How does a pregnancy test work?

When were pregnancy tests invented?

Home pregnancy tests have only been available to buy over the counter since the late 1970s. They were available to doctors from 1970, however had a two hour test time, and prior to this, a diagnosis of pregnancy was generally assumed purely from symptoms such as missed periods, nausea etc. As far back as ancient Egypt, written information has been found describing a process of women urinating on grain; if it grew she was believed to be pregnant, and if it didn’t then she wasn’t. Barley growth was deemed indicative of having a boy and wheat growth a girl.

How do pregnancy tests work?

Pregnancy tests today work by detecting the hormone hCG (human chorionic gonadotrohphin) in the urine. This hormone begins to be produced after the implantation of a fertilized egg within the womb and can be detected in either blood or urine within a week or two.

The pregnancy stick tests which you can buy over the counter in supermarkets and pharmacies have a special coating, Once they are dipped in urine, if hCG is present, a colour change can occur in the windows. The sticks are lateral flow tests, and work with similar technology to a covid test. They are filled with layers of porous paper which liquid can flow along, and are engineered to contain chemicals which react to certain proteins or hormones. As urine soaks along the paper, it passes through microscopic beads coated with antibodies. HCG binds to the antibodies. As it continues to flow towards the test strip, further binding takes place which stops the beads from travelling any further, and as they accumulate in one place the colour becomes visible. A further “control” window may change colour to indicate that the test has worked correctly.

What is HCG?

HCG supports the new pregnancy in its early development by stimulating the production of progesterone within the corpus luteum (an important cyst which remains in the ovary after ovulation). This continued supply of progesterone is important to maintain the pregnancy in the lining of the uterus and prevents the next menstrual bleed.

HCG production is regulated by the pituitary gland and the hypothalamus in the brain. In the early stages of a healthy viable pregnancy growing in the right place, hCG levels more or less double every 48 hours. After around week 10 or 11, the levels plateau and remain stable.

hCG is produced by the trophoblast cells of the placenta. These cells are formed after the fertilized egg implants in the uterus and begins to grow. Initially, hCG levels are low, but they increase rapidly in the first few weeks of pregnancy.

How accurate are pregnancy tests?

In the very early stages of pregnancy, urine pregnancy tests may not be accurate. It may be too early for HCG to be detected, although the sensitivity of tests today mean pregnancy can often be detected even a day or two before the period is due. Blood tests are far more accurate, and used frequently in early pregnancy clinics when a pregnancy has not been visualised by ultrasound scan. These blood tests can be very helpful in determining whether or not a pregnancy is continuing to develop, and the best management plan for a patient who has been diagnosed with a pregnancy of unknown location.

Pregnancy tests are usually very accurate when used correctly if the woman already has a missed period. However, the accuracy of the test can depend on several factors, including the timing of the test, the quality of the test, and the individual woman’s physiology. Sometimes tests are taken too early, such as prior to implantation, or in the days following when HCG is not significant enough to register.

Urine tests are less sensitive than blood tests and may not detect pregnancy as early. They are also more susceptible to user error, such as not following the instructions correctly, or taking the test too early. Generally according to manufacturers, urine tests are around 97-99% accurate when used correctly. The NHS states that a positive test result is unlikely to be wrong, however warns that a negative result is less reliable. Even if the positive line is very faint, that is still a positive, although it is important to note at this stage that that does not necessary mean the pregnancy is viable and ongoing. Many pregnancies fail at a very early stage, however women can continue to test positive for days or even weeks afterwards. If that is the case, the positive line may start to fade as her hormones gradually drop.

Blood tests are far more accurate than urine tests and can detect a pregnancy as early as 7-12 days after conception. They are also less prone to user error and can provide more precise information about hCG levels. However, blood tests are more invasive than urine tests and require a clinician to draw blood, and healthcare providers will only perform blood tests if there is a clinical indication that it is needed, for example in a pregnancy of unknown location

Can you get a false positive with a pregnancy test?

A false positive result occurs when the test indicates that a woman is pregnant when she is not. This can be caused by a variety of factors, including medications that contain hCG, certain medical conditions, or a defective test.  As mentioned above, following a miscarriage or early failed pregnancy, hormones may still remain high for some time. Hormones present may also be indicative of some retained tissue following a miscarriage or termination.

Certain medications including those taken for infertility may result in a false positive. Other drugs including some anti-psychotic and anti-seizure meds, along with some anxiety medications, anti sickness and anti Parkinsons prescriptions can also trigger a false positive.

There are several medical conditions which may lead to a false positive, although most of these are unusual. They include some disorders of the pituitary gland and various cancers. There is also a condition called Phantom HCG which is described as “persistent mild elevations of HCG” , meaning ongoing low levels of HCG in the blood which are not rising, although urine may test negative.