Luteinizing Hormone (LH) is a crucial chemical messenger produced by the pituitary gland in the brain. This hormone plays a significant role in the female reproductive system, orchestrating key events within the menstrual cycle. Understanding LH’s behavior, especially concerning pregnancy, helps clarify its function.
Luteinizing Hormone’s Role in Ovulation
LH is instrumental in regulating the menstrual cycle. During the first half of the cycle, LH stimulates ovarian follicles to produce estrogen. As a dominant follicle matures and estrogen levels rise, a dramatic increase in LH occurs, known as the “LH surge.”
This LH surge typically happens around the middle of the menstrual cycle, signaling the ovary to release a mature egg (ovulation). Following ovulation, LH supports the transformation of the ruptured follicle into the corpus luteum. The corpus luteum then produces progesterone, a hormone that prepares the uterine lining for potential pregnancy.
LH Levels During Pregnancy
When a woman becomes pregnant, Luteinizing Hormone levels change considerably. LH levels do not increase; instead, they remain low or are suppressed throughout gestation. This reduction is a natural physiological adjustment, allowing other hormones to take precedence in supporting the pregnancy.
Studies indicate that LH levels can drop to less than 1.5 IU/L in pregnant women. This notable decrease ensures that LH’s primary role in triggering ovulation is suspended, preventing further egg release. The body’s hormonal focus shifts to maintaining the uterine environment and nurturing the developing embryo.
The Hormonal Shift in Pregnancy
LH suppression during pregnancy is due to human chorionic gonadotropin (hCG). After a fertilized egg implants, the embryo produces hCG. This hormone is structurally similar to LH, sharing an identical alpha subunit, which allows it to bind to the same receptors in the ovary.
hCG maintains the corpus luteum, ensuring it continues to produce progesterone and estrogen. These rising levels, in turn, exert a negative feedback effect on the hypothalamus and pituitary gland, suppressing LH production and preventing cyclical LH surges.
LH Tests and Pregnancy Detection
Due to the structural similarity between LH and hCG, questions arise about using Luteinizing Hormone tests, such as ovulation predictor kits (OPKs), for pregnancy detection. While OPKs identify the LH surge before ovulation, they are unreliable for pregnancy. An OPK might show a positive line if hCG levels are high in early pregnancy, as the test may mistakenly detect hCG as LH.
OPKs are not calibrated to measure hCG, and their sensitivity varies. For accurate pregnancy detection, use dedicated home pregnancy tests. These are designed to detect human chorionic gonadotropin and provide reliable results.