01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Pelvic ultrasound can examine uterus, endometrium, ovaries, adnexa, bladder and pelvic fluid. Transvaginal imaging gives higher-resolution views of early intrauterine structures and adnexa; transabdominal imaging provides a wider field and is an alternative when an internal scan is declined or inappropriate. Consent, privacy and an offered chaperone according to policy matter.
In early pregnancy the core questions are location, viability and complication. An intrauterine gestational sac requires convincing features rather than a nonspecific fluid collection. An adnexal mass moving separately from the ovary, an extrauterine yolk sac or embryo and haemoperitoneum support ectopic pregnancy. Failure to see a pregnancy in either location produces a pregnancy of unknown location, not a final diagnosis.
Viability thresholds are deliberately conservative. Measurements, imaging route and repeat interval must be recorded. The mean gestational sac diameter and crown-rump length guide when absent structures become diagnostic, but stable uncertain cases are rescanned rather than rushed. Symptoms always take precedence because rupture can occur at low or falling hCG.
Key points
- Offer transvaginal ultrasound to identify pregnancy location and fetal pole or heartbeat; use transabdominal imaging when transvaginal scanning is declined or unsuitable and explain its limitations.
- Describe an empty uterus with a positive pregnancy test as pregnancy of unknown location until follow-up establishes intrauterine or ectopic location.
- In pregnancy of unknown location, prioritise changing symptoms over hCG values and give written twenty-four-hour emergency access advice.
- NICE advises two serum hCG measurements as near as possible to forty-eight hours apart, not earlier, to guide follow-up rather than determine location.
- With transvaginal CRL below 7 mm and no heartbeat, repeat scanning after at least seven days; conservative rules also apply to an empty small gestational sac.
- Never let serial testing delay emergency gynaecological review when the patient is unstable or rupture is clinically suspected.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
A yolk sac or embryo within an intrauterine gestational sac confirms location, while viability depends on cardiac activity and validated measurements.
An adnexal mass separate from the ovary, especially with yolk sac or embryo, strongly supports an extrauterine pregnancy.
Positive pregnancy testing with no pregnancy seen on transvaginal ultrasound remains potentially ectopic until location is established.
Moderate or large pelvic fluid, especially echogenic blood with pain or instability, raises concern for rupture.
An embryo or gestational sac below conservative diagnostic thresholds without expected cardiac activity or fetal pole requires repeat scanning.
Corpus luteum, haemorrhagic cyst, torsion and endometrioma can cause pain and adnexal findings; anatomy and Doppler are interpreted together.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Transvaginal ultrasoundFirst step - Why
- Identify early pregnancy location, embryo, heartbeat, adnexa and pelvic fluid.
- Interpretation and limitations
- NICE offers this assessment; consent is required and a negative scan may result in pregnancy of unknown location.
- 02
Transabdominal ultrasound - Why
- Provide a wider pelvic view or alternative when transvaginal scanning is unacceptable.
- Interpretation and limitations
- Lower early-pregnancy sensitivity requires explicit explanation and may necessitate repeat or specialist assessment.
- 03
Serial serum hCG - Why
- Guide follow-up intensity in pregnancy of unknown location.
- Interpretation and limitations
- Take two values as close as possible to forty-eight hours apart and not sooner. A rise greater than 63% leads to repeat transvaginal ultrasound in 7–14 days, with an earlier scan considered at hCG 1,500 IU/L or more; a fall greater than 50% leads to a urine pregnancy test 14 days after the second sample; a rise below 63% or fall below 50% requires early-pregnancy review within 24 hours. hCG never determines location or overrides symptoms.
- 04
Repeat transvaginal scan - Why
- Resolve location or viability after an appropriately timed uncertain study.
- Interpretation and limitations
- On transvaginal scanning, CRL below 7 mm without heartbeat or mean sac diameter below 25 mm without a fetal pole requires a repeat scan after at least 7 days; at or above either threshold, seek a second opinion and/or repeat after at least 7 days before diagnosis. If cardiac activity or a fetal pole is absent on transabdominal scanning, repeat after at least 14 days before diagnosis.
- 05
Full blood count and group - Why
- Assess blood loss and prepare for intervention when bleeding or ectopic pregnancy is possible.
- Interpretation and limitations
- A normal early haemoglobin does not exclude significant acute haemorrhage; physiology and scan findings remain decisive.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked casePain with empty uterusFirst stepA stable patient has pelvic pain, a positive pregnancy test and no pregnancy identified on transvaginal ultrasound.+
- 1Context: assess haemodynamics, bleeding, shoulder-tip pain, syncope, previous ectopic risk, gestational history and consent for scanning.
- 2Reasoning: classify pregnancy of unknown location, examine adnexa and free fluid and avoid inferring location from one hCG value.
- 3Outcome: obtain two hCG values about forty-eight hours apart and arrange repeat specialist assessment according to trend and symptoms.
- 4Verification: provide written emergency access advice and confirm a named early-pregnancy service owns follow-up until location is known.
02PUL routeLocate the pregnancy safelyPregnancy testing is positive but transvaginal ultrasound shows neither definite intrauterine nor ectopic pregnancy.+
- 1Explain that ectopic pregnancy remains possible, that hCG cannot establish location and that changing symptoms require direct twenty-four-hour review regardless of prior results.
- 2Arrange two serum hCG measurements as close as possible to forty-eight hours apart and not sooner.
- 3If hCG rises by more than 63%, arrange transvaginal ultrasound in 7–14 days and consider an earlier scan at 1,500 IU/L or more; if it falls by more than 50%, advise a urine pregnancy test 14 days after the second sample and review if positive; if it rises by less than 63% or falls by less than 50%, arrange early-pregnancy clinical review within 24 hours.
- 4Continue follow-up until an intrauterine pregnancy, ectopic pregnancy or resolved pregnancy is established, with symptoms always taking priority over hCG.
03Uncertain viabilityAvoid premature diagnosisAn intrauterine embryo or gestational sac lacks expected viability features but measurements are below diagnostic thresholds.+
- 1Confirm transvaginal versus transabdominal route and remeasure crown-rump length or mean sac diameter carefully; do not use last menstrual period alone to decide when cardiac activity must be visible.
- 2On transvaginal scanning, repeat after at least 7 days when CRL is below 7 mm without heartbeat or mean sac diameter is below 25 mm without a fetal pole.
- 3When transvaginal CRL is 7 mm or more without heartbeat, or mean sac diameter is 25 mm or more without a fetal pole, seek a second opinion and/or repeat after at least 7 days before diagnosis.
- 4If a transabdominal scan shows no heartbeat or no fetal pole, repeat after at least 14 days before diagnosis; explain that waiting for the repeat scan does not harm the pregnancy outcome.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Maintain active follow-up for every pregnancy of unknown location until outcome and location are resolved.
- Give written instructions to return immediately for worsening pain, syncope, shoulder-tip pain or heavy bleeding.
- Record scan route, consent, key measurements, adnexal findings, free fluid and heartbeat.
- Compare hCG values using the correct interval and avoid using the trend as proof of location.
- After a viability diagnosis, document the threshold, repeat interval and second-opinion requirements used.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Dates can be wrong
Cycle variation, uncertain ovulation and recall mean last menstrual period cannot independently establish when a heartbeat should be visible.
hCG is not an address
A rise or fall describes trophoblastic activity but cannot tell whether the pregnancy is intrauterine or extrauterine.
Consent is ongoing
Explain the internal scan, offer alternatives and stop if consent is withdrawn; transabdominal limitations should be documented.
Free fluid needs context
A small physiological volume can be normal, whereas echogenic or extensive fluid with pain and instability suggests bleeding.
Doppler does not exclude torsion
Preserved ovarian flow can occur with torsion because dual supply and intermittent twisting limit the value of a single negative sign.
Thresholds prevent harm
Conservative repeat-scan rules accept short uncertainty to avoid misclassifying and treating a potentially viable intrauterine pregnancy.
07Common pitfallsFrequent interpretation and management errors.
- 01
Calling an empty uterus proof of completed miscarriage.
- 02
Using one hCG value to locate a pregnancy or exclude rupture.
- 03
Diagnosing non-viability from menstrual dates alone.
- 04
Ignoring scan route when applying size thresholds and repeat intervals.
- 05
Allowing outpatient serial testing to delay surgery in an unstable patient.