Luteinizing Hormone (LH) is a primary hormone regulating the female menstrual cycle and fertility. LH levels do not stay high during pregnancy; they drop significantly following ovulation. The hormone’s primary surge occurs only to release a mature egg, and levels return to a low baseline shortly afterward. A different hormone entirely takes over the role of sustaining a pregnancy.
The Role of Luteinizing Hormone in Ovulation
The primary function of Luteinizing Hormone (LH) is to prompt the ovary to release an egg, known as ovulation. LH is produced by the pituitary gland, located at the base of the brain. During the first half of the menstrual cycle, LH works with Follicle-Stimulating Hormone (FSH) to encourage the growth of ovarian follicles.
As the egg matures, the follicle releases increasing amounts of estrogen, signaling the pituitary gland. This culminates in a rapid spike in LH concentration, referred to as the LH surge. The surge triggers the rupture of the dominant follicle and the release of the egg, typically within 24 to 36 hours. Once the egg is released, the LH surge quickly subsides, and hormone levels fall back to a low baseline.
Hormonal Changes Following Conception
After the egg is released, the follicle remnants transform into the corpus luteum. The corpus luteum produces progesterone, the hormone responsible for preparing and maintaining the uterine lining for potential pregnancy. Progesterone becomes the dominant hormone during the second half of the cycle, known as the luteal phase.
The corpus luteum requires a signal to continue producing progesterone beyond its natural lifespan of about 14 days. Without this signal, it disintegrates, progesterone levels plummet, and menstruation begins. If fertilization and implantation occur, a new hormonal messenger must arrive to rescue the corpus luteum. This ensures the uterine lining remains intact and maintains the pregnancy until the placenta takes over hormone production.
The True Marker of Pregnancy: Human Chorionic Gonadotropin
The hormone that maintains the corpus luteum is Human Chorionic Gonadotropin (HCG). HCG is often called the “pregnancy hormone” because it is produced by the cells that eventually form the placenta. HCG binds to the same receptors as LH on the corpus luteum, mimicking LH’s function but with a stronger, longer-lasting effect.
By binding to these receptors, HCG signals the corpus luteum to continue producing progesterone and estrogen, preventing the uterine lining from shedding. HCG is the substance measured by clinical blood tests and at-home urine pregnancy tests. HCG levels rise rapidly in early pregnancy, often doubling every two to three days, confirming the presence of a developing embryo.
Understanding LH Test Misinterpretation
Confusion about LH staying high during pregnancy often stems from Ovulation Predictor Kits (OPKs) showing a positive result in early pregnancy. This happens due to the structural similarities between LH and HCG. Both are glycoprotein hormones composed of alpha and beta subunits.
The alpha subunits of LH and HCG are nearly identical. Many inexpensive OPKs are designed to detect the surge level of LH and may not differentiate between the alpha subunits of the two hormones. When HCG levels rise in early pregnancy, this cross-reactivity can cause the OPK to register a positive result. However, a positive OPK is an unreliable indicator of pregnancy, as it detects HCG due to molecular overlap, not a true physiological LH surge.