Studies have been mixed on the association of abnormal appearing YSs with the risk of pregnancy failure. Lindsay et al27 and Cho et al28 found an increased risk of pregnancy failure with abnormal appearing YSs, but both studies followed only a small number of affected pregnancies (7 and 5, respectively). Tan et al25 monitored 31 women with abnormal appearing YSs and found no statistical association with pregnancy failure. In many studies which followed abnormal appearing YSs, an embryo with cardiac activity continued normally to term. These findings would support that abnormal YSs are not consistently associated with pregnancy failure.
The presence of a YS within a GS is reassuring; however, in the absence of an embryo, future viability is uncertain. Abdallah and colleagues followed 1060 pregnancies prospectively for viability and in the subgroup of pregnancies with a YS, but without an embryo, the false-positive rate to diagnose pregnancy failure was 2.6% at a GS diameter of 16 mm and 0.4% at a cutoff of 20 mm. There were no false positives when the GS was ≥21 mm. Taking into account the interobserver error, a cutoff of ≥25 mm GS size was recommended to diagnose pregnancy failure when a YS is seen without an embryo.7
Observation of the location, appearance, and activity of the embryo can provide clues to inevitable pregnancy failure. Abnormalities of embryonic size and growth have been closely linked with pregnancy failure.
Embryonic motion can be visualized in the early first trimester by TVS and tends to be rapid jerking motions due to embryonic nervous system immaturity.3 Goldstein et al3 identified embryonic body movements starting at 8 weeks gestation, with a sensitivity and PPV of 100% and 94%.
The embryo has a characteristic appearance as it grows from a thickening along the YS into a fetus with recognizable head and limbs. Initial demonstration of the embryo on TVS occurs when it reaches 2 to 3 mm size. It has the appearance of a straight echogenicity along the YS wall. At about day 21, the embryo develops a C-shape as the caudal neuropore elongates. A visible CRL is not identified until about 49 days with the embryo measuring 18 mm.31 There is no rigorous research assessing pregnancy outcomes in the absence of the previously discussed normal developmental landmarks, but their delay or absence can provide guidance clinically. A straight appearing 4 mm embryo should raise concern for possible pregnancy failure, especially when cardiac activity is not visualized, and a follow-up exam should be considered. TVS can image the shape of the embryo, and this information can be used with other findings to determine pregnancy failure.
Investigators have assessed normal embryo size compared with menstrual age and from these data nomograms and regression formulas for embryonic growth have been developed. These studies found a normal embryo growth rate of approximately 1.3 mm/d.32 A study by Bottomley et al33 evaluated embryonic growth and found that embryo growth is not linear, and their study did not support an absolute growth rate for determining fetal viability. Reljic34 determined that when the CRL was >2 SD below the mean for gestational age and ≤18 mm, there was a 6.5-fold increased risk of pregnancy failure compared with those at or above the mean and this risk increased as the discrepancy increased. However, Reljic did not show a similar association when the CRL was >18 mm. Stern and Coulam35 evaluated pregnancy failure after confirmation of embryonic cardiac activity and found that CRL gestational age lagged by more than 0.6 weeks behind menstrual gestational age in 86% of women studied. Mukri et al36 prospectively studied the embryo/fetal growth in 292 pregnant women and 61% of pregnancy failures had CRLs that were >2 SD below the mean and there was a direct relationship between an increasing discrepancy and the risk of pregnancy failure. At a threshold of 2 SD below the mean, the sensitivity and PPV for pregnancy failure were 61% and 31%, respectively.
Structural anatomic anomalies can be detected in the first trimester with increasing frequency using TVS (Figs. 15–18). An anomaly does not always result in pregnancy failure, but certain anatomic abnormalities may be associated with aneuploidy, which increases the risk for pregnancy failure. The sonologist needs to exercise caution when diagnosing anomalies in the first trimester as there are developmental changes in the embryo and early fetus which can be misinterpreted as anomalies (Figs. 19, 20).
Several studies have determined that an EHR <85 to 100 bpm at a gestational age below 8 weeks is associated with pregnancy failure.5,38–40 A large prospective study by Stefos and colleagues evaluated 2164 women and determined a threshold EHR of 85 bpm for predicting pregnancy failure at <6 weeks 3 days gestation. This threshold is increased to 125 bpm between 7 weeks 4 days and 8 weeks 0 day.38 As gestational age increases, studies report an increase in the threshold EHR. The risk for pregnancy failure increases as the EHR decreases, most notably between 6 and 9 weeks gestation.38,41,42 An EHR <90 bpm observed at 6 to 7 weeks gestation, carries a risk of pregnancy loss of about 25% in multiple studies, even in subjects where the EHR was in the normal range at an 8-week follow-up exam.41,42 An EHR >2 SD above the mean has not been shown to increase the risk for pregnancy failure.41
Sonographic assessment of the first trimester pregnancy has dramatically improved the diagnostic capabilities of clinicians over the past 4 decades. First trimester pregnancy was once a hidden, mysterious part of pregnancy, but is now open to investigation and examination by TVS. Imaging of the stages of embryonic and fetal development can provide the clinician with valuable information that can be used to screen for aneuploidy, evaluate for anomalies, and identify markers predictive of pregnancy failure. The best predictors of pregnancy failure are the absence of an embryo once the mean GS size reaches 25 mm and the absence of cardiac activity once the embryo is ≥7 mm. These thresholds are justifiably conservative but allow physicians to reassure patients with confidence of the sonographic diagnosis of pregnancy failure.
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