Luteinizing Hormone (LH) is a chemical messenger produced by the pituitary gland, a small structure at the base of the brain. In the context of female reproductive health, LH is typically associated with regulating the menstrual cycle and stimulating ovulation. However, when pregnancy occurs, the body undergoes substantial hormonal shifts. This article explores how LH levels change during pregnancy and why these changes are a normal and expected part of gestation.
Luteinizing Hormone’s Pre-Pregnancy Role
Before conception, luteinizing hormone is instrumental in the menstrual cycle. Early in the cycle, LH works with follicle-stimulating hormone (FSH) to promote the growth of ovarian follicles, which contain eggs. As a follicle matures, it produces increasing levels of estrogen. This rise in estrogen triggers a sudden and dramatic increase in LH, known as the “LH surge.” This surge in LH is the direct signal for the ovary to release a mature egg, a process called ovulation, typically occurring around the second week of the menstrual cycle.
After ovulation, LH then prompts the remaining follicle cells to transform into a structure called the corpus luteum. The corpus luteum is responsible for producing progesterone, a hormone that prepares the uterine lining for a potential pregnancy. If pregnancy does not occur, the corpus luteum breaks down, progesterone levels drop, and menstruation begins, restarting the cycle.
The Hormonal Transition in Early Pregnancy
After fertilization and implantation, a new hormone emerges: human chorionic gonadotropin (hCG). This hormone is produced by the trophoblast cells, which eventually form part of the placenta. A primary function of hCG in early pregnancy is to signal the corpus luteum to continue producing progesterone. This sustained progesterone production is crucial for maintaining the uterine lining and preventing menstruation, supporting pregnancy.
HCG achieves this by mimicking the action of LH, as both hormones share a similar structure and can bind to the same receptors. High levels of hCG then suppress the pituitary gland’s production of LH. This suppression leads to a significant decrease in LH levels in the bloodstream, a normal change in early pregnancy.
LH Levels Throughout Pregnancy
Beyond the initial weeks of pregnancy, LH levels remain suppressed. As the pregnancy progresses, the placenta takes over the role of progesterone and estrogen production, typically around 8 to 12 weeks of gestation. These placental hormones continue to exert negative feedback on the pituitary gland, keeping LH levels low. The continuous production of progesterone and estrogen by the placenta ensures that the body does not attempt to ovulate or initiate another menstrual cycle during pregnancy. Therefore, low levels of LH are a normal and expected finding throughout the remainder of pregnancy.
Practical Implications of LH Testing During Pregnancy
Given the suppression of LH during pregnancy, using ovulation predictor kits (OPKs) to detect pregnancy can be misleading. OPKs are designed to identify the LH surge that precedes ovulation, not pregnancy. However, because of the structural similarity between LH and hCG, some OPKs might show a positive result if a person is pregnant. Despite this possibility, OPKs are not reliable pregnancy tests, and a negative or low LH reading on an OPK during pregnancy is normal. For accurate pregnancy confirmation, it is always recommended to use a dedicated pregnancy test, which specifically detects hCG.