic review and bivariable meta-analysis,” Canadian Medical Association Journal, vol. 178, no. 6, pp. 701–711, 2008. View at: Publisher Site | Google Scholar
  19. K. Guzin, S. Tomruk, Y. A. Tuncay et al., “The relation of increased uterine artery blood flow resistance and impaired trophoblast invasion in pre-eclamptic pregnancies,” Archives of Gynecology and Obstetrics, vol. 272, no. 4, pp. 283–288, 2005. View at: Publisher Site | Google Scholar
  20. R. Madazli, A. Somunkiran, Z. Calay, S. Ilvan, and M. F. Aksu, “Histomorphology of the placenta and the placental bed of growth restricted foetuses and correlation with the Doppler velocimetries of the urine and umbilical arteries,” Placenta, vol. 24, no. 5, pp. 510–516, 2003. View at: Publisher Site | Google Scholar
  21. W. B. Robertson, T. Y. Khong, I. Brosens, F. de Wolf, B. L. Sheppard, and J. Bonnar, “The placental bed biopsy: review from three European centers,” The American Journal of Obstetrics and Gynecology, vol. 155, no. 2, pp. 401–412, 1986. View at: Google Scholar
  22. S. C. Robson, H. Simpson, E. Ball, F. Lyall, and J. N. Bulmer, “Punch biopsy of the human placental bed,” The American Journal of Obstetrics and Gynecology, vol. 187, no. 5, pp. 1349–1355, 2002. View at: Publisher Site | Google Scholar
  23. R. Pijnenborg, J. M. Bland, W. B. Robertson, G. Dixon, and I. Brosens, “The pattern of interstitial trophoblastic invasion of the myometrium in early human pregnancy,” Placenta, vol. 2, no. 4, pp. 303–316, 1981. View at: Publisher Site | Google Scholar
  24. I. Brosens, H. G. Dixon, and W. B. Robertson, “Fetal growth retardation and the arteries of the placental bed,” The British Journal of Obstetrics and Gynaecology, vol. 84, no. 9, pp. 656–664, 1977. View at: Google Scholar
  25. R. Madazli, E. Budak, Z. Calay, and M. F. Aksu, “Correlation between placental bed biopsy findings, vascular cell adhesion molecule and fibronectin levels in pre-eclampsia,” The British Journal of Obstetrics and Gynaecology, vol. 107, no. 4, pp. 514–518, 2000. View at: Google Scholar
  26. S. Lin, I. Shimizu, N. Suehara, M. Nakayama, and T. Aono, “Uterine artery doppler velocimetry in relation to trophoblast migration into the myometrium of the placental bed,” Obstetrics and Gynecology, vol. 85, no. 5, pp. 760–765, 1995. View at: Publisher Site | Google Scholar
  27. C. E. Kleinrouweler, P. M. Bossuyt, B. Thilaganathan et al., “Value of adding second-trimester uterine artery Doppler to patient characteristics in identification of nulliparous women at increased risk for pre-eclampsia: an individual patient data meta-analysis,” Ultrasound in Obstetrics & Gynecology, vol. 42, no. 3, pp. 257–267, 2013. View at: Publisher Site | Google Scholar
  28. A. E. Wallace, A. J. Host, G. S. Whitley, and J. E. Cartwright, “Decidual natural killer cell interactions with trophoblasts are impaired in pregnancies at increased risk of preeclampsia,” The American Journal of Pathology, vol. 183, no. 6, pp. 1853–1861, 2013. View at: Publisher Site | Google Scholar
  29. R. Fraser, G. S. Whitley, A. P. Johnstone et al., “Impaired decidual natural killer cell regulation of vascular remodelling in early human pregnancies with high uterine artery resistance,” Journal of Pathology, vol. 228, no. 3, pp. 322–332, 2012. View at: Publisher Site | Google Scholar
  30. G. S. J. Whitley, P. R. Dash, L. J. Ayling, F. Prefumo, B. Thilaganathan, and J. E. Cartwright, “Increased apoptosis in first trimester extravillous trophoblasts from pregnancies at higher risk of developing preeclampsia,” The American Journal of Pathology, vol. 170, no. 6, pp. 1903–1909, 2007. View at: Publisher Site | Google Scholar
  31. F. Prefumo, N. J. Sebire, and B. Thilaganathan, “Decreased endovascular trophoblast invasion in first trimester pregnancies with high-resistance uterine artery Doppler indices,” Human Reproduction, vol. 19, no. 1, pp. 206–209, 2004. View at: Publisher Site | Google Scholar
  32. J. W. Meekins, R. Pijnenborg, M. Hanssens, I. R. McFadyen, and A. van Asshe, “A study of placental bed spiral arteries and trophoblast invasion in normal and severe pre-eclamptic pregnancies,” The British Journal of Obstetrics and Gynaecology, vol. 101, no. 8, pp. 669–674, 1994. View at: Google Scholar

Copyright © 2014 Murat Akbaş et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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