HCG, or human chorionic gonadotropin, is a hormone produced during pregnancy that signals the body to sustain the conditions needed for a developing embryo. It’s the hormone that pregnancy tests detect, and it plays a central role in early pregnancy, fertility treatments, and certain medical diagnoses. Outside of pregnancy, HCG can also appear as a marker for specific cancers and is used therapeutically in men with hormonal conditions.
How HCG Works in Early Pregnancy
HCG is produced by trophoblast tissue, the cells that form in the earliest days after fertilization and later become part of the placenta. Its primary job is to keep the corpus luteum alive. The corpus luteum is a small structure in the ovary that forms after ovulation and pumps out progesterone, the hormone responsible for maintaining the uterine lining. Without HCG, the corpus luteum would break down within about two weeks, progesterone would drop, and the pregnancy could not continue.
HCG works by binding to the same receptor as luteinizing hormone (LH), one of the brain’s key reproductive signals. This shared receptor is why HCG can stand in for LH in several medical contexts. Once the placenta is mature enough to produce its own progesterone (around 8 to 12 weeks), the corpus luteum becomes less critical, and HCG levels begin to plateau and eventually decline.
HCG Levels Week by Week
Blood HCG levels rise rapidly in the first weeks of pregnancy, roughly doubling every two to three days in the earliest stages. That doubling time isn’t constant, though. It slows as the pregnancy progresses and HCG concentrations climb higher. By the time levels reach the tens of thousands, the rate of increase is noticeably slower than it was in the first few weeks.
Here’s a general guide to what blood HCG levels look like during the first trimester:
- 4 weeks: 0 to 750 µ/L
- 5 weeks: 200 to 7,000 µ/L
- 6 weeks: 200 to 32,000 µ/L
- 7 weeks: 3,000 to 160,000 µ/L
- 8 to 12 weeks: 32,000 to 210,000 µ/L
These ranges are wide because every pregnancy is different. A single HCG reading matters less than the trend over time. In a healthy early pregnancy, levels should rise by at least 53% over 48 hours. A rise slower than that can suggest a nonviable pregnancy, though it doesn’t pinpoint the cause on its own.
What Abnormal HCG Trends Can Mean
When HCG levels don’t follow the expected pattern, it raises a few possibilities. Levels that rise too slowly (below that 53% threshold over two days) may indicate either a miscarriage in progress or an ectopic pregnancy, where the embryo implants outside the uterus. Serial blood draws can’t always distinguish between the two, but the trend helps narrow things down.
If a pregnancy is ending on its own, HCG levels typically decline by 35% to 50% within two days, depending on how high they were initially. A decline slower than that can point to retained tissue or an ectopic pregnancy that needs treatment. By seven days, a normally resolving pregnancy shows HCG drops of 66% to 87%. These thresholds give clinicians a framework for deciding when further imaging or intervention is needed.
How Pregnancy Tests Detect HCG
Home pregnancy tests work by detecting HCG in urine. Most become reliable around the time of a missed period, which corresponds roughly to the 4-week mark when blood levels may still be relatively low. Blood tests are more sensitive and can detect HCG earlier, which is why they’re used to monitor pregnancies that need close tracking, such as after fertility treatments or when there’s concern about an ectopic pregnancy.
HCG in Fertility Treatments
Because HCG mimics LH, it’s widely used in fertility medicine as a “trigger shot” to induce ovulation at a precisely timed moment. When a developing egg follicle reaches the right size (typically around 16 mm on ultrasound), an HCG injection triggers the final steps of egg maturation and release. Ovulation then occurs about 36 to 48 hours after the injection, which is more predictable than a natural LH surge, where ovulation can happen anywhere from 25 to 56 hours later.
This predictability is the whole point. In intrauterine insemination, for example, the procedure is typically scheduled about 24 and 36 hours after the HCG injection to align with the ovulation window. The same principle applies in IVF, where egg retrieval needs to happen at a precise stage of follicle development.
HCG Therapy for Men
HCG has important uses in men’s health, specifically for maintaining testosterone production inside the testicles and preserving sperm production. In men, LH signals cells in the testes to produce testosterone. HCG, because it binds to the same receptor and has a much longer half-life (about 36 hours compared to LH’s 30 minutes), acts as an effective substitute.
This matters most for men on testosterone replacement therapy. Supplemental testosterone tells the brain that hormone levels are adequate, so the brain stops sending LH. Without LH, the testes shrink and sperm production drops dramatically. HCG can counteract this by maintaining the signal to produce testosterone locally within the testes, keeping sperm production alive even while a man is on testosterone therapy.
HCG is also used to help men recover natural testosterone production after stopping anabolic steroids, to treat a condition called hypogonadotropic hypogonadism (where the brain doesn’t produce enough LH), and to improve sperm retrieval in certain types of infertility. The dosing and timing vary depending on whether a man is planning for pregnancy soon or further in the future.
HCG as a Cancer Marker
Outside of pregnancy and fertility, elevated HCG in the blood can signal certain cancers. In testicular cancer, tumor cells can transform into a type of cell normally found in the placenta, which then secretes HCG. Normal levels in a non-pregnant person are below 5 IU/L. Levels above 5,000 IU/L in testicular cancer generally indicate a more aggressive tumor type and are associated with a worse prognosis.
About 15% of seminomas (one of the main types of testicular cancer) produce HCG, though in these cases the elevated level doesn’t worsen the outlook. HCG can also be elevated in cancers of the liver, lung, pancreas, and stomach. Because HCG is so closely related to LH, men with low testosterone can sometimes show falsely elevated HCG readings due to high LH levels. Marijuana use has also been linked to mildly elevated HCG.
The HCG Diet: Why It’s Considered Unsafe
HCG has been marketed as a weight loss aid, typically as drops or injections paired with an extremely low-calorie diet of around 500 calories per day. The FDA has explicitly warned consumers to avoid these products. HCG is not approved for weight loss, and the prescription drug label itself states there is no substantial evidence that it increases weight loss beyond what caloric restriction alone would produce, changes fat distribution, or reduces hunger.
Any weight loss people experience on these programs comes from eating 500 calories a day, not from the HCG. That level of caloric restriction is dangerous on its own, carrying risks of gallstone formation, electrolyte imbalances that affect heart and muscle function, and irregular heartbeat. Over-the-counter HCG products sold as drops, pellets, or sprays are not FDA-approved and may not even contain real HCG.