Human Chorionic Gonadotropin (HCG) is a hormone produced during pregnancy, supporting early fetal development. This hormone is routinely measured in prenatal screening tests, providing information about pregnancy progression. Understanding HCG and its role in these screenings offers insights into maternal and fetal health.
The Hormone HCG in Pregnancy
Human Chorionic Gonadotropin is primarily produced by the placenta, the organ that forms during pregnancy to provide oxygen and nutrients to the developing fetus. Immediately following the implantation of a fertilized egg into the uterine wall, the cells that will eventually form the placenta begin to secrete HCG.
HCG’s main function is to maintain the corpus luteum, a temporary ovarian structure that produces progesterone. Progesterone thickens and sustains the uterine lining, supporting embryo growth and preventing menstruation. HCG ensures progesterone supply until the placenta takes over production, around 8 to 12 weeks of gestation. HCG levels rise rapidly in early pregnancy, doubling every 48 to 72 hours.
HCG Levels in Prenatal Screening
HCG levels are a marker measured in prenatal screening tests, such as the first-trimester screen and the quad screen. These tests assess the risk of certain chromosomal conditions and are not diagnostic tools. HCG interpretation depends on the gestational week, as the hormone’s concentration changes throughout pregnancy.
HCG levels increase rapidly during the first trimester, peaking around 8 to 11 weeks of gestation. After peaking, levels gradually decline and plateau for the remainder of pregnancy. This pattern allows healthcare providers to compare an individual’s HCG level to the expected range for their specific week, identifying variations that might warrant further investigation.
Understanding HCG Levels and Down Syndrome Risk
High HCG levels, particularly when expressed as multiples of the median (MoM), can indicate an increased risk for Down Syndrome (Trisomy 21). This risk assessment is made in conjunction with other markers. In the first-trimester screen, high HCG is evaluated alongside low Pregnancy-Associated Plasma Protein-A (PAPP-A) and increased nuchal translucency from an ultrasound.
For the quad screen, performed in the second trimester, elevated HCG levels are considered with low alpha-fetoprotein (AFP), low unconjugated estriol (uE3), and elevated Inhibin-A. HCG levels alone are not a diagnosis of Down Syndrome. Instead, they contribute to a statistical risk assessment, indicating a higher probability that further testing might be beneficial.
Next Steps After Screening
An increased risk result from prenatal screening, which might include elevated HCG levels, does not confirm Down Syndrome. Screening tests only identify a higher likelihood. If screening results suggest an increased risk, healthcare providers discuss options for further, more definitive testing.
These diagnostic tests include non-invasive prenatal testing (NIPT), chorionic villus sampling (CVS), and amniocentesis. NIPT is a blood test analyzing fetal DNA fragments in the mother’s bloodstream, performed as early as 10 weeks. CVS involves taking a small placental tissue sample, between 10 and 13 weeks, to test for chromosomal abnormalities. Amniocentesis, performed between 15 and 20 weeks, collects amniotic fluid for genetic analysis. Discussing these options and their implications with a healthcare provider or genetic counselor aids informed decisions.