The ultrasound measurements described form the basis of an assessment of viability. Clearly, any false-positive diagnosis of miscarriage is potentially catastrophic, as it may lead to the inadvertent termination of a pregnancy. On the other hand, a false-negative diagnosis leads only to a delay in potential intervention. Thus, the goal when assessing viability should be to achieve 100 % specificity when making a diagnosis of miscarriage.

In 2011, Jeve and colleagues published a systematic review of the evidence for the ultrasound cutoff values for CRL and MSD used to make a diagnosis of miscarriage [1]. This showed that the guidance at that time was based on an inadequate number of poor quality studies, with no consistent methodology. The authors concluded that an urgent reassessment of the diagnostic criteria used to define miscarriage was required.

In a landmark paper, Abdallah et al. examined 1060 pregnancies of uncertain viability (PUV) and showed that the false-positive rate for MSD measurements of 16 and 20 mm to define miscarriage was 4.4 % and 0.5 %, respectively, and 8.3 % for an embryo CRL measurement of 5 mm [2]. The same group investigated the reproducibility of ultrasound measurements, by both the same operator and different operators [3], and found significant intra- and interobserver variability. When comparing two consecutive observers, at the pre-2011 decision boundary of 20 mm, there was a 95 % prediction interval of 16.8–24.5 mm for MSD, while for an embryo with a CRL of 5 mm, there was a 95 % prediction interval of 4.5–5.6 mm (see Table 12.2).

Based on data from these papers, the Royal College of Obstetricians and Gynaecologists (RCOG) in the United Kingdom immediately changed its guidance to define miscarriage as an empty gestational sac of MSD ≥25 mm or embryo with CRL ≥7 mm with no heartbeat [4]. In December 2012 the UK National Institute for Health and Care Excellence adopted the recommendations of the RCOG [5] with the American College of Radiology also changing in 2013 [6].

These proposed new thresholds led to a consensus meeting of the Society of Radiologists in Ultrasound (SRU) in the United States which adopted these values in a consensus paper published in the New England Journal of Medicine [7].

More recently, Preisler et al. reported on an extension of the study by Abdallah et al. on 2854 pregnancies [8]. This confirmed the poor performance of previously used cutoff values of MSD and CRL to define miscarriage and showed high levels of specificity for the new cutoff values proposed, with narrow confidence intervals (see Table 12.3).

Pregnancies are assumed to grow at a standard rate in the first trimester. This is the basis on which ultrasound dating relies [9]. An embryo size consistent with that expected according to the last menstrual period (LMP) offers reassurance, while a smaller than expected CRL may be a sign of impending miscarriage – with the risk of miscarriage increasing with greater discrepancy [10].

However, the potential for inaccurate recall of dates, together with variations in the length of the menstrual cycle, makes using gestation age as a criterion on which to base a diagnosis of miscarriage difficult. Furthermore, even when both the LMP and time of ovulation are known, the ovulation-implantation (I-O) interval may also introduce variation. Mahendru and colleagues showed that this varies from 9 to 20 days in pregnancies that remained viable [11]. Thus, two different women may have the same gestation according to LMP, but have significantly different I-O intervals: this has clear implications for assessing viability.

Other factors may also be associated with variation in embryo growth including gender, maternal age and ethnic origin [12, 13]. Slow growth is also associated with pathology: triploidy, trisomy 18 and possibly trisomy 13 [12].

Preisler and colleagues recently assessed where a safe margin could be set in order to diagnose miscarriage based on gestational age in women who reported “certain” dates [8] (see Table 12.4). They proposed that, if presenting at 70 days after the last menstrual period, providing the MSD is ≥18 mm for gestational sacs without an embryo or there is an embryo with CRL ≥3 mm without visible heart activity, miscarriage could be safely diagnosed.

Irrespective of the thresholds discussed above, all practitioners make mistakes. Accordingly in general, it is good practice when making a diagnosis of miscarriage to have the ultrasound findings checked by a second operator to further reduce the possibility of error.

When safe scan criteria for the diagnosis of miscarriage on the basis of one ultrasound scan are not met, a repeat scan at an interval is indicated. During this time, the pregnancy is defined as being of “uncertain viability” – a PUV. The key question however is what the examiner should expect to see on such a further scan in order to define pregnancy failure and at what time interval such scans should be performed.

Abdallah and colleagues demonstrated considerable overlap between gestational sac growth rates between viable and nonviable pregnancies, so using a cutoff value for growth to define miscarriage is unhelpful [14]. However, the paper by Preisler et al. has better defined the time interval required between scans if misdiagnosis is to be avoided [8]. Data from this study suggests that in the case of a gestation sac with an MSD of <12 mm (with no embryo) on an initial scan, a repeat scan should be carried after 14 days. At this point a failure of MSD to double or heart activity to be visualized is diagnostic of miscarriage. Otherwise, if the MSD is ≥12 mm or when an embryo with no heartbeat is seen, a scan interval of 7 days is appropriate, at which point the absence of embryo heart activity is diagnostic of miscarriage. The performance of currently used diagnostic criteria and proposed new criteria on which to base a diagnosis of miscarriage is seen in Table 12.4 taken from Preisler et al.

A considerable number of initial early pregnancy scans will show neither a viable pregnancy nor features diagnostic of a miscarriage and thus be classified as a pregnancy of uncertain viability (PUV). There is evidence that giving women information about the likely outcome in these circumstances may be psychologically helpful [15].

Findings suspicious for, but not diagnostic of, miscarriage are detailed in Table 12.5. The majority relate to gestation sac and embryo size, as well as gestational age and time intervals between scans, approaching the decision boundaries. The “empty amnion” (Fig. 12.1) is a sign of discordant growth: an amnion should only be visualized at the point at which a 7-week embryo is visible; if the amnion is empty or contains a much smaller CRL than expected, this strongly suggests miscarriage. Other worrisome signs of discordant growth identified by Doubilet et al. are an enlarged yolk sac (>7 mm) or a large CRL for the size of the MSD (Fig. 12.2) [7]. In addition, irregularity or low placement of a gestational sac or evidence of bleeding around a sac can be subtle indicators of likely pregnancy demise (see Fig. 12.3).