Luteinizing Hormone (LH) is a protein hormone produced by the pituitary gland, a small organ located at the base of the brain. Its primary function is directly related to fertility, specifically triggering the release of a mature egg from the ovary. People track LH levels, often using at-home test strips, to pinpoint the window of time each month that offers the best chance for conception. The question of what an LH chart looks like when pregnant arises from the common confusion between LH and the hormone that actually confirms and sustains pregnancy. This confusion is understandable, as the tools used to track LH can sometimes produce unexpected results once conception has occurred.
LH: The Hormone of Ovulation
The menstrual cycle is orchestrated by a complex interplay of hormones, with LH playing a central role in the days leading up to ovulation. During the follicular phase of the cycle, LH works alongside Follicle-Stimulating Hormone to promote the growth and maturation of fluid-filled sacs called follicles within the ovaries. As a dominant follicle matures, it produces increasing amounts of estrogen, which signals the pituitary gland.
When the estrogen reaches a certain threshold, it triggers a dramatic and rapid increase in Luteinizing Hormone known as the LH surge. This surge typically lasts for about 24 to 48 hours and is the direct trigger for the ovary to release the egg, an event known as ovulation. After the egg is released, the LH hormone helps transform the remaining follicle into the corpus luteum, which begins producing progesterone to prepare the uterine lining for potential implantation.
The purpose of an LH chart, created by using Ovulation Predictor Kits (OPKs), is to capture this pre-ovulatory surge. A successful chart clearly shows a transition from low baseline LH levels to a dark test line, signifying that ovulation is imminent. If no pregnancy occurs, the corpus luteum breaks down, progesterone and estrogen levels fall, and the cycle resets, with LH levels returning to their low baseline state.
LH Levels After Implantation
Once a fertilized egg successfully implants in the uterine wall, the body shifts its hormonal priorities to support the developing pregnancy. The newly formed embryonic tissue begins to produce a completely different hormone, Human Chorionic Gonadotropin (HCG). HCG takes over the function of sustaining the corpus luteum, which is now needed to produce high levels of progesterone and estrogen.
This sustained production of progesterone and estrogen creates a powerful negative feedback loop that signals the pituitary gland to significantly decrease its own hormone production. The pituitary gland is suppressed, and the secretion of LH essentially halts. Throughout the majority of a healthy pregnancy, Luteinizing Hormone levels are extremely low or negligible.
Therefore, the actual physiological LH chart of a person who is pregnant would show low, flat lines, a complete contrast to the high spike seen just before ovulation. The maintenance of low LH levels is a necessary biological mechanism to ensure that no further follicles are stimulated and no new ovulation event occurs during gestation.
Why LH Tests React to Pregnancy
The confusion surrounding the LH chart stems from the fact that an OPK test strip can often produce a dark, positive result when a person is pregnant. This phenomenon is known as hormonal cross-reactivity and is rooted in the structural similarities between LH and HCG. Both Luteinizing Hormone and Human Chorionic Gonadotropin belong to a family of hormones called glycoproteins, and they share an identical alpha subunit.
The two hormones differ only in their unique beta subunit, which is why they bind to the same receptors in the body. Ovulation predictor kits are immunoassays, which means they use antibodies to detect a specific protein structure—in this case, the structure of LH. Because HCG shares a high degree of structural homology with LH, the antibodies in the LH test strip cannot reliably distinguish between the two hormones.
When a pregnancy is established, HCG levels rise rapidly, reaching concentrations far higher than the peak LH surge. This abundance of HCG essentially “tricks” the OPK into registering a positive result. The test is detecting the pregnancy hormone, not the ovulation hormone it was designed for, leading to the misleading appearance of a “positive LH chart” during pregnancy.
Monitoring Pregnancy: The HCG Chart
While an LH test can offer an early indication of pregnancy due to cross-reactivity, it is not a tool for monitoring a pregnancy’s health or progression. The hormone medically relevant for this purpose is Human Chorionic Gonadotropin (HCG), and its levels are tracked via blood tests to create an HCG chart. This chart provides quantifiable data on the status of the early pregnancy.
In a viable, developing pregnancy, the HCG concentration in the blood rises exponentially in the very first weeks. For levels below approximately 1,200 milli-international units per milliliter (mIU/ml), the expected doubling time is about every 48 to 72 hours. As the pregnancy progresses and HCG levels climb higher, the rate of increase naturally slows.
HCG levels continue their steep ascent, typically peaking between 8 and 11 weeks of gestation. After this peak, the levels will gradually decline and then plateau for the remainder of the pregnancy. Monitoring the HCG chart, specifically the doubling rate, helps healthcare providers assess the health of the placental development and determine if the pregnancy is progressing as expected.