Having a positive pregnancy test but being told that your pregnancy cannot be seen on ultrasound can be worrying. You may wonder whether the pregnancy is simply too early to see, whether you have miscarried or whether it could be developing outside the uterus.
This situation is called a pregnancy of unknown location (PUL). It means that a pregnancy test is positive, but the ultrasound examination has not identified a definite pregnancy either inside or outside the uterus.
A pregnancy of unknown location is not a diagnosis of miscarriage or ectopic pregnancy. It describes a situation in which further assessment is needed to establish where the pregnancy is located and whether it is developing normally.
Key points
- A pregnancy of unknown location is diagnosed when a woman has a positive pregnancy test, but a transvaginal ultrasound does not demonstrate a definite intrauterine or extrauterine pregnancy.
- The most common reason is that the ultrasound examination has been performed too early.
- The next steps depend on your symptoms, ultrasound findings and pregnancy hormone levels.
- The likelihood of seeing a gestational sac within the uterus generally increases as the hCG level rises.
What is a pregnancy of unknown location?
A pregnancy of unknown location is diagnosed when a woman has a positive pregnancy test, but a transvaginal ultrasound does not demonstrate a definite intrauterine or extrauterine pregnancy. There are three main possible explanations.
1. The pregnancy is developing normally but is too early to see
This is common when the scan is performed very early, particularly before five to six weeks of pregnancy. The pregnancy may be developing within the uterus, but the gestational sac is still too small to identify reliably.
2. The pregnancy has stopped developing
Sometimes a very early miscarriage occurs before a pregnancy has become visible on ultrasound. This is often called a biochemical pregnancy or a very early pregnancy loss. The pregnancy hormone level may subsequently fall, and the pregnancy may resolve without any treatment.
3. The pregnancy is developing outside the uterus
An ectopic pregnancy occurs when the pregnancy implants outside the uterine cavity, most commonly within a fallopian tube. An ectopic pregnancy may initially be too small to identify on ultrasound. This is why careful follow-up is essential until the pregnancy’s location has been established or the pregnancy has resolved.
Why might my pregnancy not be visible on ultrasound?
The most common reason is that the ultrasound examination has been performed too early.
Pregnancy dates are usually calculated from the first day of the last menstrual period, assuming a 28-day cycle and ovulation occurring on approximately the 14th day. However, ovulation may occur earlier or later than expected, even in women with regular menstrual cycles.
As a result, a pregnancy thought to be five or six weeks may be several days younger. During the earliest stages, this difference can significantly affect what is visible. A gestational sac may first become visible on transvaginal ultrasound at approximately four and a half to five weeks, but this varies. An abdominal ultrasound is generally less sensitive during these very early stages.
Not seeing a pregnancy on an early scan does not automatically mean that something is wrong. However, the possibility of an ectopic pregnancy must remain under consideration until further assessment clarifies the situation.
Read more: What Can You See on a Pregnancy Scan at 5, 6, 7 and 8 Weeks?
What happens after a pregnancy of unknown location is diagnosed?
The next steps depend on your symptoms, ultrasound findings and pregnancy hormone levels.
If you are clinically well, your clinician will usually arrange blood tests to measure human chorionic gonadotrophin (β-hCG).
Two measurements are commonly taken approximately 48 hours apart. The change in β-hCG helps guide the timing of further assessment. Your clinician should also explain which symptoms require urgent medical attention and provide a clear plan for follow-up. Follow-up is needed until the location is established or the pregnancy has been shown to resolve.
Importantly, a pregnancy of unknown location should not be considered resolved simply because bleeding has stopped or you feel well.
How are hCG blood tests used?
β-hCG is a hormone produced during pregnancy.
During the earliest weeks of a normally developing pregnancy, levels generally rise rapidly. However, the rate of increase varies considerably. In women with a pregnancy of unknown location, the recommendation is to measure serum β-hCG approximately 48 hours apart to help guide further management. The results are interpreted according to how much the hormone level has increased or decreased.
If hCG rises by more than 63% over 48 hours
A developing intrauterine pregnancy is likely, although an ectopic pregnancy cannot be excluded. In this scenario, a repeat transvaginal ultrasound should be done approximately 7–14 days later to identify the pregnancy’s location. An earlier scan may be considered when the hCG level is at least 1,500 IU/L.
If hCG falls by more than 50% over 48 hours
The pregnancy is unlikely to continue. If you are clinically well, the recommendation is to do a urine pregnancy test 14 days after the second blood test. If this is negative, no further follow-up is usually required. If it remains positive, you should return for clinical review.
If hCG rises by less than 63% or falls by less than 50%
The result does not provide sufficient reassurance about how the pregnancy is developing. The recommendation is for a clinical review within 24 hours to determine the next steps, which may include further blood tests, ultrasound examinations or specialist assessment as necessary.
These thresholds guide follow-up. They do not confirm the pregnancy’s location or independently diagnose miscarriage or ectopic pregnancy.
Read more: Understanding hCG Levels in Early Pregnancy
Can a normal rise in hCG exclude an ectopic pregnancy?
No.
Although a substantial rise in hCG makes a developing intrauterine pregnancy more likely, some ectopic pregnancies also demonstrate increasing hormone levels. Similarly, a falling hCG level does not completely exclude an ectopic pregnancy.
This is why the hormone results must be interpreted alongside your symptoms and ultrasound findings.
Your clinical symptoms are more important than the hCG result when deciding whether urgent assessment is necessary. Even if your blood tests initially appear reassuring, new or worsening abdominal pain, dizziness or fainting requires prompt medical attention.
Read more: Ectopic Pregnancy: Symptoms, Ultrasound and Diagnosis
At what hCG level should a pregnancy be visible on ultrasound?
The likelihood of seeing a gestational sac within the uterus generally increases as the hCG level rises. Historically, clinicians have used an hCG discriminatory level, above which an intrauterine pregnancy would usually be expected to be visible on transvaginal ultrasound.
However, there is no single hCG concentration at which every normally developing intrauterine pregnancy must be visible. Visibility depends on gestational age, ultrasound equipment, the quality of the examination and individual pregnancy characteristics.
A pregnancy should not be diagnosed as ectopic solely because no gestational sac is visible above a particular hCG level. Where the findings are uncertain, repeat ultrasound and appropriate clinical follow-up are essential.
Can progesterone blood tests help?
Progesterone is a hormone that supports the lining of the uterus during early pregnancy.
Progesterone levels are often higher in normally developing pregnancies and lower when a pregnancy is failing. However, there is considerable overlap between the levels seen in viable pregnancies, miscarriages and ectopic pregnancies.
A low progesterone level may suggest that a pregnancy is unlikely to continue, but it cannot determine whether the pregnancy is located inside or outside the uterus.
For this reason, NICE recommends that serum progesterone measurements should not be used as an additional test to diagnose a viable intrauterine pregnancy or an ectopic pregnancy in women with a pregnancy of unknown location who are undergoing serial hCG testing.
Progesterone blood tests cannot replace ultrasound and appropriate follow-up.
What if the repeat ultrasound still does not show a pregnancy?
Sometimes the location remains uncertain after the first repeat scan.
The clinician will review the change in hCG, any bleeding or pain, and the ultrasound findings before deciding what further assessment is needed. Possible outcomes include:
- A developing intrauterine pregnancy. A gestational sac becomes visible within the uterus, followed by the yolk sac and embryo as the pregnancy develops.
- A resolving pregnancy of unknown location. The pregnancy hormone gradually falls and eventually becomes negative without the location ever being identified. This is sometimes called a failed pregnancy of unknown location.
- An ectopic pregnancy. Further ultrasound findings or clinical assessment may establish that the pregnancy is developing outside the uterus.
- A persistent pregnancy of unknown location. The pregnancy remains unlocated and the hCG level does not resolve as expected. This requires specialist assessment because a small ectopic pregnancy remains a possibility. Management depends on the clinical circumstances and may involve continued monitoring or treatment when indicated.
Is a pregnancy of unknown location more common after IVF?
A pregnancy of unknown location may also occur following IVF or other fertility treatments. The timing of embryo transfer provides more accurate information about gestational age than menstrual dates alone. However, an ultrasound performed very early may still be unable to identify the pregnancy.
An ectopic pregnancy remains possible following IVF, even when an embryo has been transferred directly into the uterus.
Rarely, a pregnancy may develop both inside and outside the uterus at the same time. This is called a heterotopic pregnancy and is more common following assisted reproductive treatment than in spontaneous pregnancies.
Women who conceive following IVF should therefore have appropriate early ultrasound assessment and follow-up if the pregnancy’s location remains uncertain.
When should I seek urgent medical attention?
Most women with a pregnancy of unknown location can undergo planned follow-up while they remain clinically well. However, an ectopic pregnancy cannot be excluded until the pregnancy’s location is established or the pregnancy has resolved.
Seek urgent medical assessment if you experience:
- Severe or worsening abdominal or pelvic pain.
- Significant pain on one side of the lower abdomen.
- Shoulder-tip pain.
- Heavy vaginal bleeding.
- Dizziness, fainting or feeling unusually weak.
These symptoms may indicate internal bleeding or a ruptured ectopic pregnancy.
If you experience collapse, severe pain or heavy bleeding with faintness, call 999 or attend an Emergency Department immediately.
Do not wait for your next scheduled blood test or ultrasound if your symptoms change.
Understanding your pregnancy of unknown location
A pregnancy of unknown location means that a pregnancy test is positive, but the pregnancy cannot yet be identified on ultrasound.
In many cases, this occurs because the pregnancy is too early to see. In others, it represents a very early pregnancy loss or an ectopic pregnancy.
The purpose of follow-up is to establish the pregnancy’s location, assess its development and identify any complications as early as possible.
The most important principle is that a pregnancy of unknown location requires appropriate follow-up until the pregnancy is located or has safely resolved.
Serial hCG measurements, repeat ultrasound examinations and careful attention to symptoms provide the safest approach while the diagnosis remains uncertain.
Related pregnancy guides
Frequently asked questions
What is a pregnancy of unknown location?
A pregnancy of unknown location is diagnosed when a woman has a positive pregnancy test, but a transvaginal ultrasound does not demonstrate a definite intrauterine or extrauterine pregnancy. There are three main possible explanations.
Why might my pregnancy not be visible on ultrasound?
The most common reason is that the ultrasound examination has been performed too early.
What happens after a pregnancy of unknown location is diagnosed?
The next steps depend on your symptoms, ultrasound findings and pregnancy hormone levels.
How are hCG blood tests used?
β-hCG is a hormone produced during pregnancy. During the earliest weeks of a normally developing pregnancy, levels generally rise rapidly.
Can a normal rise in hCG exclude an ectopic pregnancy?
No. Although a substantial rise in hCG makes a developing intrauterine pregnancy more likely, some ectopic pregnancies also demonstrate increasing hormone levels.
At what hCG level should a pregnancy be visible on ultrasound?
The likelihood of seeing a gestational sac within the uterus generally increases as the hCG level rises. Historically, clinicians have used an hCG discriminatory level, above which an intrauterine pregnancy would usually be expected to be visible on transvaginal ultrasound.
Early pregnancy care at The Pregnancy Clinic
At The Pregnancy Clinic, we provide consultant-led early pregnancy ultrasound examinations, including assessment when the location of a pregnancy is uncertain.
Where appropriate, serum β-hCG testing and follow-up ultrasound examinations can help clarify pregnancy development and guide further care.
Our approach is to explain the findings clearly, provide individualised advice and ensure that women requiring further investigation or urgent NHS assessment are directed to the appropriate service.
Book an early pregnancy consultation → Book an early pregnancy scan →The Pregnancy Clinic is not an emergency service. A pregnancy of unknown location requires reliable follow-up arrangements and access to urgent NHS assessment if symptoms develop. If you experience severe pain, heavy bleeding, fainting or other acute symptoms, please seek emergency medical attention.