eMedicine Specialties > Radiology > Obstetrics/Gynecology

Embryonic Demise: Imaging

Author: Faye C Laing, MD, Professor of Radiology, Department of Radiology, Division of Ultrasound, Harvard Medical School, Brigham and Women's Hospital
Contributor Information and Disclosures

Updated: Apr 30, 2009

Ultrasonography

Findings

Visualizing a dead embryo

• 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,¹⁰ 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 vaginal ultrasound scans can detect cardiac activity with a smaller embryonic CRL. According to the experience of a different group of investigators who used a transvaginal approach,¹¹ 18% of the time a normal IUP lacked visible embryonic heart motion when the embryonic CRL was 4 mm or less. Based on their experience, these investigators recommended that when a transvaginal approach was used, 4 mm be considered the discriminatory embryonic length for detecting cardiac motion (see Image 1). Other investigators suggest 5 mm as the discriminatory embryonic size for detecting cardiac motion.¹²
• 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 two 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 ultrasound examination should be performed when the expected embryonic CRL exceeds the discriminatory value. Alternatively, or additionally, the level of serum 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 vaginal transducer does not guarantee as favorable an outcome as detecting cardiac activity when using an abdominal transducer. With a transvaginal 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.^11,13
• Several factors account for these less favorable statistics. First, the vaginal 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.¹⁴

Predicting a poor outcome

• Bradycardia: At 5-6 weeks GA, the mean embryonic heart rate is 101 beats per minute (bpm) (see Image 2). This rate increases to 143 bpm by 8-9 weeks GA and subsequently plateaus at approximately 140 bpm.¹⁵ 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.¹⁶
• Small sac size: From 5.5-9 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% (see Image 3).¹⁷ The etiology for first trimester oligohydramnios is unclear, but this observation suggests that with suboptimal first trimester gestational sac growth, a high likelihood of pregnancy loss exists.
• Subchorionic hemorrhage: As many as 18% of women with vaginal bleeding during the first half of pregnancy have sonographic evidence for a subchorionic hemorrhage as the etiology for their bleeding (see Image 4).¹⁸ The clinical significance of this type of hemorrhage is controversial, with some investigators reporting an increased incidence of spontaneous abortion.^19,20 while others conclude this condition does not adversely affect pregnancy outcome.¹⁸ Several authorities have suggested that the size of the blood clot can be used to predict the outcome¹⁹ ; this has not been universally accepted.²¹
• 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.²² This corresponds to a GA of 7 weeks. Transvaginal transducers can uniformly detect the yolk sac when the MSD is 8 mm or larger.²³ This corresponds to a GA of 5.5 weeks (see Image 5). Failure to visualize a yolk sac when the GA has reached these discriminatory values signals the pregnancy is not progressing normally. An abnormal appearing yolk sac also can predict subsequent demise. Abnormal features include large size (diameter greater than 6 mm), calcification or echogenic material within the yolk sac, and a double appearance to the yolk sac (see Image 6).^24,25
  • 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.²⁶
• Doppler findings: To date, conflicting reports exist with regard to the usefulness of first trimester Doppler for predicting pregnancy outcome. Some reports suggest if the resistive index is measured at the subchorionic level and exceeds .55, a high likelihood of spontaneous abortion exists²⁷ ; however, others claim that Doppler analysis of these vessels are not predictive of outcome.^28,29,30

Visualizing an "empty" gestational sac

• An "empty " gestational sac is the result of 1 of 3 entities: (1) a normal early IUP, (2) an abnormal IUP, or (3) a pseudogestational sac in a patient with an ectopic pregnancy. Based on careful ultrasound sac analysis, it may be possibly to distinguish among these entities. The earliest appearance for a normal sac is a small fluid collection surrounded by high-amplitude echoes embedded in the decidualized endometrium. This appearance has been termed the "intradecidual sac sign" (IDSS) (see Image 7). Not infrequently, because it may be difficult or impossible to distinguish these 3 conditions, a follow-up ultrasound examination should be considered if clinically feasible.³¹

Abnormal sac criteria

• Size
  • 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 d GA).³²
  • 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 equal to or greater than 10 mm, failure to detect a yolk sac when the MSD is equal to or greater than 20 mm, or failure to detect an embryo when the MSD is equal to or greater than 25 mm.²²
  • Using vaginal ultrasound, a normal intrauterine gestational sac can be detected reliably at 4-5 weeks GA, at which time the MSD approaches 5 mm. Using vaginal 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.²³
• 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 (see Image 8). In normal gestation, mean sac growth is 1.13 mm/d; in comparison, mean sac growth in an abnormal intrauterine gestation is 0.70 mm/d.³³ Based on these observations, abnormal sac growth can be diagnosed confidently if the gestational sac fails to grow by at least 0.6 mm/d.
• Choriodecidual appearance
  • This refers to the sonographic 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 (see Image 9).

Visualizing a central cavity complex

• If the uterus appears normal on sonography, or if the central echoes are prominent, most often the outcome will be unfavorable. This is because most patients with recognized pregnancy loss are approximately 11 weeks pregnant (GA) when the ultrasound 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. If the patient desires to continue her pregnancy, the clinical status should determine whether serial tests (pregnancy and/or ultrasound) should be performed or if laparoscopy or laparotomy is required.

Degree of Confidence

When performed by an experienced examiner, and with state of the art equipment, vaginal ultrasound 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 ultrasound 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 ultrasound examination should be performed to obviate any risk of terminating a normal intrauterine pregnancy.

False Positives/Negatives

Prior to visualizing the yolk sac, it is often not possible to be certain if a small intrauterine saclike structure is due to an early intrauterine pregnancy (normal or abnormal), or a pseudosac associated with an ectopic pregnancy. This is because it may not be possible to clearly identify the intradecidual sac sign (IDSS). Under these circumstances, a follow-up examination should be performed if clinically feasible.

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 for 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 for an abdominal wall defect such as an omphalocele. In normal patients, the diameter of the base of the cord (that contains 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 ultrasound imaging.

• Search for CME/CE on This Topic »

• Targeting eNOS and Beyond: Emerging Heterogeneity of the Role of Endothelial Rho Proteins in Stroke Protection