hCG: what it is and how it works

Human chorionic gonadotrophin, or hCG, is often called ‘the pregnancy hormone’. It’s what every pregnancy test looks for, and it does an important job in the first weeks.

Where hCG comes from

hCG is made by the developing embryo, by the cells that go on to form the placenta. It starts once the embryo has implanted in the lining of your womb, about 6 to 12 days after ovulation, and it can be found in your blood from about a week after conception. It then passes into your wee, which is how home tests find it.

What hCG does

After you ovulate, the part of the ovary that released the egg (the corpus luteum) makes progesterone, which keeps the lining of your womb ready for a pregnancy. If you’re not pregnant, it stops after about two weeks, progesterone falls and you get your period.

If you are pregnant, hCG tells the corpus luteum to keep making progesterone, so the lining stays in place. This carries on until the placenta is ready to take over, at around 12 weeks.

How hCG levels change

Just after implantation: only small amounts, often too little for a home test.
The first few weeks: levels rise quickly. In early pregnancy, the amount in your blood often roughly doubles about every 48 hours.
Up to about 8 to 10 weeks: levels keep rising steeply.
After that: levels fall, then stay lower for the rest of the pregnancy.

Doubling is only a rough guide. NICE guidance, which early pregnancy units in England and Wales follow, treats a rise of more than 63% over 48 hours as a sign of a pregnancy that’s likely to be developing in the womb. So a rise that’s less than double isn’t necessarily a problem.

Levels also vary a lot from person to person, so there’s no single ‘right’ number. See hCG levels and test sensitivity.

hCG blood tests

You don’t usually need a blood test to confirm a pregnancy. But if you have pain or bleeding in early pregnancy, or a scan can’t yet show where the pregnancy is (called a pregnancy of unknown location), an early pregnancy unit may measure your hCG twice, 48 hours apart. The change between the two results says more than either result on its own.

A rise of more than 63%: the pregnancy is likely to be developing in the womb, though an ectopic pregnancy can’t be ruled out yet. You’ll usually be offered a scan.
A fall of more than 50%: the pregnancy is unlikely to continue, though this isn’t certain yet. You’ll usually be asked to do a urine pregnancy test 14 days later. Follow-up can vary between hospitals.
Anything in between: you’ll be seen again by the early pregnancy unit within 24 hours.

Ectopic pregnancies often make lower levels of hCG than pregnancies in the womb. Your early pregnancy unit will explain what your results mean for you.

Positive tests that aren’t from an ongoing pregnancy

After a pregnancy ends. It’s common for hCG to stay in your body for several weeks after a birth, miscarriage or termination. Some hospitals ask you to do a test about 3 weeks after a miscarriage, and to get in touch if it’s still positive.
Fertility treatment. Some fertility medicines contain hCG. If you had a ‘trigger’ injection containing hCG, it can stay in your body for up to about 10 days and give a positive that isn’t a pregnancy.
Around the menopause. Some people make a small amount of hCG in their pituitary gland, which can rarely cause a positive test.
Rarely, some tumours and other medical conditions.

If a result doesn’t make sense, your GP can help.

Questions people ask

How long does hCG stay after a miscarriage?

It can stay for several weeks. Some hospitals ask you to test around 3 weeks after a loss, and to contact them if the test is still positive. See what is a chemical pregnancy?

What does a slow rise in hCG mean?

It doesn’t always mean something is wrong. It can happen in a healthy pregnancy, an ectopic pregnancy or a pregnancy that isn’t continuing, which is why the early pregnancy unit will check you again within 24 hours and may offer a scan.

When to get help

Mild cramps that ease with rest are usually nothing to worry about. But if you’re pregnant, or think you could be, get urgent advice from your maternity unit, an early pregnancy unit or NHS 111 if you have:

any bleeding or spotting, even if it’s light
tummy pain that’s severe, on one side, or hasn’t eased after resting for 30 to 60 minutes
pain in the tip of your shoulder

Call 999 if you have bleeding along with severe tummy pain, pain in your shoulder, or feeling sick, faint or dizzy, or if the bleeding is heavy (soaking a pad soon after putting it on). Also call 999 if you have sudden, sharp and intense tummy pain and you feel very dizzy or faint, feel sick or look very pale. These can be signs of an ectopic pregnancy or a miscarriage that need treating straight away.