Myometrial Thickness in Pregnancy: Longitudinal Sonographic Study

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1 Myometrial Thickness in Pregnancy: Longitudinal Sonographic Study Shimon Degani, MD, Zvi Leibovitz, MD, Israel Shapiro, MD, Ron Gonen, MD, Gonen Ohel, MD The purpose of this study was to evaluate in vivo the changes in myometrial thickness throughout pregnancy. Myometrial thickness was measured in 25 singleton uncomplicated pregnancies. Ultrasonographic sagittal and transverse sections were used to measure uterine wall thickness from the low anterior wall (lower segment) and the anterior, posterior, right, and left walls of the upper segment and from the fundus. In each case four measurements were made in the second and third trimesters. Myometrial thickness of the upper uterine segment remains fairly constant in the first and second trimesters of pregnancy, whereas a significant linear trend was found between a decreasing thickness of the lower uterine segment and advancing gestational age. Myometrial thickness is significantly increased behind the placental insertion site as compared to other portions of the uterine wall. These data may serve as baseline reference values for further studies in the antepartum fetal surveillance of high-risk pregnancies. KEY WORDS: Myometrium, thickness; Pregnancy; Uterus, myometrial thickness. D SD, Standard deviation ABBREVIATIONS Received March 19, 1998, from the Department of Obstetrics and Gynecology, Ultrasound Unit, Bnai-Zion Medical Center, and The Bruce Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa, Israel. Revised manuscript accepted for publication June 27, Address correspondence and reprint requests to Shimon Degani, MD, Department of Obstetrics and Gynecology, Ultrasound Unit, Bnai-Zion Medical Center, POB 4940, Haifa 31048, Israel. uring pregnancy the uterus is transformed into a relatively thin-walled muscular organ of sufficient capacity to accommodate the fetus, placenta, and amniotic fluid. By the end of pregnancy uterine capacity is 500 to 1000 times greater than in the nonpregnant state. Morphologic studies have shown that uterine enlargement involves stretching and marked hypertrophy of existing muscle cells. During the first few months of pregnancy the uterine walls become considerably thicker, but as gestation advances the walls gradually thin. 1 Accompanying the increase in size of myometrial cells are an accumulation of fibrous and elastic tissue and increase in size and number of blood vessels and lymphatics. By observing the relative positions of the attachments of the fallopian tubes and the ovarian and round ligaments it was hypothesized that uterine enlargement is asymmetrical. Current knowledge of these morphologic changes in the pregnant human uterus is based on surgical and pathologic observations. In the present study, ultrasonography was used to follow in vivo changes in uterine wall thickness throughout pregnancy by the American Institute of Ultrasound in Medicine J Ultrasound Med 17: , /98/$3.50

2 662 MYOMETRIAL THICKNESS IN PREGNANCY J Ultrasound Med 17: , 1998 MATERIALS AND METHODS A total of 28 women with singleton, low-risk pregnancies were recruited at the Bnai-Zion Medical Center, Haifa, Israel, after their first antenatal visit. Gestational age was determined from a well-defined last menstrual period, consistent with an early sonogram obtained prior to 12 weeks. Pregnancies that developed complications or that had an abnormal neonatal outcome were excluded. Criteria for abnormal pregnancy and outcome were as follows: congenital abnormalities, growth disturbances, oligohydramnios, polyhydramnios, and preterm delivery. The presence of fibroids was a contraindication to inclusion in the study. A total of 25 pregnancies had normal follow-up results and outcome by these criteria. Women were asked to come for scanning at an interval of 8 weeks, with the first scan at 14 to 16 weeks of gestation. Commercially available ultrasonographic equipment (Ultramark 9, HDI, Advanced Technology Laboratories, Bothell, WA; Diasonics Synergy, Haifa, Israel) with 3.5 or 4 7 MHz convex probes were used for all measurements. The sonographic examination was carried out transabdominally with the bladder full to allow good imaging of the entire lower segment of the uterus. Ultrasonographic sagittal and transverse sections (Fig. 1) were used to measure uterine wall thickness from the low anterior wall (lower segment) at the bladder interface (A), at the midanterior wall (B), at the fundal wall (C), at the posterior wall (D), at the right wall (E), and at the left wall (F). The myometrium was defined as a layer of homogeneous echogenicity from the serosal surface to the decidua (Fig. 2). Vascular channels above the myometrium or behind the placenta and wall segments with focal or transient thickening were not included in the measurement. At least three measurements were obtained from each uterine wall, and the average of the values was calculated. All measurements were obtained by the same observer (S.D.) at 15, 23, 31, and 39 weeks of gestation. Placental location was defined according to site in the uterine wall at which the major portion of the placenta was located (A to F). The maternal age (mean ± SD) of the study group was 28 ± 6.7 years (range, 19 to 41 years), the median gravidity was 2 (range, 1 to 7), and the median parity was 1 (range, 0 to 5). Statistical analysis used paired Student s t-test and regression analysis. A value of P < 0.05 was considered significant. RESULTS Myometrial thickness (mean ± SD) in the various sites at the four gestational age periods was as follows: A, 7.4 ± 1.8 mm; B, 9.13 ± 1.6 mm; C, 9.48 ± 1.5 mm; D, ± 1.9 mm; E, 8.95 ± 1.6 mm; F, 9.05 ± 1.5 mm. The curves of myometrial thickness for the different segments of the uterine wall are shown in Figure 3. A significant negative linear relationship was found between the myometrial thickness of the lower uterine segment and gestational age (r = 0.33, P < 0.001). Differences between measured thickness from the various uterine sites throughout pregnancy and at the same gestational age period were Figure 1 Sagittal and transverse sections of uterus used for measurement of wall thickness. A, Anterior lower segment; B, midanterior wall; C, fundal wall; D, posterior wall; E, right wall; F, left wall.

3 J Ultrasound Med 17: , 1998 DEGANI ET AL 663 A Figure 2 Transabdominal scan of uterus for measurement of myometrial thickness. A, Anterior uterine wall. B, Posterior uterine wall under the placenta. S, Serosal layer; C, chorionic or decidual layer; P, placenta. B not statistically significant. No significant difference was found in myometrial thickness with respect to gravidity. Comparison of myometrial thickness according to placental location showed that posterior wall thickness was 11.6 ± 2.3 mm (mean ± SD) in pregnancies with the placenta located on the posterior wall and 9.5 ± 1.9 mm in pregnancies with the placenta located elsewhere. Anterior wall thickness was 10.3 ± 1.5 mm with anterior wall placentas and 8.9 ± 1.4 mm with placentas located at another site. The thickness of uterine walls carrying the placenta differs significantly (P < 0.001) from thickness of placenta-free uterine walls. DISCUSSION Assessment of the uterine walls should be an integral part of routine prenatal ultrasonography. The examiner should search the walls for an unusual thinning, thickening, or vascular invasion. 2 3 Surprisingly little has been published on this subject; only a few studies used ultrasonography to measure uterine walls, and the investigators concentrated on the lower uterine segment before and during labor. We conducted a systematic morphometric sonographic study of normal uterine wall. Rozenberg and coworkers 4 performed such measurements to assess the risk of defects of scarred uterus after cesarean section. These authors showed that the risk of uterine rupture or dehiscence from a defective scar is directly related to the degree of lower uterine segment thinning measured at or around 37 weeks of gestation, and in particular this risk increases significantly when the thickness is 3.5 mm or less. Rupture of the pregnant uterus after cesarean section invariably occurs in the lower uterine segment in a form of complete rupture or dehiscence. 5

4 664 MYOMETRIAL THICKNESS IN PREGNANCY J Ultrasound Med 17: , 1998 A B C D E Figure 3 Uterine wall thickness (in mm) during pregnancy. The continuous line represents the mean and the dashed lines represent the upper and lower 95% confidence intervals. A, Anterior lower segment; B, mid-anterior wall; C, fundal wall; D, posterior wall; E, right wall; F, left wall. F Spontaneous rupture of a previously intact uterus is a rare and dangerous event. 6 Because of this rarity the diagnosis is often missed or delayed. Rupture of an unscarred uterus does not necessarily occur in the lower uterine segment: although the thinner lower segment seems to be the point of least resistance, it is not always the site of spontaneous rupture. 7 Focal thickening of uterine wall may represent a uterine myoma 2 or transient segmental contraction. 8 Gillespie 9 described three phases of uterine growth: hyperplasia, which occurs before nidation; hypertrophy, which happens during the first half of pregnancy; and dilation, which occurs during the second half of pregnancy. During the first and early second trimesters of pregnancy the mass of the uterus increases in a fairly linear fashion to the full weight that it will be at term. 1 This means that although the external dimensions of the uterus will

5 J Ultrasound Med 17: , 1998 DEGANI ET AL 665 continue to enlarge during the second half of pregnancy, the uterus will not gain additional tissue. In our study the expected thinning of uterine wall was found only in the lower segment. In contrast to the findings of Gillespie in his in vitro studies, we have not found significant changes in myometrial thickness in the other regions of the uterine wall. This discrepancy and the greater thickness of the myometrium in the area of placentation may be explained by the change in myometrial structure, namely, the increase in elastic and vascular tissue elements in the muscle layer throughout pregnancy. This network not only adds materially to the strength of the uterine wall but also keeps its thickness stable. At the area of placentation the vascular tissue elements may contribute to the increased myometrial thickness. The sonographic in vivo measurement of wall thickness reflects this aspect of maternal adaptation to pregnancy. These data may serve as baseline reference values for further studies in the antepartum surveillance of high-risk pregnancies. REFERENCES 1. Cunningham FG, MacDonald PC, Gant NF, et al: Maternal adaptation to pregnancy. In Williams Obstetrics. 20th Ed. Stamford, CT, Appleton & Lange, 1997, p Exacoustos C, Rosati P: Ultrasound diagnosis of uterine myomas and complications in pregnancy. Obstet Gynecol 82:97, Jauniaux E, Toplis PJ, Nicolaides KH: Sonographic diagnosis of a non-previa placenta accreta. Ultrasound Obstet Gynecol 7:58, Rozenberg P, Goffinet F, Philippe HJ, et al: Ultrasonographic measurement of lower uterine segment to assess the risk of defects of scarred uterus. Lancet 347:281, Soltan MH, Khashggi T, Adelusi B: Pregnancy following rupture of the pregnant uterus. Int J Gynaecol Obstet 52:37, Sweeten KM, Graves WK, Athanasiou A: Spontaneous rupture of the unscarred uterus. Am J Obstet Gynecol 172:851, Suner S, Jagminas L, Peipert JF, et al: Fatal spontaneous rupture of a gravid uterus: Case report and literature review of uterine rupture. J Emerg Med 14:181, Kessler A, Mitchell DG, Kuhlman K, et al: Myoma vs. contraction in pregnancy: Differentiation with color Doppler imaging. J Clin Ultrasound 21:241, Gillespie EC: Principles of uterine growth in pregnancy. Am J Obstet Gynecol 59:949, 1950