A Pregnancy of Unknown Location (PUL) is a temporary medical classification given when a person has a positive pregnancy test, but the location of the gestation cannot be confirmed with an initial ultrasound scan. This is a descriptive label used in the very early stages of pregnancy when the human chorionic gonadotropin (hCG) hormone is present in the blood or urine. The PUL classification means the pregnancy is either too early to be visualized, located outside the uterus, or has already failed. It serves as a starting point for further investigation to determine the ultimate status and location of the pregnancy.
Criteria for Classifying a Pregnancy of Unknown Location
The designation of a pregnancy as “unknown location” relies heavily on transvaginal ultrasound (TVS). TVS is the standard imaging technique in early pregnancy, providing the most detailed view of the uterus and surrounding structures. When a positive pregnancy test is confirmed, the sonographer attempts to locate the gestational sac, the first visible sign of an intrauterine pregnancy.
The determination of PUL is linked to the discriminatory zone for hCG. This zone is the level of the hCG hormone in the blood above which a gestational sac should be visible inside the uterus using TVS. A commonly accepted discriminatory zone is an hCG concentration of approximately 1500 mIU/mL.
If the patient’s hCG level is below this 1500 mIU/mL threshold, the lack of a visible pregnancy is expected, and the PUL classification is routine. However, if the hCG level is at or above the discriminatory zone and no intrauterine sac is seen, it raises immediate concern for an abnormal pregnancy, such as one located outside the uterus.
The Three Possible Outcomes of a PUL Diagnosis
Once a pregnancy is classified as a PUL, medical monitoring begins because the classification represents one of three distinct possibilities. The most benign possibility is an early intrauterine pregnancy (IUP), meaning the pregnancy is developing normally but is simply too small to be seen on the initial scan. This is common in individuals who are unsure of their last menstrual period or have irregular cycles.
Another possible outcome is a failed intrauterine pregnancy, often referred to as a complete miscarriage. In this scenario, the pregnancy has already ended, and the products of conception have been passed or are being resorbed, leaving no visible evidence on the ultrasound. The positive pregnancy test may linger because the hCG hormone takes time to clear from the system after the pregnancy has ceased.
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 a fallopian tube. This is a potentially life-threatening condition because the fallopian tube cannot support a growing pregnancy and can rupture. A PUL diagnosis requires close follow-up primarily to rule out this dangerous outcome.
Monitoring and Management of PUL
The goal of monitoring a PUL is to quickly and safely determine which of the three potential outcomes is occurring. This is achieved primarily through serial quantitative human chorionic gonadotropin (hCG) blood tests, taken approximately 48 hours apart. The change in the hCG level over this time indicates the pregnancy’s viability and location.
In a healthy, viable intrauterine pregnancy, the hCG concentration is expected to increase by at least 63% over 48 hours. A slower rise or a plateau suggests either a failing intrauterine pregnancy or an ectopic pregnancy. Conversely, a significant drop in hCG, typically 13% or more, is a strong indicator of a failing pregnancy or miscarriage.
If the hCG levels demonstrate a viable pattern, a follow-up transvaginal ultrasound is scheduled to confirm the presence of a gestational sac inside the uterus. If the levels indicate a likely failing pregnancy, the patient is monitored until the hCG levels return to zero. If the pattern is abnormal, suggesting a high risk for ectopic pregnancy, further intervention or treatment may be necessary.
Patients are also educated on specific warning signs to watch for during monitoring. Symptoms like severe abdominal pain, shoulder tip pain, or dizziness can signal a complication, such as a ruptured ectopic pregnancy, requiring immediate medical attention. The clinical presentation is prioritized over blood test results, as an ectopic pregnancy can become an emergency quickly.
Factors Increasing the Risk of Ectopic Pregnancy
Since an ectopic pregnancy is the most concerning outcome of a PUL, understanding the underlying risk factors is a significant part of patient care. A history of a previous ectopic pregnancy is the single strongest predictor, suggesting an underlying issue with the fallopian tubes. The risk of recurrence can be as high as 10% after one prior ectopic pregnancy.
Conditions that cause inflammation or scarring in the fallopian tubes significantly increase risk. This includes a history of Pelvic Inflammatory Disease (PID), often caused by untreated sexually transmitted infections like chlamydia or gonorrhea. Any prior surgery on the fallopian tubes, such as a tubal ligation, can also impair the tube’s function, increasing the chance of implantation there.
Other factors include smoking, which is thought to damage the cilia that move the egg through the tube, and the use of assisted reproductive technologies. While an intrauterine device (IUD) is a highly effective contraceptive, if a pregnancy occurs with an IUD in place, there is an increased likelihood that it will be ectopic.