Confirming a nonviable pregnancy can be disheartening for both physicians and patients, especially after months of IVF cycles or other attempts to conceive. Patients undoubtedly experience high levels of stress and anxiety when trying to achieve a successful pregnancy, and the wait to confirm one is perhaps even more fraught.
Standards for making this diagnosis differ globally and periodically evolve, most likely because the process of determining viability is not always straightforward. Initial results can be misleading, which underscores the need to perform careful ultrasound measurements and repeat examinations so that viable pregnancies do not end in termination.
In a viable pregnancy, an intrauterine gestational sac can be visualized, and contains an embryo with heart activity. The American College of Obstetricians and Gynecologists (ACOG) defines early pregnancy loss as "a nonviable, intrauterine pregnancy with either an empty gestational sac or a gestational sac containing an embryo or fetus without fetal heart activity" identified within the first 12 to 13 weeks of gestation.
According to a study published in The Obstetrician and Gynaecologist, complications peak around 8 to 11 weeks of pregnancy. The authors point out that using diagnostic criteria is vital for avoiding a false positive miscarriage result. Early scans may not indicate viability, and physicians should use caution to avoid terminating either an early pregnancy or a "slow-growing pregnancy" without subsequent monitoring, including additional scans at intervals of 10 to 14 days.
US Standards for Unknown Viability and Failed Pregnancy
In the United States, criteria for determining pregnancy viability are based on a consensus conference by the Society of Radiologists in Ultrasound. ACOG reiterates these criteria in its practice bulletin on early pregnancy loss.
Unknown viability is determined with the following criteria:
- The gestational sac is visualized and measures at less than 25 mm.
- Either the yolk sac or the embryonic pole can be seen, or neither can be seen.
Failed pregnancies are determined with the following criteria:
- Embryo crown-rump length (CRL) is more than 7 mm but there is no heartbeat.
- The gestational sac diameter is more than 25 mm but there is no embryo.
- A second scan after 2 weeks reveals no heartbeat and no yolk sac was visualized previously.
- A second scan after 11 or more days reveals no heartbeat and gestational sac and yolk sac were visualized previously.
UK and Global Standards for Nonviability
The National Institute for Health and Care Excellence (NICE) recommends the following diagnostic criteria when determining pregnancy nonviability:
- If there is no cardiac activity, look for the fetal pole and measure the crown-rump length. Whether the CRL is less than or greater than 7 mm, a second scan should be performed at least 7 days later. If abdominal, rather than transvaginal, ultrasound is used, the second scan should be scheduled for at least 14 days later.
- If a gestational sac is visible but not a fetal pole, a second scan should be performed at least 7 days later, regardless of whether the gestational sac is greater or less than 25 mm in diameter. When using abdominal ultrasound, the second scan should be scheduled for at least 25 days later.
Elsewhere in Europe, the European Society for Human Reproduction and Embryology references guidelines from the Royal College of Obstetricians and Gynaecologists, which are based on the NICE recommendations. A study of pregnancy viability published in the Journal of Ultrasound in Medicine suggests that similar standards are used in both the EU and Japan.
The Role of Ultrasound in Confirming a Nonviable Pregnancy
Multiple sources stress that transvaginal, not transabdominal ultrasound, must be used when confirming a nonviable pregnancy, and measurement guidelines are based on transvaginal imaging. However, not all women will consent to a transvaginal ultrasound. If a patient declines a transvaginal scan, providers may want to refer to the UK guidance for transabdominal scans, keeping in mind that this imaging will have limitations. Additionally, ectopic and intra-abdominal pregnancies must be ruled out.
When an initial ultrasound shows that a pregnancy is nonviable, or even that its viability is unknown, providers need to know which differences to look for in the initial and follow-up scans.
One study published in The British Medical Journal reports that the ultrasound findings most consistent with a nonviable pregnancy at initial scan are:
- An empty gestational sac with a diameter of 25 mm or more.
- A CRL of 7 mm or more with no heart activity.
- A gestational sac diameter of 18 mm or more but no embryo after 70 days gestation.
- A CRL of 3 mm or more with no heart activity after 70 days gestation.
However, these findings require confirmation using a second scan. On repeat scan, the following were associated with nonviable pregnancy:
- An embryo with no heart activity after heart activity was not visible on a previous scan.
- No visible embryo, and a gestational sac that is less than 12 mm in diameter and has not doubled in size after 14 days.
- No visible embryo on initial or repeat scan, and a gestational sac that is 12 mm or more in diameter.
Because false positive cases had been reported under previous guidelines — pregnancies had been diagnosed as nonviable but were actually viable — the standards for making a diagnosis have changed in the past decade.
One specific reason for this change was to account for operator error and variability in taking ultrasound measurements. One study published in Ultrasound in Obstetrics and Gynecology found that gestational sac measurements can vary by as much as 18 percent in the same pregnancy if they are taken by different sonographers. As the authors point out, overestimating the gestational sac diameter on initial ultrasound and underestimating it on repeat ultrasounds could easily lead to the conclusion that the sac had not grown as expected.
ACOG suggests that physicians include their patients in the process of deciding on the next steps in the face of an inconclusive scan for viability. Patients should be informed of the possible consequences of watchful waiting, including "unwanted spontaneous passage of pregnancy tissue," anxiety and the need for emergency or last-minute procedures or appointments.
As infertility specialists well know, a positive pregnancy test does not automatically translate into a successful pregnancy. Physicians are urged to use caution and adhere to clinical guidelines when diagnosing a nonviable pregnancy, since the initial scan may be misleading.
