Human chorionic gonadotropin (hCG) is a hormone produced by the placenta during pregnancy. Its primary job is to keep the body producing progesterone, the hormone that sustains a pregnancy in its earliest weeks. Outside of pregnancy, hCG is used as a fertility treatment, a diagnostic marker in blood tests, and even a tool for detecting certain cancers.
How hCG Works in the Body
After a fertilized egg implants in the uterine wall, cells that will eventually form the placenta begin releasing hCG into the bloodstream. This hormone acts on the corpus luteum, a temporary structure in the ovary left behind after ovulation. Without hCG, the corpus luteum breaks down within about two weeks, progesterone drops, and menstruation begins. With hCG present, the corpus luteum survives and ramps up its progesterone output, giving the embryo the hormonal support it needs to develop.
At a molecular level, hCG belongs to a family of glycoprotein hormones. It’s made of two protein chains: an alpha subunit (shared with several other hormones, including thyroid-stimulating hormone and follicle-stimulating hormone) and a beta subunit that is unique to hCG. That unique beta chain is what pregnancy tests detect. The two chains lock together through an unusual structure where the beta chain wraps around and “embraces” the alpha chain, held in place by chemical bridges.
hCG Levels During Pregnancy
hCG is the hormone that makes a pregnancy test turn positive. Most home tests can detect it once blood levels rise above roughly 20 to 25 mIU/mL, which typically happens around the time of a missed period. From there, levels climb rapidly. Here’s what the typical range looks 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
In early pregnancy, hCG typically doubles every 72 hours. As levels get higher, the doubling time slows to roughly every 96 hours. Doctors sometimes order serial blood draws two to three days apart to check whether hCG is rising at a healthy pace. A single measurement matters less than the trend. Levels that plateau or drop early on can signal a miscarriage or ectopic pregnancy, while unusually high levels sometimes point to a molar pregnancy or twins.
After the first trimester, hCG levels gradually decline. By this point, the placenta itself has taken over progesterone production and the corpus luteum is no longer needed.
hCG in Fertility Treatment
Synthetic hCG is widely used in reproductive medicine. Because hCG closely mimics luteinizing hormone (the natural signal that triggers ovulation), an hCG injection can prompt the ovaries to release a mature egg at a predictable time. This is commonly called a “trigger shot,” and it’s a standard step in both IUI (intrauterine insemination) and IVF (in vitro fertilization) cycles.
In men, hCG injections stimulate the testes to produce more testosterone and sperm. This makes it a treatment option for certain types of male infertility and for men with low testosterone caused by a signaling problem between the brain and the testes.
Ovarian Hyperstimulation Syndrome
The most significant risk of hCG injections during fertility treatment is ovarian hyperstimulation syndrome (OHSS). This happens when the ovaries overreact to the hormone, swell, and begin leaking fluid into the abdomen. Mild cases cause bloating, nausea, and tenderness around the ovaries. Severe cases can lead to rapid weight gain (more than 2.2 pounds in 24 hours), shortness of breath, blood clots, and decreased urination.
OHSS typically develops within a week of the hCG injection and can worsen if pregnancy occurs, since the body then starts producing its own hCG on top of the injected dose. Women with high estrogen levels or a large number of developing follicles before the trigger shot face a higher risk. In those cases, fertility specialists may delay the hCG injection for a few days, a strategy called “coasting,” to let estrogen levels stabilize.
hCG as a Cancer Marker
Outside of pregnancy and fertility treatment, hCG shows up in an unexpected context: cancer diagnosis. Certain tumors produce hCG, most notably germ cell tumors of the testicles. In these cancers, tumor cells can transform into a cell type normally found in the placenta and begin secreting the hormone. About 15% of seminomas (the most common type of testicular cancer) produce hCG, while non-seminomatous germ cell tumors do so more frequently.
In a healthy person who is not pregnant, hCG levels sit below 5 IU/L. Levels above 5,000 IU/L generally point to non-seminomatous germ cell tumors, and higher numbers correlate with a worse prognosis. hCG can also be elevated in cancers of the liver, lung, pancreas, and stomach. For testicular cancer patients, tracking hCG levels after treatment helps doctors monitor for recurrence, since a rising level can signal that cancer cells are still active or have returned.
Why hCG Matters Beyond Pregnancy Tests
Most people encounter hCG only as the hormone behind a positive pregnancy test, but its roles extend well beyond that. It keeps early pregnancies viable by sustaining progesterone production. It serves as a powerful tool in fertility medicine for both women and men. And it functions as a measurable signal that helps oncologists detect and track certain cancers. The same hormone that confirms a pregnancy in a bathroom at home can, in a different context, guide treatment decisions in a cancer clinic. That versatility is what makes hCG one of the most clinically useful hormones in medicine.