01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Early-pregnancy testing is time-dependent. Ultrasound answers anatomy and developmental appearance; hCG measures trophoblastic activity. Neither a single scan nor a single blood result should be asked to answer every question. The safe sequence is clinical stability first, then expert transvaginal imaging, then appropriately timed repeat imaging or paired hCG where uncertainty remains. Pregnancy dates from the last menstrual period are estimates because ovulation and implantation vary.
Obtain specific consent for transvaginal ultrasound, explain the probe and privacy arrangements, offer a chaperone and allow the patient to stop. The patient inserts the probe in some services if preferred. Transvaginal imaging provides greater early resolution. If declined, offer transabdominal imaging and explain that a fuller bladder may be required, small structures may be missed and the minimum interval before a definitive repeat is longer. Refusal does not justify withdrawing care.
A complete early-pregnancy scan examines the uterus in multiple planes, endometrial cavity, cervix, both ovaries and adnexa, and fluid in the pouch of Douglas and upper pelvis when indicated. Document gestational sac location and shape, yolk sac, embryo, crown–rump length, heartbeat and any subchorionic bleed. An intrauterine fluid collection without yolk sac or embryo may be a pseudosac. A mass moving separately from the ovary and an extrauterine sac with yolk sac or embryo support tubal ectopic pregnancy.
When an embryo is visible, use CRL. If CRL is below 7 mm and no heartbeat is seen on transvaginal scan, repeat after a minimum of seven days before diagnosing miscarriage; further scans may be required. If CRL is 7 mm or more with no heartbeat, obtain a second opinion and/or repeat after at least seven days. Measurement variability around the boundary is why one exact number must not become an automatic treatment instruction.
When no embryo is visible, use mean gestational sac diameter. Below 25 mm with no fetal pole, repeat transvaginal imaging after a minimum of seven days; at 25 mm or above, seek a second opinion and/or repeat after at least seven days before diagnosis. For transabdominal scans without heartbeat or fetal pole, repeat after a minimum of 14 days. Never use last menstrual period alone to say a heartbeat must be visible.
In a PUL, take two quantitative serum hCG samples as close as possible to 48 hours apart and not earlier. A rise above 63% makes developing intrauterine pregnancy likely but does not exclude ectopic; repeat scan in 7–14 days and consider earlier imaging at hCG 1,500 IU/L or higher. A fall above 50% leads to a urine test 14 days after the second sample. A lesser rise or fall needs early-pregnancy review within 24 hours.
The discriminatory zone is a probability concept, not a diagnostic boundary. When hCG is above a level at which an intrauterine sac is commonly visible, an empty uterus raises concern and may justify earlier expert review, but does not prove ectopic pregnancy. Multiple pregnancy, uncertain dating, fibroids, equipment, operator and patient anatomy influence visibility. Methotrexate can harm an early viable intrauterine pregnancy and therefore requires a definitive ectopic diagnosis at first presentation under NICE.
Do not use serum progesterone to distinguish viable intrauterine, failing or ectopic pregnancy in PUL. Low values overlap and do not locate trophoblast. Similarly, hCG doubling language is misleading: viable rises vary and ectopics can rise substantially. Report actual percentage change and the time interval. If symptoms worsen, repeat clinical assessment immediately rather than trusting a previously reassuring curve.
Every investigation needs closure. Results should record the clinical question, exact measurements, limitations and the next appointment. A named team must review paired hCG and repeat ultrasound, contact the patient when expected attendance fails and provide a 24-hour number. Communicate uncertainty without withholding information: explain the possible outcomes, why waiting is safer than a premature diagnosis and which symptoms require emergency care.
Key points
- Transvaginal ultrasound is first-line for early pregnancy location and viability; offer transabdominal ultrasound if it is declined and explain reduced sensitivity and longer repeat intervals.
- Scan the endometrial cavity, both adnexa and pelvis for a gestational sac, yolk sac, embryo, heartbeat, adnexal mass and free fluid; do not stop at an empty uterus.
- Use crown–rump length when an embryo is visible and mean gestational sac diameter when it is not; do not use last menstrual period alone to decide that a heartbeat should be present.
- For transvaginal imaging with CRL below 7 mm and no heartbeat or mean sac diameter below 25 mm and no embryo, repeat after at least seven days before diagnosis.
- At or above those CRL or sac thresholds without expected development, seek a second opinion and/or repeat transvaginal ultrasound after at least seven days; do not diagnose from one image reflexively.
- After transabdominal uncertainty, repeat ultrasound after at least 14 days before diagnosing pregnancy failure because the approach is less sensitive early.
- For PUL, take paired hCG approximately 48 hours apart, never earlier, and use changes to select follow-up rather than to determine location.
- Waiting for the guideline repeat scan does not harm pregnancy outcome; clear explanation and 24-hour safety-netting reduce distress and unsafe premature intervention.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
An intrauterine gestational sac containing a yolk sac or embryo establishes location, although assisted conception can rarely produce concurrent heterotopic pregnancy.
An adnexal gestational sac with yolk sac or embryo separate from the ovary establishes extrauterine pregnancy and directs specialist management.
Positive pregnancy testing with neither definite intrauterine nor extrauterine pregnancy on competent transvaginal scanning mandates follow-up rather than a final label.
An intrauterine sac or embryo is seen but measurements and cardiac activity do not yet meet safe criteria for viable pregnancy or miscarriage.
Moderate or large echogenic free fluid with pain or haemodynamic change supports intraperitoneal bleeding and urgent ectopic assessment.
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
- Establish pregnancy location and developmental features with the highest early-pregnancy resolution.
- Interpretation and limitations
- Use CRL, mean sac diameter and cardiac activity with minimum seven-day repeat rules; document adnexa and free fluid, not the uterus alone.
- 02
Transabdominal ultrasound - Why
- Provide alternative or complementary imaging when internal scanning is declined, the uterus is enlarged or other pelvic pathology is present.
- Interpretation and limitations
- Explain lower early sensitivity. An uncertain viability diagnosis requires a minimum 14-day repeat interval rather than transvaginal thresholds.
- 03
Paired serum hCG - Why
- Direct follow-up in PUL by measuring trophoblastic change over a defined interval.
- Interpretation and limitations
- Take about 48 hours apart, not earlier. Report percentage change; >63% rise and >50% fall enter distinct pathways, while the middle range needs 24-hour review.
- 04
Urine pregnancy test - Why
- Confirm biochemical resolution after a strongly falling PUL or managed miscarriage at the recommended interval.
- Interpretation and limitations
- For >50% PUL decline, test 14 days after the second serum sample; after expectant or medical miscarriage management, test at three weeks.
- 05
Serum progesterone - Why
- It is sometimes requested as a viability marker but should not be used in the NICE PUL pathway.
- Interpretation and limitations
- Overlapping values cannot reliably distinguish viable intrauterine pregnancy, miscarriage and ectopic pregnancy or identify anatomical location.
04Clinical next stepsHow the result changes management or prompts escalation.
01Location scanImage the whole pelvisFirst stepPain, bleeding or pregnancy uncertainty leads to early-pregnancy ultrasound.+
- 1AlternativeObtain informed consent for transvaginal imaging, offer a chaperone and a transabdominal alternative with limitations when internal scanning is unacceptable.
- 2Assess uterus, cervix, both adnexa and free fluid and document sac, yolk sac, embryo, CRL, mean sac diameter and heartbeat using trained measurements.
- 3Classify the result as definite intrauterine, definite ectopic, unknown location or uncertain viability and link that classification to an explicit pathway.
02Uncertain viabilityRepeat without premature diagnosisAn intrauterine pregnancy is seen but embryo or sac measurements and cardiac activity are inconclusive.+
- 1Use CRL if an embryo is visible and mean sac diameter if it is not, checking image quality and obtaining a second opinion near or above diagnostic thresholds.
- 2Repeat transvaginal scanning after at least seven days, or transabdominal scanning after at least 14 days, before diagnosing miscarriage.
- 3Explain that waiting does not harm the pregnancy, give the exact appointment and provide 24-hour advice for worsening pain, bleeding or collapse.
03Unknown locationPair hCG and rescanA positive test accompanies no definite pregnancy on transvaginal imaging.+
- 1Take the first quantitative hCG and a second as close as possible to 48 hours later while prioritising any clinical symptom change.
- 2Apply NICE percentage-change branches to schedule ultrasound, a 14-day urine test or review within 24 hours, without using hCG to assign location.
- 3EscalationContinue tracking until a definite location or negative result is documented and escalate immediately if pain, bleeding or physiology worsens.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Record exact CRL, mean sac diameter, cardiac findings, adnexal findings, free fluid, scan route and the operator’s diagnostic classification.
- Compare hCG samples by actual hours and percentage change, avoiding vague doubled or failed-to-double descriptions that obscure the NICE thresholds.
- Track every repeat scan, serum result and home urine test to a named reviewer and actively contact patients who miss follow-up while ectopic remains possible.
- Reassess pain, bleeding, dizziness, shoulder-tip pain and observations at each attendance because symptoms overrule a previously reassuring test trend.
- Audit premature miscarriage or ectopic diagnoses and methotrexate given without definitive criteria as serious diagnostic safety events.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Seven days protects wanted pregnancies
The repeat interval absorbs normal biological and measurement variation and prevents a borderline scan from becoming an irreversible false miscarriage diagnosis.
Fourteen days belongs to abdominal scans
Lower resolution makes a longer interval necessary before absence of expected development can safely establish failure.
One threshold has two uncertainties
Both the hCG concentration and ultrasound visibility vary, so the discriminatory zone raises suspicion but cannot prove extrauterine location.
The adnexa complete the scan
Reporting only an empty uterine cavity misses the anatomical question most relevant to ectopic pregnancy and haemoperitoneum.
A result needs an owner
Serial testing is safe only when someone is accountable for reviewing the trend, contacting the patient and initiating the next step.
07Common pitfallsFrequent interpretation and management errors.
- 01
Diagnosing miscarriage from last menstrual period and one scan without using CRL or mean sac diameter and the minimum repeat interval.
- 02
Applying transvaginal seven-day rules to an uncertain transabdominal scan that requires at least 14 days.
- 03
Using an hCG discriminatory zone as proof of ectopic pregnancy or permission for methotrexate.
- 04
Ordering serum progesterone to locate a PUL despite overlapping values.
- 05
Stopping after examining the uterus without documenting both adnexa and free fluid.
- 06
Collecting serial results without named review, active non-attendance follow-up and symptom safety-netting.