An HCG pregnancy test checks human chorionic gonadotropin (HCG) levels in the blood or urine. This measurement means that an HCG test can determine whether a person is pregnant, as well as whether their body is producing the right level of pregnancy hormones.

Typically, HCG levels increase steadily during the first trimester, peak, then decline in the second and third trimesters as the pregnancy progresses.

Doctors may order several HCG blood tests over several days to monitor how a person’s HCG levels change. This HCG trend can help doctors determine how a pregnancy is developing.

Key points to know about HCG pregnancy tests include the following:

  • Home pregnancy tests are about 99% accurate when a person takes them correctly.
  • For the most accurate results, a person should not take an HCG test until after the first missed period.
  • A home test cannot detect pregnancy complications.

This article looks at HCG levels and how they relate to pregnancy. We also examine the potential results and accuracy of an HCG pregnancy test.

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Many people have very low levels of HCG in their blood and urine when they are not pregnant. HCG tests detect elevated levels.

Some tests may not detect pregnancy until HCG has risen to a certain level. Tests that can detect lower levels of HCG may diagnose pregnancy earlier.

Blood tests are typically more sensitive than urine tests. However, many home urine tests are highly sensitive. A 2014 analysis found that four types of home pregnancy tests could detect HCG levels up to 4 days before the expected period, or about 10 days after ovulation for many people.

Cells that become the placenta produce the hormone HCG. A person’s HCG levels quickly rise during the first few weeks of pregnancy.

HCG levels not only signal pregnancy but are also a way to measure whether a pregnancy is developing correctly.

Very low HCG levels may point to a problem with the pregnancy, be a sign of an ectopic pregnancy, or warn that pregnancy loss could occur. Rapidly rising HCG levels can signal a molar pregnancy, a condition that causes a uterine tumor to grow.

Doctors require multiple HCG measurements to track the development of a pregnancy.

HCG levels stop rising late in the first trimester. This leveling out may be why many people experience relief from pregnancy symptoms, such as nausea and fatigue, around this time.

There are two types of HCG tests: qualitative and quantitative.

Qualitative HCG tests

A person can use this type of test to check for elevated HCG levels in the urine or blood. Urine tests are about as accurate as blood tests. A high level of HCG indicates that a person is pregnant.

A negative qualitative HCG test means a person is not pregnant. If they still suspect that they are pregnant, a person should repeat the test after a few days.

False-positive results may occur if hormone levels are high due to menopause or hormone supplements. Some ovarian or testicular tumors may also raise a person’s HCG levels.

Learn more about false-positive pregnancy tests here.

Quantitative HCG tests

Also called a beta HCG test, this blood test measures the specific HCG hormone in your blood in international units per liter (IU/L). The level of HCG helps determine the age of the fetus.

HCG levels rise in the first trimester and then slightly drop. They typically peak at 28,000–210,000 IU/L around 12 weeks after conception.

If HCG is higher than the average pregnancy level, it could indicate more than one fetus.

People must read the urine test instructions and follow them carefully. Most tests use lines to show when a test is positive. The test line does not have to be as dark as the control line to be positive. Any line at all indicates the test is positive.

An individual must check the test within the time frame the instructions indicate. This is typically around 2 minutes.

Test strips can change color as they dry. Some people notice an evaporation line after several minutes. This is a very faint line that may look like a shadow.

Learn everything you need to know about pregnancy tests here.

Each pregnancy is different, but home pregnancy tests are close to 99% accurate if a person uses them as instructed. False-positive results are rarer than false-negative results.

Due to how long it takes for HCG levels to rise, a person can be pregnant and still get a negative test. A positive result usually appears after retesting a few days later.

However, because home pregnancy tests are increasingly sensitive, some can detect very early pregnancies with low HCG levels.

The best time to use an HCG test

For the most accurate results, a person should not take a home pregnancy test until 1–2 weeks after a missed period.

Implantation usually happens a week or so after ovulation. It can take several days for HCG levels to rise high enough for a test to detect the hormone. The earlier a person takes the home pregnancy test, the more difficult it is to detect this hormone.

HCG blood tests can detect pregnancy about 10 days after conception, while urine tests generally take 2 or more weeks.

Although some home pregnancy tests can provide results before the first missed period, the results are usually more accurate after the first day of the missed period.

In addition to testing too early, the following factors can cause a false negative with a urine HCG test:

  • drinking lots of water so that the urine is very diluted
  • getting too much or too little urine on the test strip
  • testing with urine late in the day when it may be weaker

Tests that have passed their expiry date may produce false positives or negatives. Reused tests are not accurate.

In rare cases, a person can have abnormally high levels of HCG even though they are not pregnant. The most common reasons for this include:

Less common causes include:

HCG tests are very safe. The urine test is risk-free. The blood test can cause brief pain and occasional bruising at the puncture site.

Urine HCG tests can return false-negative results, particularly very early in pregnancy. This can be stressful and demoralizing to people having difficulty becoming pregnant. Blood tests are typically more accurate, though even these may fail to pick up low levels of HCG in early pregnancy.

It is also possible to get an early positive result. Some people get positive tests very early in pregnancy and then experience a pregnancy loss a few days later.

If they had not had an early HCG test, they might not have known they were pregnant. This can be a stressful and alarming experience, with some people experiencing intense grief over the lost pregnancy.

If a person receives a positive HCG pregnancy result, they should make an appointment as soon as possible with a doctor, nurse, or family planning clinic. They will be able to take a follow-up test to confirm the pregnancy and detect any possible complications.

A person can use these appointments to discuss pregnancy with a health care professional who can provide resources and counseling.

Learn more by visiting our pregnancy and parenthood hub here.

Most people first learn about a pregnancy through a home HCG pregnancy test, and many confirm that the pregnancy is healthy with blood HCG testing. Though imperfect, the test is typically reliable, particularly as a pregnancy progresses.

The information that a single HCG test provides cannot differentiate between ectopic pregnancies, molar pregnancies, or other pregnancy complications. Doctors will need further information to identify these pregnancy issues and may request further blood HCG tests to observe a person’s HCG trend.

Although unusually high or low HCG levels can indicate a problem with the pregnancy, this is not always the case.

People who have concerns about the development of the embryo or the pregnancy should ask a healthcare professional about additional testing options.