Human Chorionic Gonadotropin (hCG) is a hormone primarily associated with pregnancy, detected by both at-home urine tests and quantitative blood tests. It is produced by trophoblast cells, the precursors to the placenta, shortly after a fertilized egg implants in the uterine wall. The main job of hCG is to sustain the corpus luteum, which produces progesterone necessary to maintain the uterine lining and support the growing embryo until the placenta takes over hormone production. Understanding the expected pattern of this hormone is important because deviations from the norm can signal potential issues with the pregnancy.
How hCG Levels Change During a Typical Pregnancy
In a healthy pregnancy, the concentration of hCG in the blood rises rapidly in the first few weeks. This increase is often described by the “doubling time,” where levels typically double every 48 to 72 hours. Healthcare providers monitor this rapid rate of increase to assess the viability of an early pregnancy.
The peak concentration of hCG is usually reached toward the end of the first trimester, specifically between eight and eleven weeks of gestation. After reaching this maximum point, the hormone levels begin a slow decline. By approximately 16 weeks, the concentration will plateau and remain constant for the remainder of the pregnancy until delivery.
Factors That Result in Abnormally Low hCG Levels
When hCG levels are lower than expected for the gestational age or fail to rise appropriately, it can indicate a problem with the pregnancy. One common reason for an initial low reading is a miscalculation of the gestational age, meaning the pregnancy is not as far along as initially estimated. This dating error is often corrected when hormone levels are monitored over a few days or confirmed with an ultrasound.
A more concerning reason for low or slowly rising hCG is an ectopic pregnancy, where the fertilized egg implants outside the uterus, most often in a fallopian tube. In these cases, the hormone-producing tissue is compromised, leading to levels that increase insufficiently or fail to double as expected. An abnormally slow rise, sometimes defined as less than a 35% increase over 48 hours, often signals an ectopic pregnancy or a non-viable intrauterine pregnancy.
Falling or plateauing hCG levels after an initial rise may indicate a miscarriage or a blighted ovum. A blighted ovum occurs when the fertilized egg implants and the gestational sac develops, but the embryo itself fails to form or develop. In these situations, the hormone-producing tissue eventually fails, causing the hCG levels to drop.
Conditions That Cause Abnormally High hCG Levels
Levels of hCG significantly higher than the expected range can be caused by the presence of multiple fetuses, such as twins or triplets. More than one embryo means a larger total mass of placental tissue, resulting in greater production of the hormone. A dating error where the pregnancy is farther along than assumed can also result in a high initial hCG reading.
High hCG levels can also signal a molar pregnancy, a rare condition known as Gestational Trophoblastic Disease (GTD). This involves the abnormal and uncontrolled growth of the tissue that would normally form the placenta. Because this tissue proliferates excessively, it produces massive amounts of hCG, often leading to dramatically elevated levels compared to a normal singleton pregnancy.
In some instances, a high hCG result may not be related to a pregnancy at all. Certain non-gestational cancers, including specific types of testicular, ovarian, or lung cancer, can produce the hormone as a tumor marker. This production occurs outside of the reproductive cycle and aids in the diagnosis and monitoring of these malignancies.
Medications and Measurement Variables
External factors like fertility treatments can temporarily interfere with the interpretation of hCG results. Certain fertility medications, often called “trigger shots,” contain exogenous hCG to stimulate ovulation. If a pregnancy test is taken too soon after receiving one of these injections, the residual medication can cause a false-positive result until the administered hormone is cleared from the body.
The specific type of test performed also affects the results; quantitative blood tests provide an exact number, while qualitative urine tests indicate only presence or absence of the hormone. In extremely rare cases, the “hook effect” can occur when hCG concentrations are exceptionally high, such as in advanced molar pregnancies. The overwhelming abundance of the hormone can saturate the testing antibodies, leading to a misleadingly low or even false-negative result on some rapid assays.