A diagnosis of Pregnancy of Unknown Location (PUL) is a temporary classification given to a person with a confirmed pregnancy test but whose pregnancy cannot be visualized on an initial ultrasound scan. This situation is common in very early pregnancy. The term simply describes an inconclusive finding at a single point in time, requiring follow-up tests to determine the final outcome. The subsequent diagnostic process aims to swiftly resolve the status, ultimately classifying the pregnancy as either developing normally, miscarrying, or growing outside the uterus.
Defining a Pregnancy of Unknown Location
A Pregnancy of Unknown Location is strictly a descriptive term, not a final diagnosis. It is applied when a blood test confirms the presence of human chorionic gonadotropin (HCG), but a transvaginal ultrasound does not show a definitive pregnancy sac either inside or outside the uterus. The fundamental criteria for a PUL are a positive pregnancy test and a non-diagnostic ultrasound.
The inability to visualize the pregnancy on the first scan can be due to one of three main possibilities. The most common reason is that the pregnancy is simply too early to be seen. This means it is an early, normal intrauterine pregnancy that has not yet grown large enough for the ultrasound to detect. A developing gestational sac is typically only visible when HCG levels reach a certain threshold, often termed the discriminatory zone, which is generally around 1,500 to 2,000 mIU/mL.
Another possibility is a failing pregnancy or very early miscarriage, where the gestational tissue has passed, but HCG hormone levels remain temporarily detectable in the blood. The third, and most serious, possibility is an ectopic pregnancy, where the fertilized egg has implanted outside the main cavity of the uterus, most often in the fallopian tube. Even an ectopic pregnancy may not be visible on a very early ultrasound, which is why the PUL classification necessitates careful follow-up.
The Diagnostic Process
To move beyond the temporary classification of PUL, medical professionals rely on serial monitoring. This strategy primarily tracks the quantitative levels of HCG in the blood. This involves taking blood samples, typically 48 hours apart, to measure the change in the hormone level over time. The pattern of this change provides strong clues about the pregnancy’s location and viability.
In a healthy, developing pregnancy within the uterus, the HCG level should increase rapidly, generally doubling approximately every 48 to 72 hours in the first four to six weeks. A minimal rise of at least 35% over 48 hours can still be considered normal for a successful intrauterine pregnancy. A rise that is significantly slower than this expected doubling rate raises suspicion for an abnormal pregnancy.
If the HCG trend suggests a normal trajectory, a repeat transvaginal ultrasound is scheduled, usually one to two weeks later. This allows the gestational sac time to grow large enough for visualization. If the HCG level is decreasing, it suggests a failing pregnancy, and monitoring continues until the hormone level returns to zero.
Potential Outcomes and Next Steps
The serial monitoring of HCG levels and repeat ultrasound scans eventually resolve the status of a Pregnancy of Unknown Location into one of three definitive outcomes.
Confirmed Intrauterine Pregnancy (IUP)
The most desired outcome is a confirmed Intrauterine Pregnancy (IUP). This is diagnosed when a subsequent ultrasound visualizes a gestational sac, and potentially an embryo, inside the uterine cavity. This confirms that the initial PUL diagnosis was simply due to the pregnancy being too early to detect visually.
Spontaneously Resolving or Failing Pregnancy
The second common outcome is a Spontaneously Resolving or Failing Pregnancy, which represents a miscarriage. This is indicated by HCG levels that show a significant, rapid decline. Medical management typically involves expectant monitoring until the HCG level is confirmed to be zero, indicating the complete resolution of the pregnancy.
Ectopic Pregnancy
The third, and most urgent, outcome is an Ectopic Pregnancy. This occurs when the rising HCG levels do not follow the normal doubling pattern, instead rising slowly, plateauing, or falling at an abnormally slow rate. This atypical trend, combined with the continued absence of an intrauterine sac, strongly suggests the pregnancy is growing in an unsafe location outside the uterus. Because an ectopic pregnancy can become life-threatening, immediate treatment is necessary. Management options include administration of the medication methotrexate, which stops the growth of the pregnancy, or surgical intervention, often a laparoscopy, to remove the ectopic tissue.