Human Chorionic Gonadotropin (HCG) is a hormone produced by the placenta shortly after a fertilized egg implants in the uterine wall. It is often referred to as the pregnancy hormone because its presence is what home pregnancy tests detect to confirm a pregnancy. HCG plays a significant role in early pregnancy by signaling the body to maintain the uterine lining and to continue producing progesterone and estrogen, hormones essential for supporting the developing fetus. Progesterone, in particular, helps thicken the uterine lining and keeps the uterus relaxed, which is important for a healthy pregnancy.
Understanding HCG Level Increases
In a healthy early pregnancy, HCG levels typically show a rapid increase. The concentration of HCG usually doubles approximately every 48 to 72 hours, particularly during the first four to six weeks of gestation. This doubling rate can vary somewhat, and as pregnancy progresses, specifically after HCG levels exceed 1,200 mIU/mL, the doubling time may extend to 72-96 hours. Beyond 6,000 mIU/mL, the doubling time can be even longer, sometimes taking more than four days.
HCG levels peak around 8 to 11 weeks of pregnancy, after which they naturally decline and then level off for the remainder of the gestation. While there is a wide range of what is considered “normal” for HCG levels at different gestational stages, the pattern of increase is generally more indicative of pregnancy progression than a single measurement. For instance, at 3-4 weeks, levels might range from 5-426 mIU/mL, and by 5-6 weeks, they can be between 1,080-56,500 mIU/mL. Monitoring the rate of rise over several days provides a more accurate assessment.
When HCG Rise Doesn’t Indicate Viability
While rising HCG levels are generally a positive sign, they do not definitively guarantee a viable pregnancy. Miscarriage remains a possibility even with increasing HCG, as specific scenarios exist where HCG levels may rise but the pregnancy is not viable.
One such scenario is an ectopic pregnancy, where the fertilized egg implants outside the uterus, most commonly in a fallopian tube. In these cases, HCG levels often rise, but typically at a slower rate or may plateau, increasing less than 66% in 48 hours.
Another situation is a blighted ovum, also known as an anembryonic pregnancy. Here, a fertilized egg implants in the uterus, and the gestational sac and placenta begin to develop, producing HCG, but no embryo forms within the sac. Consequently, HCG levels may rise, even though the pregnancy is not viable.
Molar pregnancies, an abnormal growth of placental tissue, also cause HCG levels to rise, often to abnormally high concentrations. In a complete molar pregnancy, no embryo forms, and the abnormal placental tissue grows rapidly, leading to very high HCG levels.
Finally, a rise in HCG that is significantly slower than expected, even if consistently increasing, can indicate a non-viable pregnancy that may lead to miscarriage. A rise of less than 35% over 48 hours can be considered abnormal. Such slow-rising levels suggest the embryo may not be developing properly, requiring further testing to determine the prognosis.
Comprehensive Pregnancy Assessment
HCG levels represent only one component in assessing early pregnancy viability. Other diagnostic tools, particularly ultrasound, play a role in providing a more complete picture. Ultrasound findings are interpreted in conjunction with HCG levels to determine the health and location of a pregnancy.
A transvaginal ultrasound can typically detect a gestational sac when HCG levels reach approximately 1,500 to 2,000 mIU/mL, usually around 5 to 6 weeks of gestation. A yolk sac can often be seen by 6 weeks, or when the gestational sac reaches about 10 mm. The presence of a fetal pole with heart tones is generally visible by the completion of 7 menstrual weeks, or when HCG levels exceed 10,800 mIU/mL.
If HCG levels are rising but an ultrasound does not show expected structures for the gestational age and HCG level, it can raise concerns about conditions like ectopic pregnancy or blighted ovum. Clinical symptoms such as vaginal bleeding, cramping, or the disappearance of pregnancy symptoms (like nausea or breast tenderness) are also important indicators that prompt further investigation. A comprehensive assessment involving HCG monitoring and ultrasound imaging helps diagnose and manage early pregnancy outcomes.