Pregnancy test

Human chorionic gonadotropin (hCG) is a hormone produced by the embryo's trophoblast (the cells that later give rise to the placenta) as soon as the fertilized egg implants in the uterus. Its presence in blood or urine indicates pregnancy, which is why it is the basis of virtually every pregnancy test available.

The test is done on a blood sample drawn at the laboratory. It specifically detects the beta subunit (beta-hCG): the part that distinguishes hCG from the pituitary hormones LH, FSH and TSH, which share a common alpha subunit and could otherwise cause false positives.

The qualitative beta-hCG blood test answers a single question: are you pregnant, yes or no? The result is reported as positive or negative, and it's the simplest option when all you want is to confirm whether you're pregnant.

The quantitative beta-hCG test, on the other hand, measures the exact concentration of the hormone in international units per liter (IU/L or mIU/mL). It's the right choice when a precise number is needed — for example, so your healthcare provider can estimate how many weeks pregnant you are, or to track how the value changes over the first few weeks. Those numbers must always be interpreted by a doctor.

The body starts producing hCG a few days after the fertilized egg implants, and the hormone can appear in the blood around 10 days after conception, according to sources such as MedlinePlus (NIH).

In practice, the blood test is more sensitive than a urine test: it can come back positive before your missed period, whereas a urine test is recommended starting on the first day of your missed period, as advised by the Ministerio de Salud de la Nación (Argentina's national Ministry of Health). If your result is negative but you still suspect you're pregnant, it's best to repeat the test one week later, because hCG levels rise gradually during the first few weeks.

A positive result confirms the pregnancy, but it doesn't tell you where the pregnancy is located or how it is progressing. That's why it's best to see an obstetrician or gynecologist as soon as possible to start prenatal care. The Ministerio de Salud suggests starting prenatal check-ups during the first trimester, ideally before 12 weeks.

If the result is negative and your period is still late, it's a good idea to repeat the test a week later or talk to your doctor. And if you have any warning signs (severe abdominal pain, bleeding, dizziness), don't wait for a test result: go to a gynecology emergency department.