Human Chorionic Gonadotrophin (hCG) hormone is unique to pregnancy. It is the hormone that is detected in the blood and/ or urine of a woman who is pregnant.
Tests for hCG tend to be extremely sensitive and it only requires the smallest concentration of hCG to be present for a positive pregnancy diagnosis to be made. This is why it is impossible to have a “false positive” pregnancy test, because if hCG is present, it is a sure sign of pregnancy. However, it is possible to have a “false negative” test result, primarily in cases where it is too early in the pregnancy for sufficient levels of hCG to be detectable. Otherwise, this can occur if there has been an insufficient sample of blood or urine to test or the urine was too diluted.
Research has shown that most embryos implant sometime between days 8 to 10 after ovulation, but this is variable of course. Some women conceive right after they have ovulated and others 5 days later.
When is hCG produced?
Human Chorionic Gonadotrophin is produced very early on after fertilisation and subsequent implantation of the zygote into the wall of the mother’s womb. It is produced from the cells that eventually become the placenta. The role of hCG is to nourish the zygote (which becomes the embryo), and to support it until the placenta has developed to the stage where it takes over this function. It also acts as a signal to the body not to menstruate as the thickened lining of the womb is needed to nurture the growing baby.
Blood tests, rather than urine tests, provide an earlier detection of the presence of hCG. Urine may be too diluted to determine if hCG is present. This is one of the reasons why a common recommendation from the manufacturers of home pregnancy tests is to use a sample of the first urine passed in the morning.
Even before a woman has missed her period, hCG is being produced. As early as 9-12 days after conception, it is possible to detect the presence of this hormone in the blood and within 12-14 days when it is excreted in the urine. If you are having twins, then your hCG levels are likely to be different to another pregnant woman who is only carrying 1 baby. But this is dependent on the stage of pregnancy and the individual woman.
Only pregnant women produce hCG. Interestingly, this has been one way of detecting if a urine sample has been interfered with when testing for drug use in sports or criminal activities. The presence of hCG in a falsely tendered urine sample has, over time, caused many laboratory staff to question the authenticity and identity of an individual’s sample. This is especially the case when it is supposed to have come from a man!
How frequently does hCG multiply?
Generally, hCG doubles in concentration every 3 days (72 hours), but only within early pregnancy. Levels tend to peak between 8-10 weeks of pregnancy and then they start to decline. As the pregnancy advances, lower levels tend to be the consistent feature. This is one of the reasons why the symptoms of nausea and tiredness, so familiar within the first trimester of pregnancy, begin to settle after the first few months.
By around 4 months of gestation, the placenta has taken over the function of supporting the developing embryo and there is no need for such high hCG levels. Everything tends to settle down and the relative “calmness” of the second trimester, when women are said to “bloom”, occurs. But, not always, of course.
It is important to remember that there is no consistent “normal” level of hCG – every pregnant woman and her baby are unique. This is why there is a large variation of numbers and concentration of hCG given for each age and stage of gestation. Some women have very low levels of hCG, but progress to delivering perfectly healthy and normal babies. Others have very high, elevated levels, which may alert their healthcare provider to suspect a mother is pregnant with twins or triplets. But in fact, she may be carrying just one baby.
When there is doubt over hCG levels, it is common for a screening ultrasounds to be done. These can more finely assess foetal age and growth, rather than blood hCG levels, which are prone to a lot of variation. It is more common for a trans-vaginal scan to be done, as this is more accurate than an abdominal scan.
When there are consistent low levels of hCG and an absence of other pregnancy symptoms, it is more likely that there are pregnancy complications. For example, an Ectopic (tubal) Pregnancy, a Blighted Ovum, or a non-viable pregnancy may be present. Sometimes, low levels of hCG are detected when there has been a mistake made with calculating how long it has been since conception. A screening ultrasound may be ordered with a “wait and watch” approach. Around 1 in 5 pregnancies do not progress and result in spontaneous miscarriage and, although disappointing, it is a harsh reality for many pregnant women. If the level of hCG is very high, this may be because of miscalculation of dates, a molar pregnancy, or a multiple pregnancy.
How will I know what my hCG levels are?
You won’t, unless your healthcare practitioner orders regular tests and advises you what your levels are. If you’ve been having fertility treatment, are threatening to miscarry, or there is doubt over the stability of your pregnancy, you are more likely to have regular hCG testing. But in the main, regular hCG measurements are not routinely done.
Twin pregnancy hCG levels
When a mother is carrying a twin pregnancy, she generally has higher hCG levels than a woman who is pregnant with one baby. But this is not a guarantee; there is a big cross-over between singleton and twin pregnancy hCG levels and between individual women. Comparison is generally not helpful.
In general, pregnant women do not know what their individual hCG levels are. They know they are pregnant from their own clinical judgement and because they’ve either confirmed this with a home pregnancy test or their pregnancy has been confirmed by their health-care provider. Home pregnancy tests do not give a precise reading of hCG levels, they only detect whether hCG is present or absent in the sample. However, for women who are undergoing fertility treatment and have regular blood tests, monitoring their hCG levels is likely to be an easier process because of pathology results.
In women who are having a multiple pregnancy, their hCG levels may be higher than those with a singleton pregnancy. But this elevation is not evident until after the first period is missed.
A very rapid increase in hCG levels can be cause for concern. When an embryo implants in the ovary or, there is a rapid growth of either placental tissue/ not foetal, then high hCG levels can reflect this. There are also some types of (very rare) cancers which can impact on hCG concentrations.
What is the measurement of hCG in pregnancy?
For a pregnancy to be confirmed the hCG level needs to be above 25 mlU/ml. A reading less than 5 mlU/ml is considered negative. These measurements mean that the hCG concentration is measured in milli-international units per millilitre of the mother’s blood.
What is important, is not so much the individual readings and concentration of hCG, but a measureable increase over time. This is a sign that the placenta is secreting sufficient hCG to maintain the zygote/ embryo and support it.
Generally there are 2 types of hCG tests done; a qualitative hCG test actually detects if hCG is present in the blood; and the other, a quantitative test, measures the actual amount present. If there is doubt about the viability of the pregnancy or the healthcare practitioner suspects abnormally low or high readings, then a quantitative hCG blood test is ordered.
My hCG level is high – does that mean I’m having twins?
No, not necessarily. There are many reasons for a high reading and, remember, there is a wide range of normal when it comes to hCG levels. These levels should not be used to estimate pregnancy gestation either, as they are inexact and can vary widely. One isolated reading does not give an accurate insight into the status of a pregnancy. If there is any doubt, multiple testing over a few days will give more of an indication of the viability of a pregnancy.
Even if you are pregnant with twins, this does not guarantee you will produce hCG any sooner than if you were pregnant with 1 baby. But because your levels may be higher, it is possible to detect a pregnancy slightly earlier due to higher levels of hCG.