Human chorionic gonadotropin (hCG) is a hormone your body produces during pregnancy to keep the pregnancy viable in its earliest weeks. It works by binding to receptors in the ovaries that signal them to continue producing progesterone, the hormone that maintains the uterine lining and prevents menstruation. Without hCG, the pregnancy would end before the embryo could sustain itself. This same hormone is what pregnancy tests detect, and it’s also used medically to trigger ovulation during fertility treatments.
What hCG Is Made Of
hCG is a glycoprotein built from two linked pieces called subunits: an alpha subunit and a beta subunit. The alpha subunit (92 amino acids) is nearly identical to the one found in several other hormones, including luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). What makes hCG unique is its beta subunit, which is 145 amino acids long and includes an extra 30-amino-acid tail that the other hormones lack. This tail is the reason pregnancy tests can specifically detect hCG without confusing it for LH or other related hormones.
How hCG Keeps a Pregnancy Going
After an egg is released from the ovary, the structure it leaves behind (called the corpus luteum) starts producing progesterone. Progesterone thickens the uterine lining and prepares it for a fertilized egg. If no pregnancy occurs, the corpus luteum breaks down after about two weeks, progesterone drops, and your period starts.
When an embryo implants, the developing placenta begins secreting hCG almost immediately. hCG binds to receptors on the cells of the corpus luteum, essentially overriding the signal to shut down. It switches on enzymes inside those cells that ramp up progesterone production in a dose-dependent way: the more hCG present, the more progesterone the corpus luteum produces. This rescue mission continues until roughly weeks 8 to 10 of pregnancy, when the placenta itself becomes capable of producing enough progesterone on its own.
Interestingly, hCG is about five times more potent than LH at activating the signaling pathway responsible for progesterone production. That extra potency matters because the corpus luteum needs a strong, sustained signal to survive well beyond its normal two-week lifespan.
How hCG Levels Rise During Pregnancy
hCG levels increase rapidly in early pregnancy, nearly doubling every three days for the first 8 to 10 weeks. Here’s what typical levels look like by week (measured from the last menstrual period):
- Week 3: 5 to 50 mIU/mL
- Week 4: 5 to 426 mIU/mL
- Week 5: 18 to 7,340 mIU/mL
- Week 6: 1,080 to 56,500 mIU/mL
- Weeks 7 to 8: 7,650 to 229,000 mIU/mL
- Weeks 9 to 12: 25,700 to 288,000 mIU/mL
- Weeks 13 to 16: 13,300 to 254,000 mIU/mL
- Weeks 17 to 24: 4,060 to 165,400 mIU/mL
- Weeks 25 to 40: 3,640 to 117,000 mIU/mL
Notice how levels peak somewhere around weeks 9 to 12, then gradually decline for the rest of pregnancy. That peak corresponds with the transition period when the placenta takes over progesterone production and the corpus luteum is no longer the primary source. The wide ranges at each week are normal. A single hCG reading matters less than whether levels are rising appropriately over time.
How Pregnancy Tests Detect hCG
Home pregnancy tests use antibodies that react specifically with hCG’s unique beta subunit. Most tests on the market claim a sensitivity between 10 and 25 mIU/mL. To reliably detect 95% of pregnancies on the day of a missed period, a test needs to pick up concentrations as low as about 12.4 mIU/mL. For 99% accuracy from the day of the expected period, a test should consistently detect 25 mIU/mL.
Blood tests are slightly more sensitive and can detect hCG earlier. Serum concentrations reach approximately 10 mIU/mL between 9 and 10 days after ovulation. That’s why a blood draw can sometimes confirm pregnancy a few days before a home urine test turns positive.
hCG in Fertility Treatments
Because hCG mimics LH so closely (they bind the same receptor), doctors use injectable hCG to trigger ovulation in fertility cycles. When follicles in the ovaries have grown to at least 18 mm on ultrasound, an injection of hCG acts like a surge of LH, telling the mature follicle to release its egg. Follicle rupture and ovulation typically happen 36 to 38 hours after the injection, which is why insemination procedures are usually scheduled 34 to 36 hours post-injection to coincide with the egg’s release.
After ovulation, hCG also supports the corpus luteum in these treatment cycles, just as it would in a natural pregnancy. This dual role, triggering the egg’s release and then supporting progesterone production afterward, makes it a cornerstone of protocols for both IUI and IVF.
hCG’s Effect on Testosterone
hCG doesn’t only work in the female body. In men, it stimulates Leydig cells in the testes, the same cells that respond to LH from the pituitary gland. When hCG binds to LH receptors on Leydig cells, those cells increase testosterone production. This is why hCG is sometimes prescribed for men with low testosterone or delayed puberty, and why it’s used clinically to evaluate whether the testes are capable of producing testosterone normally. It’s also why hCG appears on lists of banned performance-enhancing substances in sports.
Elevated hCG Outside of Pregnancy
While hCG is primarily associated with pregnancy, certain tumors also produce it. Gestational trophoblastic disease, a group of rare conditions involving abnormal placental tissue, causes elevated hCG in 100% of cases. Germ cell tumors (which can develop in the ovaries or testes) produce detectable hCG in about 55% to 57% of cases. Other cancers occasionally produce it too, though less frequently, in roughly 8% to 23% of cases depending on the type. Because of this, hCG is used as a tumor marker to help diagnose these conditions and monitor whether treatment is working.
hCG and Weight Loss Claims
Products marketed as hCG supplements for weight loss have circulated for years, typically paired with an extreme 500-calorie-per-day diet. The FDA has been clear: hCG is not approved for weight loss. The prescription drug label itself states there is no substantial evidence that hCG increases weight loss beyond what calorie restriction alone would achieve, redistributes fat, or reduces hunger. Any weight loss on these programs comes from the dangerously low calorie intake, not the hormone. A 500-calorie diet carries real risks, including gallstones, dangerous electrolyte imbalances, and irregular heartbeat. The FDA advises consumers to avoid these products entirely.