“HCG is first made after the embryo implants into the lining of the uterus. This occurs about six days after ovulation, but can take up to 12 days,” says Dr. Brennan Lang, an Ob-Gyn at Baylor Obstetrics and Gynecology at Texas Children’s Pavilion for Women. While most hormones increase at the beginning of pregnancy and continue to rise throughout the 40-week journey, hCG doesn’t follow that pattern.
Instead, hCG levels usually double in concentration every 29-53 hours during the first several weeks of pregnancy, says Dr. Lang. After about 8-10 weeks, hCG levels begin to fall, and they eventually stop progressing.
"This means that at the beginning of pregnancy, your hCG number (measured in mIU/ml or IU/l ) might be two. It'll then move to four, eight, and 16, and then it really picks up speed, moving into the 25,000 to 50,000 range," says Elisa Ross, M.D., an Ob-Gyn at Cleveland Clinic Hillcrest Hospital. "When it gets to 100,000, it turns around and goes down again. For the rest of your pregnancy, your hCG level will likely remain at 10,000."
Most experts theorize that the reason for change in hCG levels is that, at a certain point in the pregnancy, the placenta takes over making the hormones estrogen and progesterone. This means hCG is no longer needed to stimulate the ovary to make hormones, says Dr. Ross.
Home pregnancy tests measure hCG levels in your urine. “Urine pregnancy tests are very accurate. They can usually detect an hCG level of at least 20-50 mIU/mL,” says Dr. Lang. If the results are positive, he says you need to confirm a pregnancy with an ultrasound of the pelvis, a pelvic exam, or a blood test at the doctor’s office.
“The blood test does have a lower threshold to be positive, measuring levels as low as about one to five mIU/mL,” says Dr. Lang. “However, one single blood test is not always very helpful. Usually, we need at least two measurements (at least 48 hours apart or more) to begin to understand how a pregnancy is progressing.” That’s because trends in hCG levels are more important that the numbers themselves in determining a healthy pregnancy.
"What makes hCG so interesting is that there is a huge range of what's normal when it comes to this number," says Michele Hakakha, M.D., an obstetrician and co-author of Expecting 411. Because the normal range is so large, it's often hard to tell much about a pregnancy from just one hCG measurement, according to Dr. Hakakha.
High or low hCG levels could still indicate a normal pregnancy. Doctors can, however, gain information from looking at trends. A normal pregnancy has hCG levels that double every 48 hours, and hCG levels usually peak around 90-100,000 mIU/mL, says Dr. Lang. Anything significantly higher or lower should be checked by your Ob-Gyn.
“Low hCG levels may represent a very early pregnancy or a pregnancy that is ending in miscarriage,” says Dr. Lang. Other causes include blighted ovum (the fertilized egg fails to develop properly) and ectopic pregnancy (the embryo implanted somewhere outside of the uterus – usually the fallopian tube). A poorly estimated gestational age could also be the culprit, meaning the pregnancy isn’t as far along as previously thought. This is more likely if a woman does not have normal menstrual cycles and is not ovulating on day 14 of her cycle.
According to Dr. Lang, extremely high levels of hCG (greater than 100,000 mIU/mL) may represent an abnormal pregnancy. Causes may include a placental tumor or molar pregnancy, in which a non-viable egg implants in the uterus and secretes the hCG hormone. Higher levels of hCG may also represent pregnancy with multiples, or an inaccurate measurement of gestational age (the pregnancy could be further along than expected).