A miscarriage is defined as the spontaneous loss of a pregnancy before the 20th week of gestation. For individuals experiencing symptoms like bleeding or cramping, or for those undergoing a routine first-trimester appointment, the ultrasound examination serves as the primary and most reliable tool for diagnosis. This imaging technique uses sound waves to create a real-time picture of the uterus, allowing healthcare providers to assess the viability of the pregnancy and determine the precise state of development. Clear and standardized visual criteria on the ultrasound are used to confirm whether a pregnancy is progressing normally or if a loss has occurred.
Primary Visual Signs of Pregnancy Loss
The ultrasound assesses the viability of the pregnancy by looking for normal signs of development. The most definitive observation is the lack of visible fetal heart activity at a stage when it should be easily detectable. Typically, a heartbeat can be seen via transvaginal ultrasound as early as six weeks gestation, often when the embryo measures just a few millimeters long.
When an embryo is clearly visualized, its length, known as the Crown-Rump Length (CRL), is measured and compared to the expected size for the gestational age. A significant discrepancy, where the embryo is much smaller than anticipated, indicates developmental arrest or growth stagnation. Another sign is a gestational sac that appears distorted, irregularly shaped, or positioned too low within the uterine cavity.
The structures within the gestational sac also offer important visual information about the pregnancy’s health. The yolk sac, which provides nutrition to the developing embryo, can appear abnormal if it is too large, too small, or has an irregular shape. Similarly, if the amnion, the membrane surrounding the embryo, is visible but appears empty, this finding, sometimes called the “empty amnion sign,” is highly suspicious for a failure of the embryo to develop.
Types of Miscarriage Defined by Ultrasound Findings
The specific appearance on the ultrasound, combined with a patient’s symptoms, determines the clinical classification of the pregnancy loss. A missed miscarriage, also known as an embryonic demise or silent miscarriage, is diagnosed when the ultrasound reveals a non-viable embryo or gestational sac, but the patient has not yet experienced bleeding or cramping. In this scenario, the body has not recognized or begun the process of expelling the tissue.
Conversely, an incomplete miscarriage is identified when the uterus still contains retained products of conception (tissue) after the process of loss has begun. On the ultrasound, this appears as heterogeneous or mixed-echogenicity tissue within the uterine cavity, often accompanied by active bleeding. This remaining material distinguishes it from other types of loss.
A complete miscarriage is diagnosed when the ultrasound shows an empty uterus with a thin, normal-appearing endometrial lining, indicating that the uterine contents have been fully expelled. This diagnosis is typically made after a patient has passed tissue and the bleeding has subsided. The term threatened miscarriage is used when a patient experiences symptoms like vaginal bleeding, but the ultrasound confirms that the embryo still has a detectable, normal heart rate, meaning the pregnancy is still viable.
Definitive Diagnostic Measurements and Thresholds
International medical bodies rely on specific, conservative measurements to definitively confirm a miscarriage. The most widely accepted size-based criterion for a non-viable pregnancy is a Crown-Rump Length (CRL) of 7 millimeters or greater without any visible cardiac activity. This measurement must be taken via transvaginal ultrasound.
Another definitive threshold involves the gestational sac itself. If the Mean Sac Diameter (MSD), the average of three internal measurements of the sac, is 25 millimeters or larger and no embryo is visible inside, the pregnancy is non-viable. These strict cut-off values ensure a 100% specificity, meaning a viable pregnancy cannot meet these criteria.
When the initial scan results fall just below these definitive thresholds—for example, a CRL of 5 millimeters without a heartbeat—the pregnancy is considered to be of uncertain viability. In these instances, a follow-up scan is mandated to confirm the diagnosis, often referred to as the “Two-Week Rule.” For a definitive diagnosis, the follow-up scan must show no growth or no development of a heartbeat after 7 to 14 days, depending on the initial findings.
Next Steps Following Ultrasound Confirmation
Once the ultrasound has definitively confirmed a non-viable pregnancy, the patient and healthcare provider discuss management options. Expectant management involves waiting for the body to naturally pass the retained tissue, a process that can take days to weeks. This is often the preferred option for patients who wish to avoid medical intervention.
Medical management uses oral or vaginal medication, typically misoprostol, to induce uterine contractions and expedite the process of expelling the tissue. This option is chosen when expectant management fails or if the patient prefers a more predictable timeline. The third option is surgical management, most commonly a procedure called dilation and curettage (D&C), which involves the surgical removal of the tissue from the uterus.
The choice among these options depends on the specific ultrasound findings, the gestational age, the patient’s clinical stability, and her personal preference. A final follow-up, sometimes including another ultrasound, is often recommended to ensure the uterus has emptied completely.