- Human chorionic gonadotropin (hCG) is a glycoprotein hormone that plays a critical role in the early stages of pregnancy.
- It is produced primarily by the syncytiotrophoblast cells of the developing placenta shortly after implantation of the embryo into the uterine wall.
- hCG is one of the first detectable signs of pregnancy and is the hormone measured in both urine and blood pregnancy tests. Its early and rapid rise in maternal circulation makes it essential for initiating and maintaining the physiological conditions necessary for embryonic and fetal development.
- Biochemically, hCG consists of two subunits: alpha and beta. The alpha subunit is structurally similar to other hormones such as luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH), while the beta subunit is unique to hCG and provides its specific biological identity. The hormone is secreted in increasing quantities during the first trimester, peaking around 8–11 weeks of gestation, after which levels decline and then stabilize for the remainder of pregnancy.
- The primary function of hCG is to support the corpus luteum in the ovary, which in turn secretes progesterone. Progesterone is vital for maintaining the endometrial lining of the uterus, creating a supportive environment for the developing embryo. Without the stimulatory effect of hCG, the corpus luteum would regress, leading to a drop in progesterone levels and, consequently, termination of the pregnancy. By sustaining progesterone production until the placenta can take over hormone synthesis, hCG acts as a hormonal bridge between maternal and embryonic systems.
- Beyond its luteotropic role, hCG has multiple additional functions. It modulates the maternal immune response to prevent the rejection of the semi-allogeneic fetus, facilitates angiogenesis in the uterine lining to ensure adequate blood supply to the placenta, and promotes trophoblast differentiation and invasion, which are essential processes for successful placental development. hCG has also been shown to stimulate thyroid activity during early pregnancy due to its structural similarity to TSH, which can explain mild physiological hyperthyroidism in some pregnant individuals.
- Clinically, hCG is of great importance not only in confirming pregnancy but also in monitoring pregnancy health. Abnormally low hCG levels can indicate a possible ectopic pregnancy, miscarriage, or threatened abortion, while excessively high levels may suggest gestational trophoblastic disease, molar pregnancy, or multiple gestation (e.g., twins or triplets). Serial hCG measurements are often used to assess the viability of early pregnancy, especially when complications are suspected.
- In medical treatments, recombinant or purified hCG is used therapeutically in assisted reproductive technologies (ART), such as in vitro fertilization (IVF), to trigger ovulation in women and stimulate testosterone production in men with hypogonadism. hCG has also been investigated for its potential roles in cancer diagnostics and therapy, particularly in tumors that aberrantly produce hCG, such as choriocarcinoma and some testicular cancers.
