Criteria used to diagnose miscarriage vary in both the United Kingdom and the United States. According to the medical literature and national guidelines, cutoff values for mean gestational sac diameter (MSD) and embryo crown-rump length (CRL) used to define miscarriage range from 13 to 25 mm for MSD and from 3 to 8 mm for CRL. These variations regarding definitions used to decide the viability of a human embryo are troubling; any error may result in an inadvertent termination of a wanted pregnancy.
The aim of this multicenter observational study was to define the false-positive rate (FPR) for a diagnosis of miscarriage for different cutoff values of MSD with and without a yolk sac and CRL. Another aim of the study was to establish cutoff values for CRL and MSD that can be used based on a single measurement to make a definitive diagnosis of miscarriage. Data were collected prospectively for 1060 women with intrauterine pregnancy of uncertain viability (IPUV) from teaching hospitals in London. IPUV was defined based on ultrasound findings as an intrauterine sac of <20 mm or <30 mm MSD or an embryo with CRL of <6 mm or <8 mm with no fetal heartbeat.
The study end point was viability of the pregnancy between 11 and 14 weeks at the time of the routine nuchal translucency scan. The sensitivity, specificity, and positive and negative predictive values were calculated using the following range of potential cutoff values to define miscarriage: 8 to 30 mm for MSD with or without yolk sac and 3 to 8 mm for CRL. Among the 1060 women with a diagnosis of IPUV, 473 (44.6%) fetuses remained viable and 587 (55.4%) were nonviable by the time of the 11- to 14-week scan. With the lack of visualization on ultrasound of both embryo and yolk sac, the FPR for miscarriage was 4.4%, using an MSD cutoff of 16 mm and 0.5% for a cutoff of 20 mm. No false-positive cases were found when the MSD was ≥21 mm. When a yolk sac was present but no embryo visualized, the FPR was 2.6% for an MSD cutoff of 16 mm and 0.4% for a cutoff of 20 mm, with no false-positive cases using an MSD cutoff of ≥21 mm. For a visible embryo without a heartbeat, the FPR for miscarriage using a CRL cutoff of 4 mm or 5 mm was 8.3%. No false-positive results were found using a CRL cutoff of ≥5.3 mm.
These findings indicate that inadvertent termination of wanted pregnancies may occur using current definitions for miscarriage. The data support the introduction of an MSD cutoff of >25 mm and a CRL cutoff of >7 to minimize the risk of a false-positive diagnosis.