During pregnancy, your body produces a hormone called human Chorionic Gonadotropin (hCG). hCG is produced by the cells that will form the placenta. hCG starts being produced once the embryo implants in the uterine wall. Pregnancy tests work by detecting hCG levels in the blood or urine sample provided.
hCG levels can usually be detected in the blood around 11 days after conception, though it takes 12-14 days for hcg levels to be detected in a urine sample. hCG levels increase rapidly at the start of the pregnancy, but will then decline slightly until around week 16 and remain steady.
If you get a positive pregnancy test result, you are most likely pregnant. False positives are very rare. A very early miscarriage could also cause a false positive if your hCG levels haven’t yet dropped, allowing the test to pick up this hormone in your body.
What Does hCG Do?
hCG is responsible for maintaining a thick uterine lining, without this hormone the lining would begin to shred putting the pregnancy at risk. hCG is the hormone responsible for preventing periods. hCG is produced by cells that form the placenta, and this means it is not usually present in the body unless you are pregnant.
In most, but not all, healthy pregnancies, hCG levels double every two or three days at the start of the pregnancy. As the pregnancy progresses, this rate may slow to every four days, but the levels are expected to continue to rise until sometime between weeks eight and 11.
It's important to remember that all pregnancies are different. Some women may have low hCG levels throughout the pregnancy, whereas others may have very high levels, and yet both can go on to have healthy babies.
An hCG level of less than 5mlU/ml (milli-international units per millilitre) will give you a negative pregnancy test result. If your level of hCG is found to be 25mlU/ml or over, then you will get a positive pregnancy test result. Fertility drugs containing hCG can affect your hCG levels, so you should discuss this with your practitioner before having your levels checked.
Some pregnancy tests use hCG levels to date the pregnancy, but this can be wildly inaccurate due to the differing levels between women. A single hCG reading is not usually enough to give an accurate diagnoses of any potential conditions. Instead, practitioners will usually take two readings a couple of days apart to allow them to draw a comparison from the levels.
hCG Levels During Pregnancy
It's not routine for hCG levels to be checked throughout pregnancy. This will only be done if you are showing symptoms of a possible problem.
Firstly, it’s important to remember that there is no such thing as ‘normal’. What is normal for you, may not be normal to the woman sat next to you in the midwife’s waiting room. And what’s normal for your first pregnancy, may not be normal for your second. The following levels are given as a rough guide, the important bit is the change in levels each week, not the individual result itself. The following chart shows an average level range for each week dated since the first day of the last menstrual period (LMP).
Women who are not pregnant: <5.0 mIU/ml
3 weeks LMP: 5 – 50 mIU/ml
4 weeks LMP: 5 – 426 mIU/ml
5 weeks LMP: 18 – 7,340 mIU/ml
6 weeks LMP: 1,080 – 56,500 mIU/ml
7-8 weeks LMP: 7, 650 – 229,000 mIU/ml
9-12 weeks LMP: 25,700 – 288,000 mIU/ml
13-16 weeks LMP: 13,300 – 254,000 mIU/ml
17-24 weeks LMP: 4,060 – 165,400 mIU/ml
25-40 weeks LMP: 3,640 – 117,000 mIU/ml
Women after menopause: 9.5 mIU/ml
What Does It Mean If You Have Low or High hCG Levels?
If your healthcare provider checks your hCG levels and finds that they are low, they will repeat the test a couple of days later so that the numbers can be compared. Low hCG levels could be indicative of:
- Incorrect pregnancy dating – you may not be as many weeks pregnant as previously assumed
- Possible miscarriage
- Ectopic pregnancy
High hCG levels will also need to be rechecked, but could be indicative of:
- Incorrect pregnancy dating – you may be further along than previously thought
- Multiple pregnancy – you could be carrying twins or more. If carrying twins, your hCG levels could be as much as 30-50% higher.
- Molar pregnancy – if a molar pregnancy occurs, there is a growing mass in the uterus but it will not develop into a baby. The mass is known as a hydatidiform mole and will produce high levels of hCG. The mole will need to be removed.
hCG Levels & Pregnancy Loss
After a pregnancy loss, hCG levels should drop down to less than 5mlU/ml within around six weeks. The time it takes to reduce will depend upon the level of the hormone at the end of the pregnancy, and how the loss occurred. If you experience a pregnancy loss, your healthcare provider will recheck your hCG levels a few weeks later to make sure they have reduced.