Luteinizing Hormone (LH) is a pituitary hormone that regulates reproductive timing. While commonly associated with predicting ovulation, LH testing is sometimes mistaken for or used in place of standard pregnancy testing. This confusion arises because the two testing methods interact with hormones that are molecularly similar. This analysis clarifies the relationship between LH levels and established pregnancy.
LH’s Role in the Menstrual Cycle
LH is secreted by the pituitary gland, maintaining low levels throughout most of the menstrual cycle. This baseline level is necessary for the development of ovarian follicles.
The most dramatic event is the “LH surge,” a rapid spike in concentration occurring roughly 24 to 36 hours before ovulation. This surge is the biological signal that triggers the mature egg release. Ovulation Predictor Kits (OPKs) are designed to detect this temporary increase in LH.
When a person is not pregnant, an LH test typically shows a faint or no line, indicating a low baseline level. A positive result (test line as dark as or darker than the control line) is expected only during the short window of the LH surge. After ovulation, LH levels return to their low baseline until the next cycle begins.
Molecular Similarities Between LH and hCG
The reason an LH test can appear positive during pregnancy is rooted in the structural biology of the hormones. Human Chorionic Gonadotropin (hCG) is produced by the developing placenta and is the marker dedicated pregnancy tests detect. Both LH and hCG belong to the family of glycoprotein hormones.
These two hormones are heterodimers, composed of two distinct chains: an alpha subunit and a beta subunit. Crucially, LH and hCG share an identical alpha subunit. This common structural component means that antibody-based tests designed to detect that subunit will react to both hormones.
The hormone-specific beta subunits are also highly similar, sharing approximately 85% amino acid sequence homology. Antibodies used in many inexpensive LH test strips cannot reliably differentiate between the high concentration of hCG and the lower concentration of LH. This cross-reactivity is the fundamental reason why a positive LH test can occur in early pregnancy.
Interpreting LH Test Results During Early Pregnancy
When a person is pregnant, the high level of hCG circulating in the body activates the LH test strip. The LH test line may appear dark or positive, mimicking a true LH surge. The test line resulting from hCG can often be significantly darker than the control line because hCG concentration in early pregnancy is much higher than LH during a typical surge.
Using an LH test for pregnancy detection is an unreliable, off-label application. LH tests are qualitative tools designed to measure a rapid temporary spike, not the gradual, sustained rise of hCG. The sensitivity of the antibodies to hCG varies significantly between different brands and batches of ovulation tests.
A dedicated hCG test or a blood test ordered by a healthcare provider is the only reliable method for confirming pregnancy. While a dark LH test line might suggest a high level of cross-reactive hormone, it should only be considered a possible indicator. A standard pregnancy test is engineered with antibodies that target the unique structure of the hCG beta subunit, ensuring a more accurate result.