Luteinizing Hormone (LH) is a reproductive hormone produced by the pituitary gland. This hormone regulates ovarian function in women and testicular function in men. For women trying to conceive, measuring LH levels is a primary method for determining fertility and the timing of ovulation. Once pregnancy is established, however, the concentration and function of LH change significantly, shifting the focus to a different hormone.
The Role of LH in the Menstrual Cycle
Luteinizing hormone (LH), along with Follicle-Stimulating Hormone (FSH), controls the processes leading to potential pregnancy. During the first half of the menstrual cycle, LH stimulates the ovarian follicles to mature. As the follicles grow, they produce estrogen, which signals the pituitary gland.
When estrogen levels reach a certain threshold, they trigger the LH surge. This surge is the direct hormonal signal that causes the most mature follicle to rupture and release an egg (ovulation). Ovulation typically occurs 24 to 36 hours after the onset of the surge.
After the egg is released, the remaining follicular tissue transforms into the corpus luteum. LH maintains the corpus luteum, ensuring it produces progesterone. Progesterone prepares the uterine lining for implantation. If fertilization and implantation do not occur, LH levels decline, and the corpus luteum breaks down, starting the next menstrual period.
The Primary Hormone of Pregnancy
If conception occurs, the temporary role of LH is quickly replaced by Human Chorionic Gonadotropin (hCG). Trophoblast cells, which eventually develop into the placenta, begin secreting hCG shortly after implantation.
The primary function of hCG is to rescue the corpus luteum from degradation. By binding to the same receptors LH uses, hCG signals the corpus luteum to continue producing progesterone and estrogen. This sustained production keeps the uterine lining stable, supporting the pregnancy. Without this signal, the corpus luteum would fail, ending the pregnancy.
HCG is structurally similar to LH, which is key to its function and detection. Both are glycoproteins composed of alpha and beta subunits. The alpha subunits are nearly identical, but the beta subunit of hCG has an added chain of amino acids, giving it a longer lifespan. This similarity allows hCG to bind to the LH receptor, while the unique beta subunit is what pregnancy tests detect.
Luteinizing Hormone Levels Once Pregnancy is Established
Once the developing embryo begins producing a substantial amount of hCG, Luteinizing Hormone levels are effectively suppressed. The high concentration of hCG acts upon the pituitary gland through a negative feedback loop, signaling it to drastically reduce its production of both LH and FSH.
In a confirmed pregnancy, LH levels fall to a low, baseline state and remain suppressed for the duration of gestation. The typical pre-ovulatory surge levels drop significantly, often to less than 1.5 IU/L. This suppression is necessary because hCG has taken over the role of signaling the ovary.
The suppression of LH ensures that no further follicular development or ovulation occurs, which is a protective mechanism during pregnancy. The low LH levels are a direct physiological consequence of the high, sustained presence of the pregnancy hormone.
Understanding Cross-Reactivity in Home Testing
The structural similarity between LH and hCG can lead to confusion when using at-home tests. Ovulation Predictor Kits (OPKs) are designed to detect the LH surge preceding ovulation. However, high levels of hCG can sometimes trick an OPK into showing a positive result due to the identical alpha subunit shared by both hormones.
This phenomenon is known as cross-reactivity, meaning the test cannot perfectly differentiate between LH and hCG molecules. If a woman is pregnant, rising hCG levels may bind to the antibodies on the OPK strip, causing a positive line to appear.
A positive result on an OPK is not a reliable method for confirming pregnancy. Ovulation tests are optimized to detect LH at the ovulation threshold, not the much higher hCG levels found in early pregnancy. For accurate confirmation, a test specifically designed to detect the unique beta subunit of hCG should always be used.