Did you know the first home pregnancy test became commercially available in 1977? Unlike the simple, mess-free kits available in pharmacies today, it looked more like a children's chemistry set — it contained a vial of purified water, a test tube, an angled mirror, and a sample of red blood cells taken from a sheep [1].
Thankfully, it's way easier these days, and there are no sheep involved. Despite the advances, it can be easy to get confused about a pregnancy test result: are you looking for a clear, bold line or is a faint one still a positive result?
Whether you're trying to conceive or experiencing early pregnancy symptoms, it's important to get familiar with how home pregnancy tests work.
How do pregnancy tests work?
Let's start with the basics. After all, home pregnancy tests are still mini chemistry sets.
Pregnancy tests first and foremost work by checking for a hormone called human chorionic gonadotropin, known as hCG [2]. This is a pregnancy hormone made by your placenta, which provides a growing baby with oxygen and nutrients from your bloodstream throughout the entire pregnancy.
During the first month of pregnancy, hCG levels will double roughly every 48 hours and continue to rise gradually until the 10-12 week mark. From here, hCG levels drop slightly until the 20-week mark and then continue the same until full term [3].
Without getting too clinical, this hormone stimulates the corpus luteum to produce progesterone to maintain the pregnancy. Smaller amounts of hCG are also produced in the pituitary gland, the liver, and the colon [4].
In the early weeks of pregnancy, hCG plays an important role in maintaining the function of the corpus luteum, which is the mass of cells that eventually forms from a mature egg into a developing embryo.
It's worth keeping in mind that like many hormones, there is a wide range of hCG levels between individuals — but the trend is relatively constant [3]. hCG alone is not enough to measure estimates of gestational age because of this variation between individuals, which is why we use ultrasound testing to check and measure pregnancy duration. Once a baby is born, hCG levels are no longer detectable after a few weeks.
What does a positive home pregnancy test look like?
A home pregnancy test is easy to do in the privacy and comfort of your home, and they all work by testing urine. Some tests require you to urinate directly onto the testing strip, and others contain a strip you dip into a urine sample. There is often a control line present to compare against.
Depending on the home pregnancy test you use, the instructions contained mean results should be easy to interpret: you may be looking for a blue line, a plus or minus sign, or a second line on the pregnancy test strip [5].
Much like the COVID-19 home testing kits most of us are now familiar with, you're essentially looking for a test line to appear.
Very faint line on pregnancy test: what does it mean?
Generally speaking, if you see a faint line on a pregnancy test this indicates a positive result [2]. Home pregnancy tests are very effective and most have an accuracy reading of up to 99% [3][4].
No matter how faint the line is, most pregnancy tests are accurate and should be treated as a positive result, and you may want to book in to see your healthcare provider to discuss results and future planning.
As these home testing kits have been around for years now, studies — both local and international — suggest false negative results are rare and if they happen, are a result of certain testing factors and not the kit itself (more on that below). A faint positive is usually still a positive.
What causes a faint line on a pregnancy test?
In most cases, a faint line on a pregnancy test means a positive test result. If you've tested within a few days before or after your period is due, you are very early in your pregnancy and your hCG levels have not increased enough to create a very dark line on a pregnancy test [6].
There are also other reasons why faint positive lines may appear on a pregnancy test [6]:
- Diluted urine: If you've had a lot of water or liquids before testing, the hCG in your urine may be diluted and give you a faint positive pregnancy test result
- The 'hook effect': As mentioned earlier, hCG levels are high in the early pregnancy stage and as pregnancy progresses, a different form of hCG becomes dominant. This means sometimes people are 5 or more weeks pregnant, but test results show a faint positive result
- Chemical pregnancy/miscarriage: During a very early miscarriage, hCG levels begin to fall. As hCG levels fall, the second line on your pregnancy tests would become fainter which could indicate an early pregnancy loss
Can a faint line still be negative?
A faint line can be negative, although it's rare. This can be caused by a chemical pregnancy, which is when a woman miscarries during early pregnancy before anything can be seen on an ultrasound [8].
It may mean someone was recently pregnant as there is remaining hCG detected as a positive test result.
In other cases, a faint positive line may actually be displaying an evaporation line, if the pregnancy test was not used properly.
What is an evaporation line?
If a test gets wet or is left out too long, an evaporation line — which is a faint streak — may appear on a pregnancy test. This can look like a faint positive line, which is why it's important to follow the instructions on home pregnancy tests and set a timer to check the result.
Evaporation lines occur when the urine that was on the test area starts to dry. The chemical composition of the sample changes due to evaporation and the test may start to show a positive line [7].
These may look like positive pregnancy tests but aren't, and if you're trying to conceive this could be heartbreaking, so remember each kit is unique and to test correctly.
How long after a faint positive should I test again?
If you get a faint positive pregnancy test result and aren't able to book in for a blood test, you can try again in the coming days.
If your pregnancy test shows only the control line, it's safe to accept this is not a positive result. But if you're actively trying, you can keep a pregnancy test (or 2 or 3) on hand to check regularly. As discussed, the tests detect hormone levels during very early stages best, so test early and test correctly.
How to avoid a false positive result on a pregnancy test
It's rare to get a false positive pregnancy result if you aren't pregnant, but it can happen. This is known as a false positive result.
This can happen if you have blood or protein in your urine. Certain medications such as tranquilisers, hypnotics, anticonvulsants, and fertility drugs can cause this too.
A false positive can also be caused by [2]:
- Using a dirty urine collection cup: For example, washing detergent or soap residue can cause false positive pregnancy test results
- A faulty test kit: If a home pregnancy test is expired or has been damaged, this may cause a faint positive line
- Recent birth or miscarriage: As mentioned earlier, hCG levels will still be present shortly after birth or miscarriage, so this pregnancy hormone may display pregnancy test lines
- An ovarian tumour or ectopic pregnancy: See your healthcare professional immediately if you suspect anything concerning is happening. You know your body best
I've done a home pregnancy test — now what?
Based on the results, consider taking the following steps:
Your test is positive or displays a faint line and you aren't sure
Make an appointment with your doctor. You might need a blood test or ultrasound to confirm the pregnancy. The sooner a pregnancy is confirmed, the sooner prenatal care can begin, which is best for maternal and child health.
Your home pregnancy test is negative
If your period doesn't begin, take the test again in a few days or one week. It's especially important to do this if you took the test before or right after a missed period.
You continue to get negative test results, but your period doesn't start
Or you still think you might be pregnant. In this case, it's best to contact your healthcare provider. Your provider may suggest you take a blood test to check for pregnancy, which may be more accurate than a home test.
Certain health problems may lead to missed periods. If you're not pregnant, your doctor can help figure out what's causing missed periods. If you have a negative pregnancy test result but suspect you're pregnant, trust your body and instincts. Avoid cigarettes and alcohol, and make an appointment to see your doctor.
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