June 23rd, 2012 by Hasham

Dichorionic diamniotic twin pregnancy

A dichorionic diamniotic (DCDA) twin pregnancy is a type of twin pregnancy where each twin has its own chorionic and amnionic sacs. This type occurs most commonly with dizygotic, and sometimes also with monozygotic twin pregnancies.
Epidemiology

DCDA pregnancies account for the vast majority (~ 76%) of all twin pregnancies. They account for all dzygotic pregancies and ~ 20% of monozygotic pregnancies.
Pathophysiology

With a dizygotic pregnancy two ova are independently fertilised by two sperm leading to two zygotes.

With a monozygotic twin pregnancy, a DCDA pregnancy results from seperation of the zygotes at ~ 1 – 4  days post fertilisation (morula) stage.
Radiographic features
Ultrasound

Sonographic assessment of chorionicity is most accurate in the first trimester.
First trimester

Features supporting a DCDA pregnancy:

* presence of two gestational sacs with a thick echogenic chorion surrounding each embryo
* a thick inter twin membrane
* twin peak sign
* two yolk sacs may be seen (this however does not differentiate a DCDA pregnancy from a MCDA pregnancy).

Second trimester

* when there is no placental fusion, two seperate placental sites may be seen 4
* a finding of two different genders for each twin is a definitive feature for a dizygotic pregnancy which in turn will invariably mean a DCDA pregnancy.

If the twins are of the same gender then it is extremely difficult if not impossible to determine if they are monozygotic or dizygotic on ultrasound.
Complications

While the complication rate is still much higher than a singleton pregnancy, A DCDA pregnancy carries the lowest rate of complications amongst twin pregnancies. Such recognised complications include

* increased risk of intra uterine growth restriction (IUGR)
* placenta related problems
o increased risk of velamentous cord insertion
o increased risk of marginal cord insertion
o increased incidence of placenta previa spectrum

Genitourinary complications in pregnancy

Pregnancy is predominantly an uncomplicated physiological state; however, extrauterine problems may arise. Imaging of these complications is a crucial part of the diagnostic work-up. This article concentrates on the pathologic processes of the genitourinary tract which may complicate a pregnancy; uterine fibroids, adnexal masses, and renal calculi.

Dr. Nelson is a Resident in Radiology and Dr. Harris is an Associate Professor of Radiology at Dartmouth Medical School and Dartmouth-Hitchcock Medical Center in Lebanon, NH.

P regnancy is predominantly an uncomplicated physiologic state; however, difficulties may arise at any time. These are mostly due to problems with the placenta or fetus throughout gestation, but extrauterine problems also may arise, usually in the second or third trimester. Imaging of these complications is a crucial part of the diagnostic work-up. 1,2 This article will concentrate on pregnancy complicated by pathologic processes of the genitourinary tract: uterine fibroids, adnexal masses, and renal calculi.

Uterine fibroids

Fibroids are the most common pelvic mass in women of child-bearing age, though they are more frequently seen in older patients. 3 These benign tumors are composed histologically of smooth muscle cells and fibrous tissue, the latter related to degeneration. 4 The incidence of fibroids within the general population is 20 to 25%, 5 while the incidence during pregnancy is 0.3 to 2.6%, 6 with 0.3 to 7% of these being symptomatic. In our society, as the average age of child-bearing increases so does the percentage of pregnant women with myomas, occurring more commonly in women in their third to fourth decades of life. 5

Most fibroids are asymptomatic and present no threat to the health of the mother or fetus. Large fibroids that are present at the onset of pregnancy or during the first or second trimester may result in spontaneous abortions or ectopic pregnancies (figure 1). 7 During the second half of gestation, the patient may experience premature delivery, premature rupture of membranes, hemoperitoneum, or dystocia. 7 Additionally, fibroids greater than 6 cm in size and/or multiple in number may cause pain, placental abruption, abnormal presentation, prolonged labor, or retained products of conception. 3,4 Lower uterine fibroids may prevent vaginal delivery (figure 2). An increased incidence of cesarean deliveries has been observed in patients with myomas greater than 10 cm. 7,8,9

Fibroids’ effect on fetal growth and development is controversial; one series related a two-fold increase in caudal dysplasia in fibroid pregnancies; 6 others have not found an association. 10 Fibroids that come in contact with the placenta are associated with a higher incidence of complications, including placental abruption, premature delivery, and post-partum hemorrhage. 7,10,11 Winer-Muram et al reported a 71% complication rate in pregnancies with a placental-fibroid relationship. They also reported bleeding within the first 18 weeks in 60% of patients (in comparison to a general population rate of approximately 25% within the first half of pregnancy), post-partum hemorrhage in 14% (vs 5% of the general population), and premature delivery in 14% (vs 8%). 11

Rice et al observed abruption in 57% of patients with retroplacental fibroids, in comparison to a 2.5% incidence in patients with fibroids unrelated to the placenta. These researchers also observed a correlation between premature delivery and fibroid size, noting a prevalence in 21% of patients with fibroids of 3 to 5 cm, and in 28% of those with fibroids greater than 5 cm. 10

Smooth muscle cells in myomas have an abundance of estrogen receptors, presumably accounting for the often rapid growth of fibroids during the hyperestrogenic state of pregnancy. 4 In one study, Aharoni and coworkers measured the volume of fibroids and reported no change in the size (±10% of initial volume) in 60%, an increase in 21% (greater than 25% change), and a decrease in size in 19% (maximum reduction of less than 20%); the lack of increase in size is in accordance with three other retrospective studies. 8

Ultrasound evaluation of uterine fibroids during pregnancy should record the number and location of fibroids (including the layer of uterus involved), the visualization of adnexa, and the examination of the kidneys to exclude nonphysiologic hydronephrosis, a rare complication of uterine fibroids in pregnancy. 13 On ultrasound, a fibroid is usually a solid-appearing mass with acoustic attenuation. As connective tissue composition increases, so does the echogenicity, as will acoustic shadowing if calcification is present. CT will reveal solid masses that are similar in density to normal myometrial tissue. With degeneration, a low-attenuation mass (with or without calcification) may be seen. 5

Many authorities highly regard magnetic resonance imaging of fibroids and find it superior in diagnosing and locating the masses. 4,14 MRI allows imaging in multiple planes, is impervious to bone and gas artifact, may distinguish uterine from ovarian tissue, and provides better visualization of the lateral and posterior pelvis, locating ovaries that may not be identified on ultrasound. An MRI protocol may include a T1-weighted image, T2-weighted images, and a STIR sequence to view possible degenerative changes. 4 MRI demonstrates well-demarcated round or oval lesions, often appearing homogeneous and with low to intermediate signal with myometrium on T1-weighted images and low signal on T2-weighted images. 15 Some such lesions may be surrounded by a high signal intensity rim on T2-weighted images; these are believed to be secondary to vascular congestion, although one study found a histologic correlation with dilated lymphatics, veins, edema, or a combination of the above. 4

After secondary changes occur in the fibroids, the typical sonographic appearance becomes more complex (figures 3A,B). As these tumors enlarge and outgrow their blood supply, smooth muscle cells degenerate into myxomatous components, increasing the fibrous content. 4,15 Puerperal fibroids, often of the carneous or fleshy type, demonstrate edema, necrosis, hyalinization, and hypertrophy of the muscle cells. 16 Edematous fibroids may resemble hydatiform moles sonographically, with multiple cystic regions within a heterogeneous echogenic area. On MRI, focal or patchy areas of high signal on T2-weighted images are present in degenerated fibroids (figure 3C), and on occasion may be mistaken for a molar pregnancy. 15 These areas also may mimic duplicated or retroverted uteri, adenomyomas, calcified fetal parts, or an adnexal mass. 4

We have found ultrasound, especially color Doppler, to be helpful in differentiating fibroids from uterine contractions and subchorionic or retroplacental hematomas. Fibroids tend to be hypoechoic, rounded, and to have one or two large feeding vessels at the periphery, whereas contractions are isoechoic to myometrium and often have hypervascular color Doppler flow, as befits contracting musculature. Hematomas have variable echogenicity and usually are completely avascular.

Several studies have investigated fibroid growth with pregnancy. 9,17 Strobelt and coworkers noted that most fibroids of less than 5 cm tended to disappear with pregnancy, whereas fibroids that were greater than 5 cm tended to remain stable or decrease in size. Multiple myomas disappeared less often than single myomas. 17 Rosati et al noted that approximately one-third of fibroids increased in volume with advancing gestation, but myoma volume was not predictive of growth patterns. However, they found a higher rate of complications in fibroids of greater than 200 cm 3 compared to those of less than 100 cm 3 . 9

A recent study suggests that color Doppler may provide clues indicating growth potential of fibroids in non-pregnant patients (figure 4). 18 Fibroids with a detectable leiomyoma artery, defined as a prominent artery identified at the periphery of the fibroid, were more likely to increase in volume during pregnancy by at least 30%, in comparison to those fibroids without a leiomyomatous artery which demonstrated little growth. Color Doppler of fibroids may be indicated to evaluate potential for growth and possible complications in pregnancy, although this requires further investigation.

Gadolinium contrast is relatively contraindicated for use in pregnant women, as little is known regarding its effects on the human fetus. It has been used to differentiate bowel from fibroids, but this is generally not a clinical problem; fibroids markedly enhance, whereas the bowel wall enhances mildly. 4 Rofsky and coworkers studied the effects of intraperitoneal gadopentetate administered to pregnant mice at 9.5 days of gestation. At 18 days of gestation, one statistically significant discrepancy was noted; the intraperitoneal saline-injected group had a fetal weight below the mean, versus controls. 19 Gadolinium has been shown to increase the incidence of skeletal malformations in rabbit fetuses given the contrast agent at 0.5 mmol/kg/day for 13 days (Omniscan package insert, Nycomed, Princeton, NJ). No human fetal studies are available.

Adnexal masses

It is estimated that 0.5 to 1.2% of pregnant women will have an adnexal mass. 20-22 Most of these are pregnancy related, corpus luteum or theca lutein cysts and will resolve by 14- to 16-weeks of gestation. A complex or persistent adnexal mass continues to be a significant clinical problem, as the chance of ovarian malignancy is a small but potentially traumatic risk to both patient and physician. The risk of ovarian malignancy in pregnancy is quite low, occurring in 0.1 to 0.4% of pregnancies. 4 However, malignancy has been detected in 2 to 5% of adnexal masses persisting after the first trimester, and 2.4 to 5% persisting after the third trimester. 23

Complications of an adnexal mass include rupture, obstruction to labor, and ovarian torsion. These occur with an incidence of less than 1 to 2.3%, and often occur in the mid to late first trimester. 20,22 Fortunately, the diagnosis is often made at an early stage and the carcinoma is of a low grade. 21 Ovarian torsion and/or rupture may be the presenting signs of ovarian cancer. 21 Similar to the non-pregnant state, surgical staging is essential for determination of therapeutic options, and CT is the most useful imaging modality for this task.

Historically, surgery was advocated for all masses persisting into the second trimester, the point at which fetal demise decreases. 24 However, with the advent of transvaginal ultrasound and MRI, operative rates are now lower, and in many situations patients may be managed conservatively. The majority of adnexal masses are identified at 13 to 20 weeks by ultrasound, and if the adnexal mass is simple or characteristic for a benign lesion, it can be safely monitored with ultrasound. 21

Some authors advocate sonographic appearance as a predictor of benignity or malignancy and the need for surgical intervention. In a series of 131 adnexal masses in 125 pregnancies, 89.3% of patients were characterized with benign-appearing adnexal masses (figure 5). 22 Complex characteristics considered suspicious for malignancy were noted in 10.7%. The features included a mass with both solid and cystic constituents, or a uniformly but irregularly filled solid lesion lacking through transmission. In this series, 95% of dermoids, 80% of endometriomas, and 71% of simple cysts were diagnosed correctly. Only one of the 14 (7%) suspicious lesions proved to be an ovarian carcinoma, an overall incidence of 0.8%. No loss of pregnancy occurred in the 24 (19%) patients that underwent laparoscopy.

However, the above statistics are in contrast to those of more surgically-oriented reports. In a surgical series of 31 patients, a 17-weeks pregnant patient experienced a miscarriage 24 hours post-operatively. 20 Neiswender and Toub advise surgical intervention if an adnexal mass enlarges, contains solid components, or measures greater than 6 cm during the second trimester of pregnancy. 25 They report a series of two patients who successfully underwent laparoscopic excision in the second trimester. These authors favored laparoscopic surgery over laparotomy due to its less invasive nature. They also advocate performing surgery during the second trimester; premature labor and delivery is less likely to occur at this time compared to the third trimester. If delivery occurs during this period of pregnancy, the fetus generally is non-viable.

Several benign adnexal lesions have a characteristic sonographic appearance. Simple cysts are anechoic, with well-defined thin posterior walls and increased through transmission. Endo-metriomas are characteristically homo-genous, with low-level echoes and increased through transmission (figure 6). Teratomas have a typical appearance of a highly echogenic or calcified solid portion with acoustic shadowing, often with cystic components.

Complex masses are more difficult to assess noninvasively and physicians may now refer patients with these types of lesions to MRI before resorting to surgery. Endometriomas often demonstrate foci of high signal intensity on T1-weighted images (T1WI), and are variable on T2WI, with a characteristic fluid-fluid level; dependent fluid may be very low signal on T2WI due to hemosiderin. 4 Serous cystadenomas demonstrate low signal on T1WI and markedly high signal intensity on T2WI, with or without septations. Pedunculated leiomyomas may mimic solid malignant ovarian tumors, but MRI usually can differentiate between these entities.

Spectral Doppler ultrasound remains controversial in its role in the diagnosis of ovarian malignancy. While conventional sonography combined with color Doppler may increase the accuracy, sensitivity, and specificity of ultrasound diagnosis, most studies show significant overlap and limited value in the use of spectral characteristics of resistive index, pulsatility index, and peak systolic velocity for differentiating malignant from benign lesions (figure 7). 26

Urolithiasis

Urolithiasis is an uncommon but perplexing diagnostic problem in pregnancy, affecting about 1 in 1,500 patients. 27 The gravid state confers no predisposition for renal stones; the incidence is the same for non-pregnant females. 28 By 9 to 11 weeks of gestation, pregnancy is a hypercalciuric and uric acid state as a result of increased renal plasma flow and glomerular filtration rate. 29 Changes in urine composition, including alkalinization, elevated levels of magnesium, citrate, and nephrocalcin, and the presence of stone formation inhibitors oppose this stone-forming state during pregnancy. 30 In particular, increased urinary citrate and magnesium excretion inhibits calcium stone formation, glycosaminoglycans and acidic glycoproteins (nephrocalcin) inhibit oxalate stones, and urine alkalinity from the respiratory alkalosis of pregnancy inhibits uric acid stones. 29 Struvite stones are seen with infection and in association with congenital abnormalities.

The vast majority of renal calculi during pregnancy are asymptomatic and are incidentally found, although they are responsible for the majority of chief complaints of abdominal pain unrelated to the fetus or placenta (figure 8). 29,30 Acute pyelonephritis and obstruction by stones are the most frequent urinary tract problems in pregnancy. 31 Most patients with stones present in their second or third trimester with a combination of pain, sepsis, and/or obstruction. Flank pain is the most common symptom, usually associated with macroscopic or microscopic hematuria. 29

Haddad and coworkers describe three states of urinary tract dilatation in
pregnancy: 1) physiological, 2) “over-distension syndrome,” and 3) “acute hydronephrosis in pregnancy.” Physiologic dilatation is asymptomatic (occurring in 80% of pregnancies, more often found on the right side than the left). 32 The overdistension syndrome, resulting from obstruction by the gravid uterus, is characterized by pain which may mimic renal colic; treatment for this syndrome is conservative. Acute hydronephrosis in pregnancy may result from a change in the position of the fetus, diuresis, or passage of a stone into the ureter. Constant pain, with occasional nausea and vomiting, that is unresponsive to conservative therapy defines this entity. Interventional treatment may be required for relief of symptoms.

Urolithiasis should be considered in any pregnant patient with fever that persists after 24 hours of antibiotics. 29 Differential diagnosis includes pyelonephritis, appendicitis, cholecystitis, and pregnancy-related diseases such as ovarian vein thrombosis, labor, placental abruption, or pregnancy-induced liver disease. Complications of urolithiasis include urinary tract infections, occurring in 10 to 20% of patients, precipitation of premature labor by renal colic, or permanent renal failure when diagnosis or intervention is significantly delayed. 28,30

Differentiation of the physiological pelvicaliectasis from obstruction, particularly on the right side, is difficult. Physiologic pelvicaliectasis may occur as early as at 6 to 10 weeks of gestation and is present in up to 90% of pregnancies by the third trimester. 29,31 It is the result of both hormonal and mechanical factors, the latter more often involving the right ureter (85% vs 15%). 27 The gravid uterus compresses the ureter at the pelvic brim near the crossing of the iliac vessels. The right ureter has been noted to take a more acute angle at this position, thus causing the higher incidence of right-sided physiological hydronephrosis. On a hormonal basis, ureteral dilatation likely results from relaxation of smooth muscle in response to progesterone.

In imaging of non-pathologic ureterohydronephrosis, the dilated ureter tapers to a normal caliber pelvic ureter; the smooth transition characteristically takes place at the crossing of the common iliac vessels adjacent to the sacrum. 31 Therefore, pathologic urinary tract dilatation is suggested by dilatation of the pelvic ureter (figure 9), a severe left-sided delayed nephrogram or hydronephrosis (figure 10), or the presence of a demonstrable stone (figure 11). Renal enlargement also may be seen as a result of elevated renal vascular volume and interstitial fluid in response to renal hemodynamic alterations. 32

Ultrasound, especially with the wide-spread use of spectral and color Doppler, has assumed primary importance in diagnosis of hydronephrosis in pregnancy. Excretory urography has an important but secondary role in diagnosis, due to the increased risk of ionizing radiation to the fetus.

Renal and bladder sonographic imaging in pregnancy should include the renal size, echogenicity, presence or absence of perinephric fluid, calculi, or ureteral jets, and at least three measurements of resistive indices of interlobar or arcuate arteries for each kidney. For Doppler waveforms, the lowest pulse repetition frequency should be used to maximize the Doppler spectrum and reduce error. 27 Transvaginal, transperineal, or transrectal ultrasound may be necessary for detection of lower ureter calculi and dilatation. Doppler may help identify the common iliac vessels and thus locate tapering of the physiological dilated ureters. 33 Absence of marked asymmetry of ureteral jets in the bladder with color Doppler over a 5- to 20-minute interval in well-hydrated patients (600 to 1000 ml of fluid p.o. or IV) is strong evidence of a moderate to high grade obstructive process (figure 12). 27,34

In 1993, Platt and coworkers reported that a mean resistive index (RI) of greater than or equal to 0.70 and/or an inter-renal difference (delta) of greater than or equal to 0.1 is indicative of acute renal obstruction. 35 However, an intra-renal resistive index (RI=peak systolic velocity-peak diastolic velocity/peak systolic velocity ) greater than 0.70 is not typically seen in physiological dilatation of pregnancy. Hertzberg et al demonstrated the lack of RI elevation in pregnancy, even in the presence of physiologic pelvicaliectasis (figure 13). 36 With ureteral obstruction, renal vascular resistance increases, diastolic blood flow decreases, and the RI increases. Resistive index in the unaffected kidney also should be obtained for comparison of inter-renal differences.

Spectral Doppler is not without controversy for diagnosis of renal obstruction. In their study, Tublin et al were unable to reproduce the high sensitivity or specificity of Doppler US for obstruction using Platt’s method and parameters. 37 They found duplex sonography to be 82% specific but only 44% sensitive for the detection of acute renal colic in an experimental animal model.

According to a review by Ellenbogen et al, ultrasound detection of symptomatic calculi is 98% sensitive and 74% specific in the non-pregnant patient, but it drops off to 34% and 86%, respectively, in the pregnant patient. 38 A KUB and limited excretory urogram (4 to 5 films) during pregnancy has been reported to be 94% sensitive and 100% specific, 39 but has associated radiation risks. Although the risk of radiation-associated anomalies and cancers is relatively low with diagnostic studies, spontaneous abortion, teratogenicity, CNS malformations, and childhood cancers are potential concerns.

In some cases, magnetic resonance urography may be useful for differentiating physiologic from pathologic pelvicaliectasis. 31,40 It offers no ionizing radiation exposure and may be more accurate than ultrasound. Roy and coworkers assessed the utility of rapid acquisition with relaxation enhancement (RARE) MR-urography (RMU) in pregnant patients with clinically suspected ureterohydronephrosis. The researchers were able to differentiate physiological from calculi-related hydronephrosis in the majority of cases. Again, detection of physiologic dilatation presented as tapering of the lower lumbar ureter, whereas obstructive ureterohydronephrosis was evident by a ureteral filling defect on heavily weighted T2 images with ureteral dilatation proximally. The level of obstruction was determined in all cases by RMU, as opposed to ultrasound, in which visualization of the blocked ureter is often not possible.