Human chorionic gonadotropin (hCG) is a hormone produced by the body during pregnancy. It supports fetal growth.

Doctors test hCG levels in the urine and blood to confirm pregnancy. They also use hCG blood tests to help determine if a person could be experiencing an ectopic pregnancy or miscarriage.

Pregnancy, ectopic pregnancy, and miscarriage will never be diagnosed based on one hCG level alone, but it’s helpful to know how these levels work in such cases.

If you think you’re pregnant, a doctor will test blood drawn from a vein to check your hCG levels.

If you don’t have any hCG present in your blood, this doesn’t necessarily mean you aren’t pregnant. You may be too early in your pregnancy for your hCG levels to increase.

HCG levels higher than 5 million international units per milliliter (mIU/mL) typically indicate pregnancy. Your first test result is considered a baseline level. This level can range from very small amounts of hCG (such as 20 mIU/mL or even lower) to larger amounts (such as 2,500 mIU/mL).

The baseline level is important because of a concept doctors call doubling time. In the first four weeks of a viable pregnancy, hCG levels will typically double about every two to three days. After six weeks, the levels will double about every 96 hours.

So, if your baseline level is higher than 5 mIU/mL, your doctor may order a repeat test a couple days later to see if the number doubles.

In the absence of certain risks, this (or one additional level) may be enough to determine pregnancy. In many cases, your doctor will then recommend you have an ultrasound sometime between 8 and 12 weeks as part of first trimester pregnancy care.

If you’re at risk for miscarriage or an ectopic pregnancy, you’re more likely to have hCG levels that don’t double. They may even decrease. Therefore, your doctor may ask you to return to their office two to three days after your baseline blood test to see if your level has doubled appropriately.

If your hCG levels don’t come close to doubling after 48 to 72 hours, your doctor may have concerns that the pregnancy is at risk. Medically, this may be called a possible “nonviable pregnancy.”

If your levels are dropping or rising too slowly, you’ll probably be sent for other testing as well. This could include progesterone blood tests and a transvaginal ultrasound to check your uterus for a gestational sac. Other symptoms, such as bleeding or cramping, will also be taken into account.

In the event of a miscarriage, hCG levels typically decrease from previous measurements. For example, a baseline level of 120 mIU/mL that’s dropped to 80 mIU/mL two days later can indicate the embryo is no longer developing and the body isn’t producing more hormones to support its growth.

Likewise, levels that aren’t doubling and are only rising very slowly — for example, from 120 mIU/mL to 130 mIU/mL over a period of two days — can indicate a nonviable uterine pregnancy in which a miscarriage may take place soon.

Levels that are slow to rise can also indicate a non-uterine pregnancy, which happens when the fertilized egg implants somewhere outside the uterus (usually the fallopian tubes). Because an ectopic pregnancy can be a medical emergency, it’s important a doctor identify this as quickly as possible.

On the other hand, it’s also possible to have doubling hCG levels with an ectopic pregnancy. This is why hCG levels alone aren’t enough to determine what’s going on with 100 percent accuracy.

A low baseline isn’t actually an indicator of any issues in and of itself. The normal ranges for hCG at various points of pregnancy are very wide.

For example, just one day after your missed period, your hCG level may be just 10 or 15 mIU/mL. Or it may be more than 200 mIU/mL. Each pregnancy is different in this regard.

What really matters is the change over time. Different people will have different baselines and still have lasting pregnancies.

If your levels are dropping, the outlook for your pregnancy isn’t usually a positive one.

It’s possible a laboratory could have made an error. It could also be the case that a preexisting condition, such as ovarian hyperstimulation syndrome (OHSS) following fertility treatments, is affecting your hormone levels.

However, in general, declining levels of hCG after a positive pregnancy result isn’t a good sign. Chances are the pregnancy is nonviable, according to the journal Fertility and Sterility.

Slowly increasing hCG levels don’t necessarily mean you’re miscarrying, though they will usually signal further testing to see if you are.

Doctors use data based on smaller-scale studies in those who conceived after pregnancy treatments, according to the journal Fertility and Sterility. The hCG numbers may be helpful in guiding next steps, but they aren’t an absolute indicator of either a miscarriage or a viable pregnancy.

Doctors chiefly use doubling times to confirm a pregnancy, not diagnose a miscarriage. According to the journal , a 53 percent or greater rise in hCG levels after two days can confirm a viable pregnancy in 99 percent of pregnancies.

An important factor to consider with doubling times is the starting hCG value. For example, those with a baseline hCG level below 1,500 mIU/mL have more “room” to increase their hCG levels.

Someone who may be further along than they think and starts at a high hCG level of 5,000 mIU/mL or greater typically doesn’t have the same rate of hCG increases, according to .

Carrying multiples (twins, triplets, etc.) can affect the rate of hCG rise, as well as how far along you are.

Ectopic pregnancy and miscarriage can result in lower hCG levels. A molar pregnancy can result in higher levels.

Doctors will use a variety of tests to confirm a miscarriage. These include:

  • performing blood tests, including hCG and progesterone
  • considering symptoms, such as pelvic cramping or vaginal bleeding
  • performing a vaginal ultrasound and pelvic exam
  • conducting fetal heart scanning (if your dates indicate a fetal heartbeat should be detectable)

Your doctor will ideally take several pieces of information into account before diagnosing a miscarriage. If the pregnancy is very early, declining hCG levels may be the only way to determine that miscarriage is likely until a little more time passes.

It’s important doctors identify a miscarriage or ectopic pregnancy as early as possible. An ectopic pregnancy can result in rupture of a fallopian tube or other injury that threatens your fertility and life. A miscarriage that results in retained tissue increases infection and bleeding risk.

For these reasons, if you’re experiencing a pregnancy loss, your doctor may recommend taking medications or having certain surgical treatments to minimize complications.

Pregnancy loss can also take an emotional toll. A diagnosis can provide closure and allow grieving and the healing process to begin.

When you miscarry (and also anytime you give birth), your body no longer produces hCG. Your levels will ultimately go back to 0 mIU/mL.

In fact, anything less than 5 mIU/mL is “negative,” so effectively, 1 to 4 mIU/mL is also considered “zero” by doctors.

If you have a miscarriage, the time it takes for your levels to go to zero varies based on how high your levels were at time of miscarriage. If you miscarry very early in your pregnancy and your hCG levels haven’t increased by much, your levels will usually return to zero within a few days.

If your hCG level was in the thousands or tens of thousands when you miscarried, it may take several weeks for your levels to return to zero, according to the American Association for Clinical Chemistry.

When you do get to zero, you’ll usually start having your period and ovulating again.

Doctors don’t usually recommend trying to get pregnant again until you have that first period after your miscarriage. This makes it easier to calculate your due date.

If you have a D and C (dilation and curettage) procedure as part of your miscarriage, your doctor may recommend waiting two or three cycles before trying to get pregnant again. This is because a D and C can thin the uterine lining, and a thicker lining is better in pregnancy. The lining will build back up over a few months.

Early miscarriage can be a painful emotional and physical experience. If you suspect you may be having a miscarriage, talk to your doctor. Your doctor can order tests, including the hCG blood test, to provide you with further information.

If you do have a miscarriage, know that it doesn’t mean you won’t go on to have a successful pregnancy. In fact, most people do.

Also know that there are many organizations that provide support for those who have experienced pregnancy loss. Talk to your doctor for more information.