Luteinizing Hormone (LH) is a glycoprotein hormone produced by the anterior pituitary gland. Its primary function is to regulate the reproductive cycle, and it is most commonly known for its role in fertility tracking. Many people track this hormone to pinpoint the most opportune time for conception. Seeing a positive reading on an ovulation test after a confirmed pregnancy can be confusing. This contradiction is due to the biological overlap between two hormones.
The Primary Function of Luteinizing Hormone
In the non-pregnant state, LH is a key regulator of the menstrual cycle, working in tandem with Follicle-Stimulating Hormone (FSH). During the first half of the cycle, LH stimulates the ovarian follicles to produce estrogen. As the dominant follicle matures, the sustained rise in estrogen triggers a sudden spike in LH secretion from the pituitary gland.
This event is known as the LH surge, and it is the direct signal that triggers ovulation, the release of a mature egg from the ovary. The surge typically occurs about 24 to 48 hours before the egg is released. After ovulation, LH stimulates the remnants of the ruptured follicle to transform into the corpus luteum. The corpus luteum then begins to produce the hormone progesterone, which prepares the uterine lining for a potential pregnancy.
Actual LH Levels During Pregnancy
Once conception occurs and a pregnancy is established, the physiological level of LH in the body drops significantly. A true LH chart during pregnancy would show very low, often undetectable, readings. This suppression is a direct consequence of the hormonal changes that maintain gestation.
The newly formed corpus luteum is initially rescued from degeneration by a different hormone, Human Chorionic Gonadotropin (hCG), which begins to be produced shortly after implantation. High concentrations of progesterone and estrogen, secreted first by the corpus luteum and later by the developing placenta, feed back to the pituitary gland. This negative feedback loop suppresses the release of both LH and FSH. The body halts LH production because its function—triggering a new cycle and ovulation—is no longer necessary during gestation.
Why Ovulation Test Strips Show Positive Results
The reason many people see a positive result on an Ovulation Predictor Kit (OPK) while pregnant is not due to a physiological rise in LH but rather a quirk of the test strip’s design. Ovulation tests are manufactured to detect Luteinizing Hormone in the urine. However, the antibodies used in these kits are not specific enough to differentiate between LH and the pregnancy hormone, hCG.
Human Chorionic Gonadotropin, produced by the developing embryo, is structurally very similar to LH. The two hormones share an identical alpha subunit, meaning they look alike to the test strip’s detection mechanism. Because of this high structural homology, the OPK cannot distinguish between the LH that signals ovulation and the high concentration of hCG present in early pregnancy. This phenomenon is known as cross-reactivity.
As the pregnancy progresses, hCG levels rise rapidly, doubling roughly every two to three days in the first few weeks. The concentration of hCG in the urine quickly reaches a level high enough to trigger a positive result on the OPK, tricking the test into reading the pregnancy hormone as an LH surge. Seeing a consistently positive result on an OPK for several days after the expected ovulation window indicates that the test is picking up rising hCG instead of a true LH surge. The test is designed to detect a single, transient LH peak, not a sustained hormonal presence.
The Difference Between LH and hCG
While LH and hCG are structurally similar enough to cause cross-reactivity on a basic ovulation test, they are distinct hormones. Both are classified as heterodimeric glycoproteins, meaning they are composed of two different protein chains: an alpha subunit and a beta subunit. The alpha subunit is identical across LH, FSH, and hCG, which explains the confusion for simple tests.
The key difference lies in their beta subunits. The beta subunit of hCG is longer than that of LH and contains a unique tail of amino acids known as the carboxy-terminal peptide (CTP). This CTP is heavily glycosylated, meaning it has sugar molecules attached, which significantly extends the hormone’s half-life in the bloodstream. This unique structure allows specialized, dedicated pregnancy tests to accurately distinguish hCG from LH.
Pregnancy tests are designed with antibodies that specifically target the unique beta subunit of hCG, ignoring the common alpha subunit and the LH hormone. Therefore, while a positive ovulation test may suggest pregnancy due to cross-reactivity, only a dedicated pregnancy test, which measures hCG exclusively, provides a reliable and definitive confirmation. Relying solely on an OPK for pregnancy status is not recommended due to the potential for misleading results.