It’s easy to get overwhelmed with emotions when you’re taking a pregnancy test. Your life may change forever depending on the results. Confirming those results with your doctor, even if they’re negative, is important.
How Pregnancy Tests Work
Pregnancy tests check the levels of human chorionic gonadotropin (hCG) in your body. This is one of the first things to develop when you’re pregnant. It's a substance that builds up when the fertilized egg embeds in the womb around 10 days after conception.
Types of Pregnancy Tests
At-home pregnancy test. The most common pregnancy test is an over-the-counter, at-home test. These tests check for hCG levels in your urine. While the tests claim to be 99% accurate if you follow the instructions, it’s important to follow up with your doctor.
At-home pregnancy tests have some advantages compared to going straight to your doctor when you first think you may be pregnant:
- At-home pregnancy test kits are inexpensive.
- You get almost immediate results.
- The instructions are simple.
Your doctor can also perform a urine pregnancy test to confirm previous results. They will also talk to you about next steps to ensure your health and that of your baby.
Blood test. This type of pregnancy test is done by your doctor. They’ll take a blood sample and send it off. A lab will check the levels of hCG in your blood.
A blood test isn’t used frequently for pregnancy because they’re expensive and provide similar results as a urine test. A blood test is typically reserved for any concerns about the pregnancy, such as if you’re using fertility treatments.
Results of a Pregnancy Test
When you take an at-home pregnancy test, you may receive a false result because of a variety of circumstances.
False-positive result. This is when the pregnancy test says you’re pregnant when you aren’t. This can be caused by:
- A contaminated urine collection cup
- A damaged, expired, or faulty pregnancy test kit
- Blood in your urine
- Protein in your urine (typically the result of kidney damage)
- Medications such as anticonvulsants, fertility drugs, diuretics, or tranquilizers
- Having recently given birth or having had a miscarriage
- An ovarian tumor
False-negative result. This is when the test says you aren’t pregnant but you are. Possible causes include:
- Taking the test too early after your missed period. Repeat the test about a week after a missed period for more accurate results.
- Not following the instructions. Follow the pregnancy test’s instructions as precisely as you can, including the amount of time you need to leave the test to do its work.
- Being too hydrated. Take the test early in the morning when your urine is concentrated. Diluted urine won’t give an accurate reading of your hCG levels.
The Hook Effect
Pregnancy tests are a type of immunoassay, a test that measures the concentration of certain particles, molecules, in the tested material. Scientists call it the "hook effect" when a weakness of the immunoassay comes to light.
The hook effect occurs when there is an abundance of analyte (what’s being measured). The analyte of a pregnancy test is hCG.
During the pregnancy test, two molecules — one called an antibody and one called an antigen — become the “sandwich bread” on either side of the "filler," the hCG molecule. The number of hCG molecules that are sandwiched determine the results of the pregnancy test.
Too much hCG messes up the jobs of the antibodies and antigens. The hCG molecules overwhelm the antibodies, preventing the molecular sandwich that is supposed to occur.
As the pregnancy test draws to a close, all the extra molecules are washed away, leaving just a few sandwiched molecules behind. Since the abundance of hCG prevented most of the molecular sandwiches from forming, the pregnancy test falsely shows that there are low levels of hCG.
After the Pregnancy Test
Concern over results. If your at-home test came up positive, or you’ve taken multiple tests and gotten conflicting results, it’s time to see your doctor. They will perform a blood test or an ultrasound to confirm your results.
Missed period. After a missed period, wait a week to use a pregnancy test. Repeat the test after a few days. The sooner you use a test after conception, the more likely you’ll get an inaccurate result.
If you haven’t gotten your period but your result was negative, don’t be alarmed. There are a few causes of a missed period:
- Significant weight loss
- Significant changes to your diet or exercise routine
- Certain drugs or medications
- Onset of menopause
Missed periods can also be the result of problems with your thyroid or ovaries. Talk with your doctor to get to the bottom of your missed periods.