If an embryo is identified on a transabdominal scan and cardiac activity is not visible, the prognosis is usually poor. It is important to realize that occasionally a very small embryo may be present in which cardiac activity cannot be confirmed.
According to the experience of one group of investigators who used a transabdominal approach, [13] 21% of the time a normal IUP lacked visible embryonic heart motion when the embryonic crown rump length (CRL) was 9 mm or less. Based on their experience, these investigators recommended that when using a transabdominal approach, 9 mm should be considered the discriminatory embryonic length for detecting cardiac motion. Used in this manner, the discriminatory level denotes the numeric value when a certain finding should always be present.
Given its superior resolution, it is not surprising that vagin*l ultrasonographic scans can detect cardiac activity with a smaller embryonic CRL.
One report recommended that, when a transvagin*l approach is used, 4 mm should be considered the discriminatory embryonic length for detecting cardiac motion. According to the investigators, who reviewed the use of the transvagin*l approach in embryonic ultrasonographic imaging, 18% of the time a normal IUP lacked visible embryonic heart motion when the embryonic CRL was 4 mm or less. [14] Other investigators suggested 5 mm as the discriminatory embryonic size for detecting cardiac motion. [15, 16]
(See the images below.)
Embryonic demise. vagin*l scanning determined the embryonic crown rump length as 5.4 mm, corresponding to 6.4 weeks' gestational age. Cardiac activity was present at 121 beats per minute.
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This was an 8 weeks pregnancy by dates, scan showed a small embryo with absent cardiac flicker and an irregular amniotic membrane.
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This was a pregnancy of over 18 weeks by dates, ultrasound shows a much smaller fetus (around 9 weeks by CRL) with generalized edema and absent cardiac activity.
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If an embryo exceeds the discriminatory length and cardiac activity is absent, a nonviable gestation should be diagnosed. Because this observation has such important clinical ramifications, this observation should be made by 2 independent observers, and interpretive caution must be exercised in any questionable case. Documentation should be available by M mode imaging and/or by obtaining a videotape or video clip.
If the length of the embryo is less than the discriminatory value, the patient should be managed expectantly, and a repeat ultrasonographic examination should be performed when the expected embryonic CRL exceeds the discriminatory value. Alternatively, or additionally, the level of serum human chorionic gonadotropin (hCG) may be useful for determining whether a normal IUP is present.
Visualizing a living embryo
Although seemingly a paradox, it is well known that detecting cardiac activity when using a vagin*l transducer does not guarantee as favorable an outcome as detecting cardiac activity when using an abdominal transducer. With a transvagin*l approach, mortality rates of 20-30% have been reported in women with threatened abortion in whom embryonic cardiac activity is documented at 6 weeks' GA. [14, 17]
Several factors account for these less favorable statistics. First, the vagin*l approach detects cardiac activity earlier when the incidence of pregnancy loss is relatively higher. In addition, a number of other important observations have been made, which, when observed with a living embryo, are predictive of a poor outcome. [18]
Bradycardia
At 5-6 weeks' GA, the mean embryonic heart rate is 101 beats per minute (bpm). This rate increases to 143 bpm by 8-9 weeks' GA and subsequently plateaus at approximately 140 bpm. [19] Therefore, it is not unusual for an initially detected embryonic heart rate to be somewhat slower than the fetal heart rate recorded later in pregnancy. An unusually slow heart rate is cause for concern. In one study, all embryos from 5+ to 8+ weeks' GA in which the heart rate was less than 85 bpm resulted in spontaneous miscarriage. [20]
(See the image below.)
At 5.5 weeks' gestational age, the embryonic heart rate was 92 beats per minute. Follow-up scan revealed embryonic demise.
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Subchorionic hemorrhage
As many as 18% of women with vagin*l bleeding during the first half of pregnancy have ultrasonographic evidence of a subchorionic hemorrhage (displayed in the image below) as the etiology for their bleeding. [21] The clinical significance of this type of hemorrhage is controversial, with some investigators reporting an increased incidence of spontaneous abortion, [22, 23] and others concluding that this condition does not adversely affect pregnancy outcome. [21] Several authorities have suggested that the size of the blood clot can be used to predict the outcome [22] ; this has not been universally accepted. [24]
A large subchorionic hemorrhage is present superior to the gestational sac (white arrow). Follow-up scan revealed embryonic demise.
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Abnormal yolk sac/amnion
The yolk sac normally forms by 28 menstrual days and is the first structure visible in the gestational sac. Normally, it should be seen on a transabdominal scan when the mean sac diameter (MSD) is 20 mm or larger. [25] This corresponds to a GA of 7 weeks. Transvagin*l transducers can uniformly detect the yolk sac when the MSD is 8 mm or larger. [26] This corresponds to a GA of 5.5 weeks. Failure to visualize a yolk sac when the GA has reached these discriminatory values signals that the pregnancy is not progressing normally. A normal-appearing yolk sac at 5.5 weeks' GA is shown in the image below.
Embryonic demise. A normal appearing yolk sac (arrow) is seen on this transvagin*l scan, performed at 5.5 weeks' gestational age. Diameter is 3 mm.
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An abnormal-appearing yolk sac also can predict subsequent embryonic demise. Abnormal features include large size (diameter greater than 6 mm, as seen in the image below), calcification or echogenic material within the yolk sac, and a double appearance to the yolk sac. [27, 28]
Very large, as well as very small echogenic yolk sacs suggest an adverse outcome, this patient had embryonic demise with no cardiac pulsation seen on gray scale or Doppler. The yolk sac is dense, collapsed and very echogenic.
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Embryonic demise. An abnormally large yolk sac is present (arrow) within this gestational sac. Diameter measured 10 mm. Follow-up imaging confirmed a failed pregnancy.
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The amnion develops somewhat earlier than the yolk sac, but because this membrane is so thin, it is more difficult to visualize than the yolk sac. Normally, the amnion is visible on transabdominal scans late in the embryonic period. If the amnion is easily seen, it is probably too thick and most likely is abnormal.
Other features consistent with pregnancy failure include a visible amnion without a simultaneously visible yolk sac, embryo, or cardiac activity. An enlarged amniotic sac is another sonographic sign that predicts a failed pregnancy or embryonic death. [29]
Doppler findings
To date, conflicting reports exist with regard to the usefulness of first-trimester Doppler for predicting pregnancy outcome. Some reports suggest that if the resistive index is measured at the subchorionic level and exceeds 0.55, a high likelihood of spontaneous abortion exists. Others claim, however, that Doppler analysis of these vessels is not predictive of outcome. [30, 31, 11, 32, 33]
Visualizing an "empty" gestational sac
An "empty " gestational sac is the product of a normal early IUP or an abnormal IUP; another alternative is that the structure is actually a pseudogestational sac in a patient with an ectopic pregnancy. Based on careful ultrasonographic sac analysis, it may be possible to distinguish which of these alternatives is correct. Not infrequently, however, it is difficult or impossible to make this determination, in which case a follow-up ultrasonographic examination should be considered if clinically feasible.
A normal sac first appears as a small fluid collection surrounded by high-amplitude echoes embedded in the decidualized endometrium. This appearance has been termed the "intradecidual sac sign" (IDSS) (seen in the image below). [34]
Embryonic demise. This very small sac (arrow) is positioned within the anterior endometrium. Note the linear central cavity echo positioned just deep to the sac. This relationship characterizes a normal-appearing intradecidual sac sign.
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Abnormal sac size
From 5.5 to 9 weeks' GA, the mean gestational sac size (MSS) is normally at least 5 mm greater than the CRL. When this difference is less than 5 mm, the subsequent spontaneous abortion rate exceeds 90%. [35] The etiology for first-trimester oligohydramnios (seen in the image below) is unclear, but this observation suggests that with suboptimal first-trimester gestational sac growth, a high likelihood of pregnancy loss exists.
This embryo was 8 weeks' gestational age. Lack of fluid surrounding the embryo resulted in a disproportionately small sac. A follow-up scan 1 week later revealed demise.
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An early normal intrauterine gestational sac often can be identified transabdominally by 31 days' GA and can consistently be identified by 35 days' GA. To confidently diagnose an IUP, most sonographers rely on the double decidual sac (DDS) finding, which is not universally present until the MSD is 10 mm (40 days' GA). [36]
Specific size criteria can be used to distinguish normal from abnormal intrauterine gestational sacs. Using a transabdominal approach, size criteria that unequivocally suggest an abnormal sac include failure to detect a DDS when the MSD is 10 mm or more, failure to detect a yolk sac when the MSD is 20 mm or more, or failure to detect an embryo when the MSD is 25 mm or more. [25]
Using vagin*l ultrasonography, a normal intrauterine gestational sac can be detected reliably at 4-5 weeks' GA, at which time the MSD approaches 5 mm. Using vagin*l transducers, criteria that suggest an abnormal sac include failure to detect a yolk sac when the MSD is 8 mm or greater and failure to detect cardiac activity when the MSD exceeds 16 mm. [26]
Abnormal sac growth rate
The term "blighted ovum" (or anembryonic pregnancy) is used to describe an abnormal IUP with developmental arrest occurring prior to formation of the embryo or at a stage when it is not detectable using currently available equipment. In normal gestation, mean sac growth is 1.13 mm/day; in comparison, mean sac growth in an abnormal intrauterine gestation is 0.70 mm/day. [37] Based on these observations, abnormal sac growth can be diagnosed confidently if the gestational sac fails to grow by at least 0.6 mm/day.
(See the image below.)
Embryonic demise. Using a vagin*l approach, the mean diameter of this sac exceeded 20 mm. Neither a yolk sac nor embryo was visible. These findings are consistent with a "blighted ovum."
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Choriodecidual appearance of sac
Choriodecidual appearance of sac refers to the ultrasonographic appearance of the echoes that surround an early intrauterine gestational sac. An abnormal appearance includes a distorted sac shape; a thin (< 2 mm), weakly echogenic, or irregular choriodecidual reaction; and absence of the double decidual sac sign when the MSD exceeds 10 mm. (Features of choriodecidual sac appearance are seen in the image below.)
Embryonic demise. Note the irregular shape of this sac. In addition, the choriodecidual reaction is somewhat thin. This pregnancy failed.
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Visualizing a central cavity complex
If the uterus appears normal on ultrasonography or if the central echoes are prominent, the outcome will most oftenbe unfavorable. This is because most patients with recognized pregnancy loss are approximately 11 weeks' pregnant (GA) when the ultrasonographic examination normally reveals intrauterine products of conception.
When the central cavity complex is abnormally thickened (and often irregularly echogenic), the differential diagnosis includes intrauterine blood; retained products following an incomplete spontaneous abortion; decidual changes secondary to an early, but not yet visible, intrauterine pregnancy; or a decidual reaction from an ectopic pregnancy. If the uterus has this appearance and the patient does not desire her pregnancy, uterine evacuation should be performed to detect the presence or absence of chorionic villi. If chorionic villi are absent, the patient remains at risk for an ectopic pregnancy.
Complex fluid in the uterine canal in a patient with an ectopic pregnancy. Note central location of the fluid and sharp pointed inferior edge that differentiate this from an early gestational sac
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If the patient desires to continue her pregnancy, the clinical status should determine whether serial tests (pregnancy and/or ultrasonographic) should be performed or if laparoscopy or laparotomy is required.
(See the images below.)
Very early pregnancy, even an ongoing one can sometimes be difficult to confidently diagnose, this patient had recently missed her periods, the ultrasound features of a teardrop shaped cystic area with a central echogenicity was confusing for a pseudogestational sac but the patient insisted on continuing with the pregnancy (next image).
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The same patient when followed up after 9 days shows a definite sac, good decidual reaction and an appropriately sized embryonic pole
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Degree of confidence
When performed by an experienced examiner with state-of-the art equipment, vagin*l ultrasonography can detect an early intrauterine pregnancy with a high degree of confidence. This is particularly true once a yolk sac is identified. Using the criteria outlined above for predicting a poor outcome, it is usually possible to determine which pregnancies will fail.
However, it is important to note that these discriminatory criteria are guidelines. If certain findings are not observed at the appropriate time, if the ultrasonographic findings are equivocal, if the examination is technically difficult, or if the sonographer is inexperienced, caution is warranted. The embryo always should be given the benefit of the doubt, and a follow-up ultrasonographic examination should be performed to obviate any risk of terminating a normal intrauterine pregnancy.
According to a study by Dooley et al, the presence of an amniotic sac without a live embryo at the initial visit had a specificity of 100% (95% CI, 98.53-100.00%) and positive predictive value of 100% (95% CI, 97.2-100.0%) for the diagnosis of early pregnancy failure. An amniotic sac in the absence of a live embryo was found in 174/1135 (15.3%) women with a pregnancy of uncertain viability at the initial ultrasound scan. [7]
False positives/negatives
Prior to visualizing the yolk sac, it is often not possible to be certain if the presence of a small, saclike intrauterine structure is the result of an early intrauterine pregnancy (normal or abnormal) or if the structure is a pseudosac associated with an ectopic pregnancy. This is because it may not be possible to clearly identify the IDSS. Under these circ*mstances, a follow-up examination should be performed if clinically feasible.
Occasionally, a subchorionic hemorrhage may resemble a second intrauterine sac. However, since most of these women are bleeding, the correct diagnosis usually can be made with careful scanning. Whenever uncertainty exists, perform a short interval follow-up examination at 5-7 days.
Later in the first trimester, several anatomic structures undergo developmental changes that can be misinterpreted as abnormal. One potential pitfall is misinterpreting the developing rhombencephalon as an abnormal intracranial cystic structure, such as hydrocephalus or a Dandy-Walker cyst. Note that because these anomalies require second-trimester imaging, these conditions should not be diagnosed during the first trimester.
Another potential source of confusion is misinterpreting physiologic herniation of the bowel within the umbilical cord as an abdominal wall defect, such as an omphalocele. In normal patients, the diameter of the base of the cord (containing herniated bowel) should be less than 7 mm; in addition, no appreciable herniation should be seen once the CRL is greater than 45 mm. In questionable cases, perform careful follow-up ultrasonographic imaging.
