Situation: A 30-year-old primigravida at 30 weeks' gestation attends a prenatal check-up at a lying-in clinic. Her pregnancy has been uncomplicated so far. She complains of heartburn after meals and low back pain at the end of the day. She takes the iron–folic acid tablet from the health center each morning. She recalls, "My home pregnancy test was positive early on, but the midwife said that result alone did not prove I was pregnant." Which explanation should the nurse give?

Pregnancy tests detect human chorionic gonadotropin (hCG), which can also be produced by a molar pregnancy and some tumors. For that reason a positive test, whether urine or serum, is a probable sign. Only findings that come from the fetus itself (fetal heartbeat, fetus seen on ultrasound, fetal movement felt by the examiner) are positive signs.

Why a positive pregnancy test is not a positive sign

Pregnancy tests, whether performed on urine or blood, detect human chorionic gonadotropin (hCG). The reason a positive result is classified as only a probable sign of pregnancy is that hCG is not exclusively produced by a normal intrauterine fetus. Other tissues can secrete this hormone, so the test result alone cannot prove that a viable pregnancy exists.

A positive hCG test is a probable sign because the hormone can be elevated in conditions unrelated to a normal pregnancy. The most clinically important examples are gestational trophoblastic disease and certain tumors. A molar pregnancy (hydatidiform mole) produces large amounts of hCG from abnormal trophoblastic tissue, and choriocarcinoma—a highly malignant trophoblastic tumor—also secretes β-hCG ^[3]. In addition, rare non-trophoblastic neoplasms can cause elevated β-hCG. A case report describes a woman with a malignant phyllodes tumor of the breast who had a positive urine pregnancy test and a serum β-hCG level of 1,152.7 mIU/mL ^[2]. These examples illustrate why a positive test cannot stand alone as proof of pregnancy.

Watch out! A positive hCG result does not tell you where the hormone is coming from. It could be a normal pregnancy, a molar pregnancy, choriocarcinoma, or even a rare non-gestational tumor.

The distinction between probable and positive signs follows a clear principle. Positive signs of pregnancy are only those findings that originate directly from the fetus itself. These include hearing the fetal heartbeat, visualizing the fetus on ultrasound, and palpating fetal movement by the examiner. Everything else—amenorrhea, nausea, breast changes, uterine enlargement, and positive hCG tests—remains a probable sign because each can have a non-pregnancy cause.

Sign type	Examples	Why it is classified this way
Probable sign	Positive urine or serum hCG, uterine enlargement, Hegar sign, Chadwick sign	Can be produced by conditions other than a viable fetus (e.g., molar pregnancy, tumors)
Positive sign	Fetal heartbeat heard, fetus seen on ultrasound, fetal movement felt by examiner	Originates directly from the fetus; cannot be simulated by other conditions

A related point concerns false-positive urine tests. One case report describes a woman with adenomyosis and persistent vaginal bleeding who had a positive qualitative urine hCG but a negative quantitative serum hCG ^[1]. This suggests that the false-positive urine result was not caused by elevated circulating hCG, but rather by interference in the urine specimen itself. The report notes that hemoglobin at 40 mg/dL in the urine was present, while 1 mg/dL of hemoglobin was not expected to interfere with the test device ^[1]. This is a separate issue from the main concept, but it reinforces that urine hCG results can be misleading for reasons beyond actual hormone elevation.

Key point! The nurse should explain that the hormone detected by pregnancy tests can come from conditions other than a fetus, which is why a positive test is a probable sign, not a positive sign.

The other answer choices are incorrect. A blood test for hCG would still be a probable sign, not a positive sign, because the hormone source remains uncertain. Urine pregnancy tests measure hCG, not progesterone, and progesterone elevation before a period is a different hormonal pattern. Urine dilution is not the primary reason a positive test fails to prove pregnancy; the biological source of hCG is the central issue.