Human Chorionic Gonadotropin, commonly known as HCG, is a hormone produced during pregnancy.
Understanding HCG in Pregnancy
HCG is produced by trophoblast cells, part of the developing placenta. Its primary function is maintaining the corpus luteum, which produces progesterone to thicken and sustain the uterine lining, preventing menstruation and supporting early pregnancy.
HCG levels are measured using either blood tests or urine tests. Quantitative blood tests provide a specific numerical value of the hormone in milli-international units per milliliter (mIU/mL). Urine tests, often used in home pregnancy kits, generally provide a qualitative result, indicating presence or absence of HCG. Blood tests can detect HCG as early as 10 to 11 days after conception, preceding detection by most urine tests.
In a typical singleton pregnancy, HCG levels rise rapidly following implantation. They commonly double every 48 to 72 hours during the first four to six weeks. As the pregnancy progresses, the doubling time slows, extending to 72 to 96 hours. HCG levels reach their peak around 8 to 11 weeks of gestation, then gradually decline and stabilize for the remainder of pregnancy.
HCG Levels in Twin Pregnancies
HCG levels in twin or multiple pregnancies are generally higher than in singleton pregnancies. This elevation is due to more placental tissue, as each developing fetus contributes to hormone production. While there is no universal standard, HCG levels in twin pregnancies are estimated to be 30% to 50% higher than for a single fetus. Some observations suggest they could even be twice as high.
While HCG levels are higher, some studies indicate a faster doubling time for twins, such as every 1.4 days, compared to 1.8 days for singletons. For instance, at around 14 days post-ovulation, HCG levels for singletons range from 142 to 211 mIU/mL, whereas for twins, they are between 213 and 316 mIU/mL. Similarly, at five weeks of gestation, a singleton pregnancy shows HCG levels around 7,000 mIU/mL, while a twin pregnancy exhibits levels closer to 15,000 mIU/mL.
There is considerable overlap in HCG ranges between singleton and twin pregnancies, and individual variations are wide. Up to 15% of singleton pregnancies can fall within the reported twin range. Therefore, elevated HCG levels alone, while suggestive, do not definitively confirm a twin pregnancy.
Confirming a Twin Pregnancy
A higher-than-expected HCG level might indicate the pregnancy is further along than initially thought, indicating inaccurate conception dating.
In some instances, unusually high HCG levels can be associated with conditions other than multiple pregnancies. These may include a molar pregnancy, an abnormal uterine growth, or certain types of tumors. Given this, healthcare providers focus on the trend of HCG levels over time rather than a single measurement, observing if levels rise as expected.
The only reliable method for confirming a twin pregnancy is through ultrasound imaging. An ultrasound can identify multiple gestational sacs, fetal poles, and distinct heartbeats. It can detect twins as early as 6 to 8 weeks, with greater clarity and accuracy by 10 to 12 weeks. For early assessments, a transvaginal ultrasound may provide clearer images.