Human Chorionic Gonadotropin (hCG) is a glycoprotein hormone often called the “pregnancy hormone” because its presence indicates a developing pregnancy. It is produced by trophoblast cells, which eventually form the placenta. The primary function of hCG is to maintain the corpus luteum, a temporary structure that secretes progesterone. Progesterone thickens the uterine lining and enriches it with blood vessels, creating a stable environment for the early pregnancy. Measuring hCG in the blood is a common method used by healthcare providers to confirm pregnancy and gain insight into its early progression.
Understanding hCG Levels and Gestational Weeks
A quantitative blood test, often called a beta-hCG test, measures the precise amount of this hormone in milli-international units per milliliter (mIU/mL). These measurements are correlated with the number of weeks since the Last Menstrual Period (LMP) to estimate gestational age. For example, at three weeks LMP, the range is typically 5–50 mIU/mL. By four weeks LMP, the range widens to 5–426 mIU/mL, and at five weeks, it can span from 18 to 7,340 mIU/mL.
These ranges demonstrate the immense variability that exists even in healthy pregnancies. By six weeks LMP, the concentration increases substantially, ranging from 1,080 to 56,500 mIU/mL. The ranges become even broader later in the first trimester, sometimes reaching over 200,000 mIU/mL between seven and twelve weeks.
This wide variation means that a single hCG number cannot pinpoint an exact date for conception or delivery. Differences stem partly from the natural variation in when a fertilized egg implants into the uterine wall, which can vary by several days. Laboratory sensitivity and reference ranges also vary slightly, making it difficult to compare results across different facilities.
For instance, an hCG level of 3,800 mIU/mL could fall within the expected range for a pregnancy at five weeks or six weeks. This variability makes a definitive age determination impossible based on this number alone.
The Significance of hCG Doubling Time
While a single hCG reading offers only a broad estimate of gestational age, the rate at which the hormone level increases provides more meaningful information about the health of the developing pregnancy. In a viable, early pregnancy, the hCG concentration typically doubles every 48 to 72 hours. This “doubling time” is evaluated by taking two blood tests two days apart to assess the trend.
A satisfactory doubling time suggests that the trophoblast tissue is growing appropriately and the pregnancy is progressing as expected. If the doubling time is significantly slower than 48 to 72 hours, it may indicate a miscarriage or an ectopic pregnancy. Conversely, an abnormally fast rise in levels can sometimes be associated with a multiple gestation, such as twins, or a molar pregnancy.
This rapid doubling rate slows down considerably as the pregnancy progresses. Once the hCG level rises above approximately 6,000 mIU/mL, the doubling time increases, sometimes taking 96 hours or more. The rate of increase continues to slow until the hormone levels peak around 8 to 12 weeks of gestation.
Why hCG Levels Alone Are Not Definitive for Dating
The wide ranges and variation in doubling rates illustrate why hCG measurements are not a definitive tool for precisely dating a pregnancy. Due to biological differences in implantation timing and hormone production, two people who conceived on the same day could have vastly different hCG levels. Relying solely on hCG values provides only an approximate gestational age.
The accepted standard for establishing the most accurate due date in the first trimester is an early obstetrical ultrasound. This method uses measurements of the developing fetus, such as the Crown-Rump Length (CRL), to determine gestational age with greater precision than hormone levels. An early ultrasound is typically performed once the hCG concentration reaches the “discriminatory zone.”
This zone, often cited as 1,500 to 2,000 mIU/mL, is the level at which a gestational sac should be visible inside the uterus via transvaginal ultrasound. If the hCG level is above this zone and no sac is visualized, it raises concern for a potential ectopic pregnancy. After 10 to 12 weeks, hCG levels naturally peak and then decline, making them ineffective for dating beyond the first trimester.