A blighted ovum, medically known as anembryonic pregnancy, is a common cause of first-trimester miscarriage. This condition occurs when a fertilized egg implants successfully, and the gestational sac begins to form, but an embryo never develops inside. Because the body registers the implantation, individuals often experience a positive pregnancy test and early symptoms like nausea or breast tenderness. The subsequent diagnosis of an anembryonic pregnancy can therefore be confusing and emotionally difficult, as the body signals a pregnancy that does not contain a developing fetus.
The Source of HCG in Anembryonic Pregnancy
The presence of human chorionic gonadotropin (HCG) in a blighted ovum relates directly to the initial stages of implantation. HCG is produced by the trophoblast, the layer of cells surrounding the fertilized egg, not by the embryo itself. This tissue attaches the egg to the uterine wall and eventually develops into the placenta and the gestational sac.
The trophoblast must be active and multiplying for implantation and sac formation to occur. As these cells proliferate, they secrete HCG into the bloodstream, causing a positive pregnancy test result. This hormonal activity is what causes the symptoms of early pregnancy, such as missed periods and morning sickness, even when a fetus is absent.
The trophoblast tissue continues to function until the body recognizes the pregnancy development is abnormal. Because the HCG-producing tissue is present, hormone levels rise initially, mimicking the start of a healthy pregnancy. The ultimate trajectory of the HCG level, however, differs significantly from a viable pregnancy.
HCG Levels: The Typical Pattern and Trajectory
The trajectory of HCG levels in an anembryonic pregnancy involves an initial rise followed by a failure to maintain the expected growth rate, eventually plateauing and declining. In a viable pregnancy, HCG levels are expected to roughly double every 48 to 72 hours during the first few weeks. This rapid doubling marks a healthy, progressing pregnancy.
With a blighted ovum, the rate of increase slows down compared to a normal gestation. Although levels initially rise, they frequently fail to meet the minimal expected increase, often cited as at least a 53% rise over 48 hours. This slower rate of increase is often the first indicator of a potential issue when serial HCG testing is performed.
In a normal, singleton pregnancy, HCG levels typically peak between 50,000 to 100,000 mIU/mL around eight to ten weeks of gestation before naturally leveling off. In an anembryonic pregnancy, the level often fails to reach this normal peak. Instead, the HCG level rises to a lower, variable point before the lack of a developing embryo causes the trophoblast tissue to fail.
The maximum level reached is highly variable, depending on how long the trophoblast continues to function before the body recognizes the non-viable state. HCG levels typically begin to drop spontaneously between the eighth and thirteenth weeks of pregnancy, often when the body begins the process of miscarriage. This eventual decline signals the body’s recognition of the non-viable pregnancy and the cessation of HCG production.
Clinical Monitoring and Confirmation of Diagnosis
HCG levels alone are never used to definitively diagnose a blighted ovum; they serve primarily as a screening and monitoring tool. Diagnosis requires confirmation through a transvaginal ultrasound, which provides a direct visual assessment of the uterus. A conclusive diagnosis is typically made when HCG levels indicate that a fetal pole or embryo should be clearly visible on the scan, but the gestational sac remains empty.
Specific ultrasound criteria are relied upon for a definitive diagnosis. If a transvaginal ultrasound reveals a gestational sac with a mean sac diameter (MSD) of 25 millimeters or greater, and no embryo is visible, this confirms an anembryonic pregnancy. This measurement is used because a sac of this size should universally contain a visible embryo in a viable pregnancy. If the gestational sac is smaller than 25 millimeters, clinicians often order a follow-up ultrasound a week to ten days later to avoid a premature diagnosis due to uncertain dating.
Serial HCG testing—measuring the hormone level multiple times over a few days—monitors the viability of the pregnancy, especially when the initial ultrasound is inconclusive. After a blighted ovum is diagnosed and resolved, whether through expectant management, medication, or a procedure, HCG levels are monitored until they return to a non-pregnant level, typically below 5 mIU/mL. This follow-up testing ensures that all the HCG-producing tissue has been cleared from the uterus, confirming the resolution of the pregnancy.