Questionable Accuracy of Miscarriage Diagnoses
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In the UK, the joint report of the Royal College of Obstetricians and Gynaecologists and the Royal College of Radiologists recommends using the following criteria to diagnose pregnancy of 'uncertain viability':
1. an intrauterine gestational sac of <20mm in mean diameter with no obvious yolk sac2. presence of a fetus or fetal echo of <6mm crown-rump length with no obvious fetal heart activity.
These thresholds have been questioned by clinicians. Professor Tom Bourne, of Imperial College London told BBC News that 'we found the cut-off values were not entirely safe because they can be associated with a misdiagnosis of miscarriage in a small number of cases, and our view is that there shouldn't be any risk.' Professor Bourne also argues that 'the cut-offs should be about 25mm instead of 20mm for the sac and 7mm for an embryo without a heartbeat. He also wants a greater emphasis on repeat scans.' I agree with this viewpoint. These pregnancies are very much 'wanted' pregnancies and any risks involved should be minimised in order to help the child be carried to term.
In contrast, Professor Siobhan Quenby of University Hospital Coventry has stated that she really doesn't 'think many mistakes are being made' but she welcomes greater attention and clarity on the issue. Regardless of how many mistakes are being made, efforts should be made to reduce this figure. Professor Quenby further points out that 'people were aware of the issues with the guidance and if there was any doubt, further tests, not a termination, would take place.' She confirmed that it is 'very common that people come back for a second scan' instead of the decision being made purely on the basis of the first scan.
A review of the literature into the accuracy of first-trimester ultrasound in diagnosing early embryonic demise was carried out by Jeve et al. They state that 'for the diagnosis of early embryonic demise there are various ultrasound criteria used which have relatively high specificity and poor sensitivity.' They further state that 'most pregnancy screening tests, such as Down's syndrome or gestational diabetes screening, strive for optimal sensitivity whilst tolerating a low false-positive rate. With threatened early pregnancy loss, it is imperative to have a highly specific test with a zero false-positive rate, as the diagnosis of embryonic demise leads to evacuation of the uterus.' They note that 'while it would be ideal to have both a highly sensitive and highly specific test for early pregnancy loss, it is critical to realise that a false-positive diagnosis of early embryonic demise is likely to result in inadvertent termination of pregnancy.' It is interesting to note that the review also highlights the fact that 'in clinical practice, operator error is common and it is not unusual for small embryos to be missed by inexperienced examiners.' If this is the case, then it would be vital to instate a process of repeat screenings in order to prevent this 'operator error' occurring.
A study carried out by Pexsters et al supports the findings of the review of Jeve et al. They note that 'ultrasound imaging technology has evolved rapidly over recent years, allowing better delineation of landmarks in the early first trimester.' Pexsters et al undertook a 'cross-sectional study of crown-rump length (CRL) and gestational sac measurements in a fetal medicine referral centre with a predominantly Caucasian population. Gestational age ranged from 6 to 9 weeks. All patients underwent a transvaginal ultrasound examination using a high resolution ultrasound machine. Two measurements of CRL and measurements of three diameters of the gestational sac were obtained by two observers.' It was found that 'interobserver agreement for CRL measurements is better than that for MSD measurements. The measurements of MSD that are specifically used when making a diagnosis of miscarriage were found to be less reproducible between two observers when performed in the same gestational age range.' It is interesting to note that 'it seems clear that CRL measurements below 8 weeks are more uniformly made, since the embryo is straight. From 8 weeks onwards, measurement differences are possible owing to the curvature of the embryo.' They note that 'possible measurement errors then range from + or -2mm in an embryo of 20mm and from + and -3mm in an embryo of 30mm, which corresponds to a difference in dating of 2 or 3 days.' It is important to be aware that 'even a difference in CRL measurement of as little as 1mm can have an impact on clinical decision-making.' The study suggests that 'great care must be taken when measurements approach the decision boundary. In general little harm is associated with repeating a scan at a later date when deciding about the potential viability of a pregnancy.' The study further suggests that 'an MSD of 20mm to define miscarriage would become 25mm to take into account possible measurement error. In this way the risk of terminating wanted viable embryos should be minimised.' By repeating scans and altering the boundaries slightly, mistakes can be reduced, which is essential.
A further observational cross-sectional study has been carried out by Abdallah et al. Prior to conducting the study, they noted that 'the variation both in the literature and in national guidelines, regarding the definitions use to make such a fundamental decision as the viability of a human embryo, is concerning, especially as any error may be associated with inadvertent termination of pregnancy.' The study found that 'among the 1060 included women with a diagnosis of intrauterine pregnancy of uncertain viability (IPUV) at initial scan, 473 (44.6%) cases remained viable at the 11-14 week scan and 587 (55.4%) cases were non-viable by this time.' They noted that the study allowed them to 'demonstrate that empty sacs with an MSD equal to or greater than those currently used to define miscarriage may remain viable and are not a definitive marker of impending pregnancy failure. It does not seem likely that the different cut-off values for MSD and CRL used to define IPUV in our study centres will have affected the results as there were no viable pregnancies observed above the minimum thresholds of 20mm for MSD or 6mm for CRL.'
The Abdallah et al study makes the following interesting comparisons: 'if we apply a cut-off of 16mm for MSD in the absence of a yolk sac to define miscarriage, this could lead to 3520 viable pregnancies in the UK each year being classified as a miscarriage and potentially undergoing termination. To put this number in context, there are 4000 stillbirths reported in the UK annually. Applying a cut-off for MSD of 20mm could lead to 400 viable pregnancies potentially being misclassified, compared with approximately 300 'cot deaths' reported in the UK each year. These numbers are significant and relate to pregnancies that would be highly likely to reach term.'
Abdallah et al note that their findings suggest that 'safe cut-off values to define miscarriage may have to be significantly increased to exclude any errors.' This correlates with the findings of Pexsters et al who suggest increasing the cut-off from 20mm to 25mm. Interestingly, Abdallah et al and Pexsters et al's finding also agree with regard to repeating the scans before a final decision is made. Abdallah et al state that 'waiting 7-10 days in order to repeat a scan is highly unlikely to lead to physical harm. The anxiety associated with being uncertain about the status of a pregnancy is very significant, but should be balanced against the possibility of inadvertent termination which is surely the worst possible outcome for any woman.' They further state that 'emphasis should be placed on the need to repeat scans when measurements are around the decision boundaries.' As Professor Tom Bourne points out, 'there's no medical cost to being more cautious in what we're doing' and this caution could prevent further mistakes being made.
Vicky Kunzli - Trainee Solicitor, Clinical Negligence Team
RoyalCollege of Radiologists, Royal College of Obstetricians and Gynaecologists, Guidance on Ultrasound Procedures in Early Pregnancy. RCR/RCOG: London, 1995
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