Luteinizing Hormone (LH) tests, commonly known as ovulation predictor kits (OPKs), detect a specific hormonal surge. These tests use a Test/Control (T/C) ratio to interpret results by comparing the intensity of a test line to a control line. Understanding how these tests function, especially when pregnancy is a possibility, helps clarify potentially confusing results.
Understanding LH and Test Ratios
Luteinizing Hormone (LH) is a hormone produced by the pituitary gland. Its primary function in the female reproductive cycle is to signal the ovary to release a mature egg, known as ovulation. LH levels typically remain low throughout most of the menstrual cycle, but they experience a rapid increase, or “surge,” 24 to 36 hours before ovulation.
At-home ovulation predictor kits (OPKs) detect this LH surge in urine. These tests feature two lines: a test line (T) and a control line (C). The T/C ratio compares the color intensity of the test line to the control line. A T/C ratio at or above 1.0, where the test line is as dark as or darker than the control line, indicates an LH surge. This signals that ovulation is likely to occur soon, marking the most fertile window for conception.
Why LH Tests Might Show a Positive Result During Pregnancy
An LH test may show a positive result during pregnancy due to structural similarities between Luteinizing Hormone (LH) and Human Chorionic Gonadotropin (HCG), the hormone produced during pregnancy. Both LH and HCG belong to the same family of glycoprotein hormones and share a nearly identical alpha subunit.
While HCG possesses a unique beta subunit, its molecular structure is similar enough to LH that antibodies in some LH test strips can cross-react with HCG. This means an LH test, designed to detect LH, can inadvertently pick up HCG in the urine. The test may then display a positive result, even though the body is producing HCG due to pregnancy, not an LH surge.
Interpreting Your LH Test Results When Pregnant
If an LH test shows a positive result when pregnancy is suspected, it is not a reliable confirmation of pregnancy. While HCG and LH cross-reactivity can cause an ovulation test to appear positive, these tests are not designed or calibrated to detect pregnancy. LH tests for HCG vary in sensitivity and are generally less sensitive than dedicated pregnancy tests.
An LH test positive during pregnancy might indicate the presence of HCG but does not confirm a viable pregnancy. Other factors can also lead to elevated LH levels, such as Polycystic Ovary Syndrome (PCOS) or perimenopause. However, in suspected pregnancy, the most probable explanation for a positive LH test is cross-reactivity with HCG.
Accurate Pregnancy Confirmation
For accurate pregnancy confirmation, use tests specifically designed to detect Human Chorionic Gonadotropin (HCG). Home pregnancy tests identify HCG in urine and are widely available and generally reliable when used correctly.
These tests are formulated to detect HCG with high sensitivity, providing a clear positive or negative result. For definitive confirmation or if uncertain about home test results, consult a healthcare professional. A doctor can perform more sensitive urine or blood tests to measure HCG levels, confirming pregnancy earlier and more precisely. Blood tests can detect pregnancy within seven to ten days after ovulation.