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Ultrasound of the empty gestation sac in threatened abortion
Clinical Radiology (1987) 38, 127-130
Ultrasound of the Empty Gestation Sac in Threatened Abortion R. F. S C O T T , T. F E A T H E R S T O N E and J. K. H U S S E Y
Departments of Diagnostic Radiology, Woodend General Hospital and Aberdeen Royal b~firmary, Aberdeen
The major concern when an empty gestation sac is encountered during threatened abortion is whether or not the pregnancy is viable. Viable pregnancies are managed expectantly, whereas non-viable pregnancies are treated by evacuation of the uterus. Early evacuation of the non-viable pregnancy spares the patient considerable anxiety and discomfort. This study shows that a single ultrasound examination is useful in differentiating viable from non-viable empty gestation sacs. The size of the empty gestation sac was found to be the most useful criterion for determining nonviability. Empty gestation sacs with sizes greater than 26 m m were non-viable, a specificity of 100%, accounting for 43% (42 out of 102) of patients in our series. Other ultrasound criteria found to be useful were shape, position, wall and decidual reaction. The positive predictive value for a successful outcome to a pregnancy was low, being only 41% (9 out of 22).
Threatened abortions occur during the first 20 weeks of pregnancy when vaginal bleeding is encountered with a closed cervical os. This occurs in 20 to 25% of pregnancies (Fantel and Shepard, 1981; Cavanagh and Comas, 1982; Hertz, 1984) affecting a large proportion of the obstetric population. The usual clinical practice is to exercise caution and to manage the pregnancy expectantly without any active intervention. The process of spontaneous expulsion may however be delayed for weeks causing prolonged bleeding and anxiety (Smith et al., 1978; Fantel and Shepard, 1981; Hertz, 1984). Ultrasound provides a simple and unique method of visualising pregnancy in its early stages. If the viability of a pregnancy can be accurately determined, a more active line of management can be pursued. Non-viable pregnancies are managed by uterine evacuation whereas viable pregnancies are observed. Ultrasound can accurately establish the presence or absence of cardiac motion in the embryo. (Robinson, 1975; Anderson, 1980; Hertz, 1984). A large proportion of threatened abortions, however, demonstrate an empty gestation sac. This appearance may occur in normal pregnancies of between 5 and 7 weeks gestational age, or in non-viable pregnancies in which the embryo died or never developed. Previous workers have shown that when an empty gestation sac is encountered, an accurate distinction between a viable and a non-viable pregnancy cannot be made and a second scan after 7 to 14 days may be required. (Donald et al., 1972; Bernard and Cooperberg, 1985). More recently it has been suggested that, using modern real-time ultrasound equipment, a high degree of specificity for predicting an abnormal outcome can
be obtained on a single examination (Nyberg et al., 1986). The purpose of this study is to determine whether, using modern high definition equipment, ultrasound criteria alone can distinguish viable from non-viable gestation sacs on a single examination.
METHODS
Over a 3 year period 346 patients presenting with threatened abortion were referred to the ultrasound department for evaluation. The patients were examined using a commercially available high resolution real-time sector scanner using a 3.5 MHz or 5 MHz transducer. Of the patients referred 102 (29.4%) were shown to have empty gestation sacs. The ultrasound appearances of these empty gestation sacs were evaluated with respect to size, shape, wall definition, position in the uterus and decidual reaction around the sac. The size of the sac was determined by measuring the largest diameter with electronic callipers. The shape was described as being either round and oval or deformed and angular. The walt of the gestation sac was carefully examined for any breaks in the continuity and loss of definition of the margins. The position of the gestation sac in the uterus was noted and described as being either fundal and miduterine or low. The presence or absence of a decidual reaction was noted. If present the continuity was assessed and a measurement of the width (widest point) was made. The viability of the gestation sac was determined by follow-up ultrasound examination and a review of the clinical records. A gestation sac was considered viable if subsequent ultrasound examinations demonstrated a live fetus or the records indicated a successful outcome of the pregnancy. The gestation sac was considered non-viable if subsequent ultrasound examinations demonstrated absent growth or spontaneous abortion occurred following the initial examination. RESULTS
Of the 102 empty gestational sacs that underwent ultrasound evaluation only 10 (9.8%) were subsequently shown to constitute a viable pregnancy. All viable gestation sacs were less than 26 mm in diameter and all had well established decidual reactions greater than 2 m m in width (Figs 1 and 2). Of the 10 viable gestation sacs nine had regular round or oval shapes, but one had an angular outline (Fig. 3). The walls of the viable gestation sacs were intact and well defined. The decidual reaction around viable sacs was con-
128
CLINICAL RADIOLOGY T a b l e 1 - T h e ultrasound a p p e a r a n c e s of viable e m p t y gestation sacs
Viable gestation sacs (9.8%)
Size Shape Wall Decidual reaction
Position
<26 mm Round/oval Deformed/angular Intact Disrupted >2 mm <2 mm Continuous Interrupted Not present Fundal/miduterine Low
No
%
10 9 1 10 0 10 0 10 0 0 10 0
100 90 10 100 0 100 0 100 0 0 100 0
meters greater than 26mm were subsequently shown to b e n o n - v i a b l e (Figs 4, 5 a n d 6). N o n - v i a b l e sacs h a d a high i n c i d e n c e o f d i s t o r t i o n of s h a p e (72 o u t o f 92) a n d d i s r u p t i o n o f t h e wall o f t h e sac (61 out o f 92) (Fig. 7). A p o o r o r a b s e n t d e c i d u a l r e a c t i o n was p r e s e n t in m o s t cases (58 out of 92), (Figs 8 a n d 9). N o low lying g e s t a t i o n sacs w e r e s h o w n to b e v i a b l e (Fig. 10). T h e s o n o g r a p h i c f e a t u r e s of t h e g e s t a t i o n sacs which w e r e s u b s e q u e n t l y s h o w n to be n o n - v i a b l e are s u m m a r i s e d in T a b l e 2.
Figs 1 and 2 - Longitudinal scans showing viable gestation sacs with normal sonographic features, including: round shape, smooth contour and prominent decidual reaction. (h=head, f=feet).
t i n u o u s a n d g r e a t e r t h a n 2 m m in width. T h e ultras o u n d a p p e a r a n c e s o f the v i a b l e g e s t a t i o n sacs are s u m m a r i s e d in T a b l e 1. T h e m a j o r i t y o f e m p t y g e s t a t i o n sacs (92 o u t of 102) p r o v e d to be n o n - v i a b l e . A l l e m p t y sacs with diaFig. 4 - Transverse scan: abnormal gestation sac (31 mm diameter), irregular shape.
Fig. 3 - Transverse scan showing an apparently abnormal gestation sac (19 mm diameter) with a distorted angular shape: this pregnancy progressed to term.
Fig. 5 - Transverse scan: abnormal gestation sac, with large size (28 mm diameter), irregular shape.
ULTRASOUND OF THE EMPTY GESTATIONSAC
129
Fig. 6 - Longitudinal scan: abnormal gestation sac, large size (48 mm diameter), irregular outline and poor decidual reaction.
Fig. 9 - Longitudinal scan: abnormal gestation sac (22mm in diameter), irregular shape and poor decidual reaction.
Fig. 7 - Longitudinal scan: abnormal gestation sac, angular shape, disrupted wall.
Fig. 10 - Longitudinal scan showing an apparently normal gestation sac but in a low uterine position. The patient subsequently aborted. T a b l e 2 - T h e ultrasound a p p e a r a n c e s of non-viable gestation sacs.
Non-viable gestation sacs" (90.2%)
Size Shape Wall Decidual reaction
Position
Fig. 8 - Transverse scan: abnormal gestation sac (16mm diameter), poorly defined margins, and no decidual reaction.
DISCUSSION U l t r a s o u n d is a n i n v a l u a b l e m e t h o d for the early a s s e s s m e n t of p a t i e n t s p r e s e n t i n g with t h r e a t e n e d a b o r t i o n . If an e m b r y o c a n n o t be identified the d i f f e r e n t i a t i o n of viable f r o m n o n - v i a b l e p r e g n a n c i e s is
8-70 mm >26 mm Round/oval Deformed/angular Intact Disrupted >2 mm <2 mm Continuous Interrupted Not present Fundal/miduterine Low
No.
%
92 42 15 77 31 61 34 58 20 40 32 74 18
100 45.6 16.4 83.6 33.6 66.4 36 64 22 43.4 34.6 80.5 19.5
n o t always straightforward ( D o n a l d et al. 1972; R o b i n son, 1975). W h e n p r e s e n t e d with a n e m p t y gestation sac the u l t r a s o n o g r a p h e r can use c e r t a i n criteria in an a t t e m p t to distinguish a n o r m a l p r e g n a n c y from a blighted o v u m or an a n e m b r y o n i c p r e g n a n c y . This study has s h o w n that a high degree of specificity can be achieved in identifying a n o n - v i a b l e gestation sac by e x p e r i e n c e d u l t r a s o n o g r a p h e r s using the criteria p r e v i o u s l y o u t l i n e d , usually o n a single exa m i n a t i o n . U l t r a s o u n d was far less a c c u r a t e in deter-
130
CLINICALRADIOLOGY
mining a successful outcome of a pregnancy having a positive predictive value of only 41% (9 out of 22). Size was found to be the most important single criterion in determining the non-viability of an empty gestation sac. A b n o r m a l l y large sacs, lacking embryos, are usually due to blighted ova and are usually secondary to chromosomal aberrations (Fujikura et al., 1966; Robinson, 1975; Fantel and Shepard, 1981). No empty sac with a diameter greater than 26 m m was subsequently shown to represent a viable pregnancy in our series, i.e. a negative predictive accuracy and specificity of 100%. This is similar to the findings of Bernard and C o o p e r b e r g (1985) and of Nyberg et al. (1986) who found the m a x i m u m diameters of viable gestation sacs to be 20 m m and 25 m m respectively. Distortion of the sac also showed close correlation with non-viability. Only one sac with a distorted and angular outline (Fig. 3) was subsequently shown to represent a viable pregnancy. It has been suggested by some workers that a disorted sac is 100% specific for non-viability (Nyberg et al., 1986). This is not entirely in keeping with our findings and those of others, however (Baker et al., 1985; Bernard and C o o p e r b e r g , 1985). Baker et al. (1985) showed that a distended bladder can cause distortion of a normal gestation sac, sometimes with loss of visualisation of the pole of the embryo. The appearances return to normal on partial voiding. Distortion of sac shape therefore, although a strong indicator of non-viability, is not a conclusive sign of abnormality and in the absence of other criteria should be interpreted with caution. A continuous decidual (trophoblastic) reaction measuring more than 2 m m in width was present in all viable gestation sacs. A continuous or partial decidual reaction was also present in 65% of non-viable gestation sacs. No gestation sac without a surrounding decidual reaction was found to be viable. This contrasts with the findings of Bernard and C o o p e r b e r g (1985) in which 13% of viable gestation sacs lacked a decidual reaction. The assessment of a decidual reaction is however somewhat subjective, being operator and equipment dependent. I m p r o p e r gain settings can obscure a decidual reaction and in early pregnancies of 4 to 5 weeks the decidual reaction m a y be sparse and have no direct correlation with pregnancy outcome, (Bernard and Cooperberg, 1985). The true absence of a decidual reaction is clearly of secondary importance and should therefore not be interpreted in isolation but in conjunction with the other findings. Large breaks in the wall of a gestation sac were not compatible with a viable pregnancy. This is a significant finding as previous workers using static grey scale equipment found that breaks in the contour of the sac,
although generally representing a blighted ovum, on occasion-proceeded to normal term (Donald et al., 1972; Kohorn and Kaufman, 1974). A low implantation site in the uterus has little effect on abortion rates and has no relation to the prognosis in threatened abortion (Smith et al., 1978). N o n e of the low positioned gestation sacs in our series (18 out of 102) were subsequently shown to represent viable pregnancies, irrespective of the sonographic appearances. The reason for this is uncertain, but it has been suggested that low implantation is a stage in, rather than the cause of, abortion and consequently a low sac in the presence of bleeding and/or an open cervix carries a poor prognosis (Donald et al., 1972; K o h o r n and Kaufman, 1974; Smith et al., 1978). In conclusion, using the ultrasound criteria above, normal gestation sacs can often be distinguished from abnormal ones on a single examination. T h e r e are still a significant proportion of e m p t y sacs, however, where no accurate distinction between normal and abnormal can be made and in these cases serial examinations should be carried out before any active m a n a g e m e n t is advocated.
REFERENCES
Anderson, SG (1980). Management of threatened abortion with real-time sonography. Obstetrics and Gynecology, 55, 259-264. Baker, ME, Mohony, BS & Bowie, JD (1985). Adverse effect of an overdistended bladder on first-trimester sonography. American Journal of Roentgenology, 145, 597-599. Bernard, KG & Cooperberg, PL (1985). Sonographic differentiation between blighted ovum and early viable pregnancy. American Journal of Roentgenology, 144, 597-602. Cavanagh, D. & Comas, MR (1982). Obstetrics and Gynecology, pp. 378-392. Harper and Row, Philadelphia. Donald, I, Morley, P & Barnett, E (1972). The diagnosis of blighted ovum by sonar. British Journal of Obstetrics and Gynaecology, 79, 304-310. Fantel, AG & Shepard, TH (1981). Principles and Practice of Obstetrics and Perinatology. Vol 1, pp. 533-563. Wiley, New York. Fujikura, T, Froehlich, LA & Driscoll, SG (1966). A simplified anatomic classification of abortions. American Journal of Obstetrics and Gynecology, 95, 902-905. Hertz, JB (1984). Diagnostic procedures in threatened abortion. Obstetrics and Gynecology, 64, 223-229. Kohorn, E1 & Kaufman, M (1974). Sonar in the first trimester of pregnancy. Obstetrics and Gynecology, 44, 473-483. Nyberg, DA, Laing, FC & Filly, RA (1986). Threatened abortion: Sonographic distinction of normal and abnormal gestation sacs. Radiology, 158, 397-400. Robinson, HP (1975). The diagnosis of early pregnancy failure by sonar. British Journal of Obsterics and Gynaecology, 82, 849857. Smith, C, Gregori, CA & Breen, JL (1978). Ultrasonography in threatened abortion. Obstetrics and Gynecology, 51, 173-177.
Ultrasound of the Empty Gestation Sac in Threatened Abortion R. F. S C O T T , T. F E A T H E R S T O N E and J. K. H U S S E Y
Departments of Diagnostic Radiology, Woodend General Hospital and Aberdeen Royal b~firmary, Aberdeen
The major concern when an empty gestation sac is encountered during threatened abortion is whether or not the pregnancy is viable. Viable pregnancies are managed expectantly, whereas non-viable pregnancies are treated by evacuation of the uterus. Early evacuation of the non-viable pregnancy spares the patient considerable anxiety and discomfort. This study shows that a single ultrasound examination is useful in differentiating viable from non-viable empty gestation sacs. The size of the empty gestation sac was found to be the most useful criterion for determining nonviability. Empty gestation sacs with sizes greater than 26 m m were non-viable, a specificity of 100%, accounting for 43% (42 out of 102) of patients in our series. Other ultrasound criteria found to be useful were shape, position, wall and decidual reaction. The positive predictive value for a successful outcome to a pregnancy was low, being only 41% (9 out of 22).
Threatened abortions occur during the first 20 weeks of pregnancy when vaginal bleeding is encountered with a closed cervical os. This occurs in 20 to 25% of pregnancies (Fantel and Shepard, 1981; Cavanagh and Comas, 1982; Hertz, 1984) affecting a large proportion of the obstetric population. The usual clinical practice is to exercise caution and to manage the pregnancy expectantly without any active intervention. The process of spontaneous expulsion may however be delayed for weeks causing prolonged bleeding and anxiety (Smith et al., 1978; Fantel and Shepard, 1981; Hertz, 1984). Ultrasound provides a simple and unique method of visualising pregnancy in its early stages. If the viability of a pregnancy can be accurately determined, a more active line of management can be pursued. Non-viable pregnancies are managed by uterine evacuation whereas viable pregnancies are observed. Ultrasound can accurately establish the presence or absence of cardiac motion in the embryo. (Robinson, 1975; Anderson, 1980; Hertz, 1984). A large proportion of threatened abortions, however, demonstrate an empty gestation sac. This appearance may occur in normal pregnancies of between 5 and 7 weeks gestational age, or in non-viable pregnancies in which the embryo died or never developed. Previous workers have shown that when an empty gestation sac is encountered, an accurate distinction between a viable and a non-viable pregnancy cannot be made and a second scan after 7 to 14 days may be required. (Donald et al., 1972; Bernard and Cooperberg, 1985). More recently it has been suggested that, using modern real-time ultrasound equipment, a high degree of specificity for predicting an abnormal outcome can
be obtained on a single examination (Nyberg et al., 1986). The purpose of this study is to determine whether, using modern high definition equipment, ultrasound criteria alone can distinguish viable from non-viable gestation sacs on a single examination.
METHODS
Over a 3 year period 346 patients presenting with threatened abortion were referred to the ultrasound department for evaluation. The patients were examined using a commercially available high resolution real-time sector scanner using a 3.5 MHz or 5 MHz transducer. Of the patients referred 102 (29.4%) were shown to have empty gestation sacs. The ultrasound appearances of these empty gestation sacs were evaluated with respect to size, shape, wall definition, position in the uterus and decidual reaction around the sac. The size of the sac was determined by measuring the largest diameter with electronic callipers. The shape was described as being either round and oval or deformed and angular. The walt of the gestation sac was carefully examined for any breaks in the continuity and loss of definition of the margins. The position of the gestation sac in the uterus was noted and described as being either fundal and miduterine or low. The presence or absence of a decidual reaction was noted. If present the continuity was assessed and a measurement of the width (widest point) was made. The viability of the gestation sac was determined by follow-up ultrasound examination and a review of the clinical records. A gestation sac was considered viable if subsequent ultrasound examinations demonstrated a live fetus or the records indicated a successful outcome of the pregnancy. The gestation sac was considered non-viable if subsequent ultrasound examinations demonstrated absent growth or spontaneous abortion occurred following the initial examination. RESULTS
Of the 102 empty gestational sacs that underwent ultrasound evaluation only 10 (9.8%) were subsequently shown to constitute a viable pregnancy. All viable gestation sacs were less than 26 mm in diameter and all had well established decidual reactions greater than 2 m m in width (Figs 1 and 2). Of the 10 viable gestation sacs nine had regular round or oval shapes, but one had an angular outline (Fig. 3). The walls of the viable gestation sacs were intact and well defined. The decidual reaction around viable sacs was con-
128
CLINICAL RADIOLOGY T a b l e 1 - T h e ultrasound a p p e a r a n c e s of viable e m p t y gestation sacs
Viable gestation sacs (9.8%)
Size Shape Wall Decidual reaction
Position
<26 mm Round/oval Deformed/angular Intact Disrupted >2 mm <2 mm Continuous Interrupted Not present Fundal/miduterine Low
No
%
10 9 1 10 0 10 0 10 0 0 10 0
100 90 10 100 0 100 0 100 0 0 100 0
meters greater than 26mm were subsequently shown to b e n o n - v i a b l e (Figs 4, 5 a n d 6). N o n - v i a b l e sacs h a d a high i n c i d e n c e o f d i s t o r t i o n of s h a p e (72 o u t o f 92) a n d d i s r u p t i o n o f t h e wall o f t h e sac (61 out o f 92) (Fig. 7). A p o o r o r a b s e n t d e c i d u a l r e a c t i o n was p r e s e n t in m o s t cases (58 out of 92), (Figs 8 a n d 9). N o low lying g e s t a t i o n sacs w e r e s h o w n to b e v i a b l e (Fig. 10). T h e s o n o g r a p h i c f e a t u r e s of t h e g e s t a t i o n sacs which w e r e s u b s e q u e n t l y s h o w n to be n o n - v i a b l e are s u m m a r i s e d in T a b l e 2.
Figs 1 and 2 - Longitudinal scans showing viable gestation sacs with normal sonographic features, including: round shape, smooth contour and prominent decidual reaction. (h=head, f=feet).
t i n u o u s a n d g r e a t e r t h a n 2 m m in width. T h e ultras o u n d a p p e a r a n c e s o f the v i a b l e g e s t a t i o n sacs are s u m m a r i s e d in T a b l e 1. T h e m a j o r i t y o f e m p t y g e s t a t i o n sacs (92 o u t of 102) p r o v e d to be n o n - v i a b l e . A l l e m p t y sacs with diaFig. 4 - Transverse scan: abnormal gestation sac (31 mm diameter), irregular shape.
Fig. 3 - Transverse scan showing an apparently abnormal gestation sac (19 mm diameter) with a distorted angular shape: this pregnancy progressed to term.
Fig. 5 - Transverse scan: abnormal gestation sac, with large size (28 mm diameter), irregular shape.
ULTRASOUND OF THE EMPTY GESTATIONSAC
129
Fig. 6 - Longitudinal scan: abnormal gestation sac, large size (48 mm diameter), irregular outline and poor decidual reaction.
Fig. 9 - Longitudinal scan: abnormal gestation sac (22mm in diameter), irregular shape and poor decidual reaction.
Fig. 7 - Longitudinal scan: abnormal gestation sac, angular shape, disrupted wall.
Fig. 10 - Longitudinal scan showing an apparently normal gestation sac but in a low uterine position. The patient subsequently aborted. T a b l e 2 - T h e ultrasound a p p e a r a n c e s of non-viable gestation sacs.
Non-viable gestation sacs" (90.2%)
Size Shape Wall Decidual reaction
Position
Fig. 8 - Transverse scan: abnormal gestation sac (16mm diameter), poorly defined margins, and no decidual reaction.
DISCUSSION U l t r a s o u n d is a n i n v a l u a b l e m e t h o d for the early a s s e s s m e n t of p a t i e n t s p r e s e n t i n g with t h r e a t e n e d a b o r t i o n . If an e m b r y o c a n n o t be identified the d i f f e r e n t i a t i o n of viable f r o m n o n - v i a b l e p r e g n a n c i e s is
8-70 mm >26 mm Round/oval Deformed/angular Intact Disrupted >2 mm <2 mm Continuous Interrupted Not present Fundal/miduterine Low
No.
%
92 42 15 77 31 61 34 58 20 40 32 74 18
100 45.6 16.4 83.6 33.6 66.4 36 64 22 43.4 34.6 80.5 19.5
n o t always straightforward ( D o n a l d et al. 1972; R o b i n son, 1975). W h e n p r e s e n t e d with a n e m p t y gestation sac the u l t r a s o n o g r a p h e r can use c e r t a i n criteria in an a t t e m p t to distinguish a n o r m a l p r e g n a n c y from a blighted o v u m or an a n e m b r y o n i c p r e g n a n c y . This study has s h o w n that a high degree of specificity can be achieved in identifying a n o n - v i a b l e gestation sac by e x p e r i e n c e d u l t r a s o n o g r a p h e r s using the criteria p r e v i o u s l y o u t l i n e d , usually o n a single exa m i n a t i o n . U l t r a s o u n d was far less a c c u r a t e in deter-
130
CLINICALRADIOLOGY
mining a successful outcome of a pregnancy having a positive predictive value of only 41% (9 out of 22). Size was found to be the most important single criterion in determining the non-viability of an empty gestation sac. A b n o r m a l l y large sacs, lacking embryos, are usually due to blighted ova and are usually secondary to chromosomal aberrations (Fujikura et al., 1966; Robinson, 1975; Fantel and Shepard, 1981). No empty sac with a diameter greater than 26 m m was subsequently shown to represent a viable pregnancy in our series, i.e. a negative predictive accuracy and specificity of 100%. This is similar to the findings of Bernard and C o o p e r b e r g (1985) and of Nyberg et al. (1986) who found the m a x i m u m diameters of viable gestation sacs to be 20 m m and 25 m m respectively. Distortion of the sac also showed close correlation with non-viability. Only one sac with a distorted and angular outline (Fig. 3) was subsequently shown to represent a viable pregnancy. It has been suggested by some workers that a disorted sac is 100% specific for non-viability (Nyberg et al., 1986). This is not entirely in keeping with our findings and those of others, however (Baker et al., 1985; Bernard and C o o p e r b e r g , 1985). Baker et al. (1985) showed that a distended bladder can cause distortion of a normal gestation sac, sometimes with loss of visualisation of the pole of the embryo. The appearances return to normal on partial voiding. Distortion of sac shape therefore, although a strong indicator of non-viability, is not a conclusive sign of abnormality and in the absence of other criteria should be interpreted with caution. A continuous decidual (trophoblastic) reaction measuring more than 2 m m in width was present in all viable gestation sacs. A continuous or partial decidual reaction was also present in 65% of non-viable gestation sacs. No gestation sac without a surrounding decidual reaction was found to be viable. This contrasts with the findings of Bernard and C o o p e r b e r g (1985) in which 13% of viable gestation sacs lacked a decidual reaction. The assessment of a decidual reaction is however somewhat subjective, being operator and equipment dependent. I m p r o p e r gain settings can obscure a decidual reaction and in early pregnancies of 4 to 5 weeks the decidual reaction m a y be sparse and have no direct correlation with pregnancy outcome, (Bernard and Cooperberg, 1985). The true absence of a decidual reaction is clearly of secondary importance and should therefore not be interpreted in isolation but in conjunction with the other findings. Large breaks in the wall of a gestation sac were not compatible with a viable pregnancy. This is a significant finding as previous workers using static grey scale equipment found that breaks in the contour of the sac,
although generally representing a blighted ovum, on occasion-proceeded to normal term (Donald et al., 1972; Kohorn and Kaufman, 1974). A low implantation site in the uterus has little effect on abortion rates and has no relation to the prognosis in threatened abortion (Smith et al., 1978). N o n e of the low positioned gestation sacs in our series (18 out of 102) were subsequently shown to represent viable pregnancies, irrespective of the sonographic appearances. The reason for this is uncertain, but it has been suggested that low implantation is a stage in, rather than the cause of, abortion and consequently a low sac in the presence of bleeding and/or an open cervix carries a poor prognosis (Donald et al., 1972; K o h o r n and Kaufman, 1974; Smith et al., 1978). In conclusion, using the ultrasound criteria above, normal gestation sacs can often be distinguished from abnormal ones on a single examination. T h e r e are still a significant proportion of e m p t y sacs, however, where no accurate distinction between normal and abnormal can be made and in these cases serial examinations should be carried out before any active m a n a g e m e n t is advocated.
REFERENCES
Anderson, SG (1980). Management of threatened abortion with real-time sonography. Obstetrics and Gynecology, 55, 259-264. Baker, ME, Mohony, BS & Bowie, JD (1985). Adverse effect of an overdistended bladder on first-trimester sonography. American Journal of Roentgenology, 145, 597-599. Bernard, KG & Cooperberg, PL (1985). Sonographic differentiation between blighted ovum and early viable pregnancy. American Journal of Roentgenology, 144, 597-602. Cavanagh, D. & Comas, MR (1982). Obstetrics and Gynecology, pp. 378-392. Harper and Row, Philadelphia. Donald, I, Morley, P & Barnett, E (1972). The diagnosis of blighted ovum by sonar. British Journal of Obstetrics and Gynaecology, 79, 304-310. Fantel, AG & Shepard, TH (1981). Principles and Practice of Obstetrics and Perinatology. Vol 1, pp. 533-563. Wiley, New York. Fujikura, T, Froehlich, LA & Driscoll, SG (1966). A simplified anatomic classification of abortions. American Journal of Obstetrics and Gynecology, 95, 902-905. Hertz, JB (1984). Diagnostic procedures in threatened abortion. Obstetrics and Gynecology, 64, 223-229. Kohorn, E1 & Kaufman, M (1974). Sonar in the first trimester of pregnancy. Obstetrics and Gynecology, 44, 473-483. Nyberg, DA, Laing, FC & Filly, RA (1986). Threatened abortion: Sonographic distinction of normal and abnormal gestation sacs. Radiology, 158, 397-400. Robinson, HP (1975). The diagnosis of early pregnancy failure by sonar. British Journal of Obsterics and Gynaecology, 82, 849857. Smith, C, Gregori, CA & Breen, JL (1978). Ultrasonography in threatened abortion. Obstetrics and Gynecology, 51, 173-177.