Missed Period & Negative Pregnancy Test? Understanding Blood Test Accuracy

Three missed periods with negative home and blood tests is unusual but possible. Consider:

Reasons for False-Negative Blood Test

• Very early pregnancy: hCG <5 mIU/mL (assay cutoff). Rare at 3 missed periods.
• Ectopic pregnancy: Low/slow-rising hCG; may not reach threshold. Emergency if abdominal pain.
• Hook effect: Extremely high hCG saturates assay antibodies → false low. Dilution corrects.
• Lab error: Sample mix-up, assay failure (rare).
• Biochemical pregnancy: Early loss before ultrasound detection; hCG rises then falls.

Non-Pregnancy Causes of Amenorrhea

• Hypothalamic: Stress, weight loss, excessive exercise, eating disorders.
• Pituitary: Prolactinoma (check prolactin), Sheehan's syndrome.
• Ovarian: PCOS (most common), premature ovarian insufficiency (FSH >25).
• Thyroid: Hypo- or hyperthyroidism.
• Adrenal: Congenital adrenal hyperplasia, Cushing's.
• Medications: OCPs, antipsychotics, opioids, chemotherapy.
• Chronic disease: CKD, liver disease.

Next Steps

1. Repeat quantitative β-hCG in 48 hrs (should double if viable intrauterine).
2. Transvaginal ultrasound (if hCG >1500–2000).
3. TSH, prolactin, FSH, LH, estradiol, free T4.
4. Progesterone challenge (if estrogenized) to assess outflow tract.

Do not assume non-pregnant; persistent amenorrhea warrants full endocrine evaluation.