Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Interpret early-pregnancy hCG and ultrasound as complementary time-dependent tests, using validated repeat intervals and avoiding false diagnoses from single thresholds or uncertain dates.
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.
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.
The repeat interval absorbs normal biological and measurement variation and prevents a borderline scan from becoming an irreversible false miscarriage diagnosis.
Lower resolution makes a longer interval necessary before absence of expected development can safely establish failure.
Both the hCG concentration and ultrasound visibility vary, so the discriminatory zone raises suspicion but cannot prove extrauterine location.
Reporting only an empty uterine cavity misses the anatomical question most relevant to ectopic pregnancy and haemoperitoneum.
Serial testing is safe only when someone is accountable for reviewing the trend, contacting the patient and initiating the next step.
Diagnosing miscarriage from last menstrual period and one scan without using CRL or mean sac diameter and the minimum repeat interval.
Applying transvaginal seven-day rules to an uncertain transabdominal scan that requires at least 14 days.
Using an hCG discriminatory zone as proof of ectopic pregnancy or permission for methotrexate.
Ordering serum progesterone to locate a PUL despite overlapping values.
Stopping after examining the uterus without documenting both adnexa and free fluid.
Collecting serial results without named review, active non-attendance follow-up and symptom safety-netting.
Your practice draft is saved in this browser.
A stable patient has a transvaginal scan showing an intrauterine embryo with crown–rump length 5 mm and no visible heartbeat. What is the correct next step?
National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes
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.