How hCG levels change by week of pregnancy

You may have heard of hCG, the pregnancy hormone, and even felt concerned about whether your body is making enough. While occasionally a drop in hCG levels can signal a problem, the good news is that most of the time there's more than enough of the hormone for a healthy pregnancy. Here's what you need to know.

What is hCG?

HCG is a hormone produced during pregnancy. The letters stand for human Chorionic Gonadotropin. After an egg is fertilized and implants in the uterus, the cells that will eventually form the placenta start making hCG.

The hormone is responsible for stopping your period and thickening your uterine lining in preparation for your embryo. HCG may contribute to your morning sickness, too.

HCG is often the first indicator of pregnancy – it's what causes a positive result on a home pregnancy test. HCG levels rise exponentially in the first few weeks of pregnancy, typically doubling every two to three days. Then they level off and drop as pregnancy progresses.

Your hCG number isn't something your doctor or midwife will normally track, however. In most cases, this isn't necessary, says Shannon Smith, M.D., an ob-gyn at Brigham Faulkner Ob/Gyn AssociatesOpens a new window in Boston and member of the BabyCenter Medical Advisory Board. It can be important if you have a high risk of miscarriage or a possible molar pregnancy or ectopic pregnancy, however.

Even if your hCG levels are outside the 'normal' range, you can still have a perfectly healthy pregnancy and baby.

- Shannon Smith, M.D., ob-gyn

A chart of hCG levels by week

While hCG levels can vary widely, they tend to fall within a range. Below is the average hCG range during pregnancy, based on the number of weeks since your last menstrual period. (Note, if your cycle is irregular, these numbers may not apply).

HCG is measured by milli-international units of hCG hormone per milliliter of blood (mIU/ml).

After delivery, it can take 7 to 60 days for hCG to drop to pre-pregnancy amounts (less than 5). Tracking the fall of the hormone can sometimes be important – after a molar pregnancy, ectopic pregnancy, or miscarriage, for example.

What hCG levels mean in pregnancy

Your hCG levels at any particular point during pregnancy don't mean much. Although there are average ranges of hCG for different stages of pregnancy, these can vary dramatically from woman to woman and from pregnancy to pregnancy.

"Even if your hCG levels are outside the 'normal' range, you can still have a perfectly healthy pregnancy and baby," says Dr. Smith.

That said, it's important that your hCG levels increase during early pregnancy. If your hCG levels don't continue to rise rapidly during the first few weeks or if they start to drop, this can signal a problem such as an ectopic pregnancy or miscarriage. Higher-than-typical hCG levels can signal that you're carrying twins or multiples or, in rare cases, have a molar pregnancy.

Often, you won't know what your hCG levels are during pregnancy. Home pregnancy tests don't give a specific hCG number. However, you may learn your hCG levels if your healthcare provider orders a blood test to confirm your pregnancy or as part of a screening test.

Your provider may also order a series of two or more hCG blood tests if they have concerns about how your pregnancy is progressing. These blood tests, taken two to three days apart, can tell your provider whether your hCG levels are trending in the right direction.

When can you first detect hCG?

A blood test can detect the hormone as early as 10 days after conception, when the fertilized egg implants in the uterus. However, most women first discover they're pregnant a bit later, using a store-bought home pregnancy test that measures hCG levels in pee.

Home pregnancy tests can detect HCG around 12 to 14 days after conception, when you're around 4 weeks pregnant – or about the time your period is due. ­ (The most common way to calculate your pregnancy due date is from the first day of your last menstrual period, which is typically two weeks before conception.)

False positives are very rare, but negative pregnancy test results can be wrong – if you took the test too early, or if your pee was very diluted, for example. If you get a negative result but think you're pregnant, wait a day or two and take another test. (First, read what not to do before taking a pregnancy test.)

Another possibility, according to the U.S. Department of Health & Human Services' Office on Women's HealthOpens a new window, is that the fertilized egg didn't implant until after the first day of your missed period. (Your body makes hCG after implantation occurs.) This happens in up to 10% of women, for whom home pregnancy tests might not be accurate as early.

It's possible (though rare) to get a false negative pregnancy test while pregnant with twins or multiples. Extremely high hCG levels might prevent a home pregnancy test from interpreting the results correctly. This phenomenon is called the "hook effect."

After doubling about every two to three days, hCG levels generally peak at around 8 to 12 weeks of pregnancy. After that they decline somewhat and usually plateau during the second and third trimester before falling.

Occasionally, hCG levels might fall instead of rise, and then rise again. While declining hCG levels often signal a miscarriage, sometimes the levels go back up and the pregnancy continues normally.

Causes of low hCG levels

Low hCG levels in pregnancy can be a sign of:

• A miscalculated due date. Providers typically calculate your baby's gestational age and your due date based on the date of your last period. But it's common to get this calculation wrong, especially if you have irregular periods or don't remember the date of your last period. Your hCG levels may be lower than expected because your pregnancy isn't as far along as your provider thought.
• Miscarriage. If your hCG levels don't go up during early pregnancy or if they start to decline, this could signal you've had or are about to have a miscarriage. If this is the case, you may experience other signs of miscarriage, such as bleeding and abdominal pain.
• Blighted ovum. This is when a fertilized egg implants in the uterus but never develops into an embryo, so hCG levels don't rise. It's a common cause of miscarriage during early pregnancy.
• Ectopic pregnancy. This is a rare but dangerous condition in which the fertilized egg implants in the fallopian tube instead of the uterus. It's usually accompanied by symptoms such as pelvic pain and vaginal spotting.

Causes of high hCG levels

A high hCG level may mean you're further along in your pregnancy than you thought. Or, it could indicate:

• A twins or multiples pregnancy
• A molar pregnancy
• Down syndrome
• Choriocarcinoma (a rare type of cancer that happens in the uterus early in pregnancy, as the placenta develops)