A Pregnancy of Unknown Location (PUL) is a temporary designation given when a pregnancy test is positive, but the location of the developing pregnancy cannot be confirmed via ultrasound scan. This status indicates the pregnancy is either too early to be visualized or has not implanted in the expected place. PUL is common in early pregnancy clinics and requires careful follow-up to determine the ultimate outcome. A PUL is not automatically an ectopic pregnancy; the classification is used until the true location and viability are established.
How a Pregnancy of Unknown Location is Identified
Identifying a PUL involves correlating transvaginal ultrasound results with quantitative blood tests for human chorionic gonadotropin (hCG). Healthcare providers measure the amount of hCG in the bloodstream; in a healthy, progressing pregnancy, this level is expected to increase by at least 63% over 48 hours. This data is compared against the “discriminatory zone,” which is the hCG level where a gestational sac should typically be visible within the uterus via ultrasound. This zone is set between 1,500 and 2,000 mIU/L. A PUL diagnosis is assigned if the hCG level is above this threshold, yet no sac is seen inside the uterus. If the hCG level is below the discriminatory zone, the pregnancy is considered too early to visualize, and the PUL designation is maintained until repeat testing.
The Three Possible Outcomes of a PUL
Once the status of a PUL is resolved through subsequent testing, it falls into one of three categories.
Failed PUL
The most common outcome is a Failed PUL, accounting for approximately 50% to 70% of cases. This represents a very early miscarriage or biochemical pregnancy where hCG levels naturally fall to zero without medical intervention.
Confirmed Intrauterine Pregnancy (IUP)
The second possibility is a confirmed Intrauterine Pregnancy (IUP), occurring when the pregnancy was too early to be seen on the initial scan. This outcome is confirmed when a follow-up ultrasound visualizes the gestational sac within the uterine cavity. A viable IUP means the pregnancy is developing normally and can proceed to routine prenatal care.
Ectopic Pregnancy
The final outcome is an Ectopic Pregnancy, occurring in about 7% to 20% of PUL cases. This condition is a medical concern because the fertilized egg has implanted outside the uterus, most often in a fallopian tube. An ectopic pregnancy cannot survive and poses a risk due to the potential for rupture and internal bleeding.
Ongoing Monitoring and Management
The standard protocol for managing a PUL focuses on sequential testing until the location is confirmed. Patients are asked to return for a repeat quantitative hCG blood test 48 hours after the initial measurement. The rate of change in the hCG level provides the first indicator of the pregnancy’s viability and location.
A rapidly increasing hCG level suggests a developing intrauterine pregnancy, prompting a repeat ultrasound scan in about one week. Conversely, a declining hCG level indicates a failed PUL or early miscarriage, and monitoring continues until the hormone level is undetectable. If the hCG level rises too slowly or plateaus, an ectopic pregnancy is suspected, even if not yet visible on an ultrasound.
If an ectopic pregnancy is confirmed or suspected, management options include expectant monitoring, medical treatment with medication like methotrexate, or surgical intervention. Expectant management is reserved for stable patients with a low risk of complications. Patients are given clear instructions on symptoms that require immediate emergency care.