A blighted ovum, also known as an anembryonic gestation, is an early pregnancy diagnosis that can be difficult for expectant parents. This diagnosis often leads to questions about the accuracy of the initial medical assessment. Understanding the diagnostic process and the potential for error is important. This article details the biological basis of the condition, the methods used to identify it, and the medical protocols in place to prevent misdiagnosis during the earliest stages of pregnancy.
Understanding Blighted Ovum
A blighted ovum is a type of early miscarriage where a fertilized egg implants in the uterine wall, and surrounding structures begin to form, but the embryo itself either never develops or stops growing very early. This condition is medically termed an anembryonic pregnancy, meaning a pregnancy without an embryo. Chromosomal abnormalities in the fertilized egg are the most common underlying cause, leading the body to recognize the non-viable nature of the pregnancy.
The gestational sac and placenta-forming tissues continue to develop for a time, producing high levels of the pregnancy hormone human chorionic gonadotropin (hCG). This hormone is detected by home pregnancy tests. Because of this hormonal production, a patient may experience typical early pregnancy symptoms, such as nausea and breast tenderness, despite the absence of a developing embryo. A positive pregnancy test result can persist for several weeks, masking that the pregnancy is not progressing normally.
Diagnostic Criteria and Tools
The primary tool used to diagnose an anembryonic pregnancy is transvaginal ultrasound, which offers a detailed view of intrauterine structures in early gestation. This imaging technique is essential for visualizing the contents of the gestational sac. During a scan, a medical professional looks for the gestational sac and attempts to identify a fetal pole, the earliest visual evidence of the embryo.
A blighted ovum is suggested when the gestational sac is visible but appears empty, lacking an embryo or yolk sac. Specific size thresholds guide the initial assessment. If a transvaginal ultrasound reveals a mean gestational sac diameter (MSD) above a certain size, yet no embryo is visible, this raises suspicion of an anembryonic gestation. These measurements categorize the pregnancy’s viability and determine the need for further evaluation.
Factors Leading to Misdiagnosis
The possibility of misdiagnosis arises mainly due to two variables: the timing of the ultrasound and anatomical considerations. The most frequent reason for an inaccurate initial diagnosis is performing the scan too early in the gestational timeline. Before six or seven weeks, the embryo may be too small to be definitively visualized, even with high-resolution transvaginal equipment.
If the pregnancy dating is slightly inaccurate, a scan performed at six weeks may actually be closer to five weeks, where the embryo is not yet expected to be visible. A miscalculation of only a few days can significantly impact the structures discernible on the screen. Additionally, the patient’s anatomy can complicate image acquisition. A retroverted, or tilted, uterus can sometimes obscure the view of the gestational sac, making accurate measurement and visualization of early embryonic structures challenging. This compromised image clarity increases the potential for an incorrect reading of an empty sac.
Protocols for Definitive Confirmation
Medical professionals follow specific protocols developed to prevent the premature and incorrect diagnosis of a blighted ovum. These guidelines emphasize a “wait and rescan” approach rather than relying on a single initial ultrasound. The American College of Obstetricians and Gynecologists (ACOG) has established strict criteria that must be met before a diagnosis of early pregnancy loss is considered definitive.
A primary criterion for a definitive diagnosis is an empty gestational sac with a Mean Sac Diameter (MSD) of \(25 \text{ mm}\) or more on a transvaginal scan. If the initial MSD is below this threshold, or if there is doubt about the presence of a developing embryo, a follow-up scan is mandated. This rescan must occur after a required waiting period, typically \(7\) to \(14\) days, to allow sufficient time for a viable embryo to become visible. For instance, if an initial scan shows a gestational sac but no yolk sac, the absence of an embryo on a repeat scan two weeks later confirms pregnancy failure. These mandatory waiting periods safeguard against terminating a viable, slow-starting pregnancy due to a hasty diagnosis.