The question “How many weeks pregnant am I?” often leads people to search for an HCG calculator, seeking a precise answer from a blood test result. Human Chorionic Gonadotropin (HCG) is known as the pregnancy hormone because its presence is what home pregnancy tests detect. While HCG measurements confirm pregnancy and offer insight into its early progression, they are not a tool for calculating exact gestational age like an ultrasound is. Understanding how this hormone works and how its levels change is the first step toward interpreting a test result.
Understanding the HCG Hormone
HCG is a glycoprotein hormone produced by the trophoblast cells that eventually form the placenta. This hormone is detectable in the blood and urine shortly after implantation of the fertilized egg into the uterine wall. The rapid appearance of HCG makes it the reliable marker used in all pregnancy tests.
The primary function of HCG in early pregnancy is to maintain the corpus luteum, a temporary structure in the ovary. By stimulating the corpus luteum, HCG ensures the continued production of progesterone and estrogen, hormones that thicken the uterine lining and prevent menstruation. This supports the developing embryo before the placenta is fully formed and takes over hormone production.
HCG testing is done in two main ways: qualitative and quantitative. A qualitative test, such as a urine strip test, simply detects whether HCG is present, giving a positive or negative result. A quantitative test, often called a beta-HCG test, is a blood test that measures the exact concentration of the hormone in milli-international units per milliliter (mIU/mL).
Tracking HCG Levels to Estimate Gestational Age
In the earliest weeks of pregnancy, the rate of HCG increase is more informative than the single absolute number. In a typical progressing pregnancy, HCG concentrations approximately double every 48 to 72 hours, particularly when levels are below 1,200 mIU/mL. This doubling time indicates that the pregnancy is developing as expected.
The rate of increase gradually slows as the pregnancy advances, and the doubling time may extend to about 96 hours once the HCG level exceeds 6,000 mIU/mL. HCG levels generally peak between 9 and 12 weeks of gestation, often reaching values over 100,000 mIU/mL. After this peak, the concentration naturally declines and levels off for the remainder of the second and third trimesters.
Since a single HCG number cannot precisely date a pregnancy, healthcare providers use broad reference ranges correlated to weeks from the last menstrual period (LMP) to provide context. For example, at 4 weeks LMP, the expected range can be 5 to 426 mIU/mL, and at 5 weeks, the range expands significantly, from 18 to 7,340 mIU/mL. These figures are guides, and healthy pregnancies can occur even at the low end of the expected range.
Limitations and Transition to Ultrasound Dating
The wide range of normal HCG values across healthy pregnancies is the main reason this number cannot serve as a precise gestational age calculator. Two women who are exactly 5 weeks pregnant can have HCG values that differ by a factor of 100, yet both can have healthy deliveries. Therefore, HCG testing is used primarily to confirm viability and progression, not to establish a due date.
Once HCG levels reach a certain threshold, the focus shifts from blood testing to physical imaging. This shift occurs because a transvaginal ultrasound can typically visualize a gestational sac when the HCG level is between 1,500 and 2,000 mIU/mL. This range is often referred to as the discriminatory zone, though modern practice recognizes that this value can vary.
The transition to ultrasound provides a far more accurate method for determining gestational age. The most accurate dating measurement during the first trimester is the Crown-Rump Length (CRL), which is the length of the embryo from the top of the head to the bottom of the buttocks. Measuring the CRL between 8 and 14 weeks of gestation offers an estimate of gestational age accurate within a few days.
After the first few weeks, the ultrasound measurement becomes the gold standard for dating because it relies on the physical size of the embryo, which is less variable than hormone levels. Once a clear CRL measurement is obtained, the estimated due date is set, and further HCG tracking is generally discontinued for dating the pregnancy.