Human Chorionic Gonadotropin (HCG) is the “pregnancy hormone” produced when a fertilized egg implants. It’s detected by home pregnancy tests. Sometimes, a positive test occurs, but a viable embryo doesn’t develop. This is known as a blighted ovum or an anembryonic pregnancy.
The Role of HCG in Pregnancy
HCG is a hormone produced by trophoblast cells, which form the placenta. It can be detected in blood around 11 days and in urine 12-14 days after conception. HCG’s primary function involves signaling the corpus luteum to produce progesterone, which maintains the uterine lining for the developing embryo.
In a healthy early pregnancy, HCG levels rise rapidly, doubling every 48 to 72 hours. This rapid rise continues until HCG peaks around 8 to 11 weeks of gestation. Levels then gradually decline and stabilize for the remainder of the pregnancy.
What is a Blighted Ovum
A blighted ovum, also known as an anembryonic pregnancy, is an early miscarriage where a fertilized egg implants and forms a gestational sac, but an embryo never develops. The fertilized egg either fails to grow into an embryo or stops developing very early and is reabsorbed by the body. Despite the absence of an embryo, the placental tissues, which include the trophoblast cells, continue to grow.
This continued growth explains why a woman might still experience pregnancy symptoms such as a missed period, morning sickness, and breast tenderness, as the placental tissue produces HCG. Blighted ova are a common cause of early pregnancy loss, accounting for about half of first-trimester miscarriages. They are often attributed to chromosomal abnormalities in the fertilized egg.
How HCG Levels Behave with a Blighted Ovum
With a blighted ovum, HCG levels rise because placental tissue is present and producing the hormone. This can lead to a positive pregnancy test and early pregnancy symptoms. The trophoblast cells, which form the early placenta, are responsible for HCG production even without an embryo.
However, the pattern of HCG rise often deviates from that of a healthy, progressing pregnancy. While HCG may initially rise, this increase can be slower than expected, indicating an abnormal trajectory. HCG levels may plateau or eventually decline. This suboptimal rise is often due to the lack of a developing embryo, which normally signals and supports optimal placental growth and hormone production.
HCG levels in a blighted ovum may not double within the typical 48 to 72 hours seen in viable pregnancies, or the doubling time might be significantly longer. For example, a rise of less than 66% over a two-day period in early pregnancy may suggest an issue. The overall pattern of HCG levels over several measurements, rather than a single value, provides more information about the pregnancy’s health. A declining HCG trend, or levels that fail to increase appropriately, can suggest a blighted ovum or other pregnancy complications.
Diagnosing a Blighted Ovum
Diagnosing a blighted ovum relies on HCG level trends and transvaginal ultrasound findings. HCG levels alone are not conclusive; healthcare providers conduct serial measurements over several days to assess the rate of increase or decrease.
A transvaginal ultrasound provides a detailed view of the uterus and its contents. With a blighted ovum, the ultrasound typically reveals an empty gestational sac, meaning it contains no visible fetal pole or yolk sac when they should be apparent. For instance, if a transvaginal ultrasound shows a gestational sac with a mean diameter greater than 25 mm without a visible embryo, it indicates an anembryonic pregnancy. An embryo should be visible on ultrasound by about six weeks after the last menstrual period. Follow-up ultrasounds confirm the absence of embryonic development, particularly if the initial scan is very early in gestation. This comprehensive approach, combining HCG trends with ultrasound imaging, allows for an accurate medical diagnosis.