Have you ever wondered how pregnancy tests work? When you first start to feel pregnant, you might instinctively head to the drug store to buy a test. These tests work by detecting the pregnancy hormone, hCG, in your urine.
Over-the-counter pregnancy tests check for the presence of the “pregnancy hormone” human chorionic gonadotropin (hCG) in your urine. The test your doctor gives you later on looks for the same hormone in your blood. This hormone begins to increase just a few days after fertilization when the egg is implanted in your uterine wall. The hCG will cause your placenta to produce progesterone after implantation, which is necessary to stop the embryo from being rejected.
Over-the-counter pregnancy tests are most accurate when taken a few days or a week after your missed period. Some brands of pregnancy tests may claim to be more accurate, but this may not be the case for everyone, because all women ovulate at different times of the year.
The most accurate test for pregnancy is done through a blood sample at your doctor’s office. Urine tests can detect lower levels of the pregnancy hormone, but a blood test can show more conclusive results. If you take a home pregnancy test, and it comes back negative, make an appointment with your doctor as soon as possible if you still have pregnancy symptoms. You can also visit Planned Parenthood offices for pregnancy tests and exams.
Approximately 25 days after fertilization, the developing fetus can be seen through transvaginal sonography – a type of ultrasound. This imaging technique allows for clear visualization of the fetus and can be used to assess fetal development.
What causes false negative test results?
If you take a pregnancy test too early, you may get a false positive result. This is because the pregnancy hormone hCG does not begin to build up in your system until after the fertilized egg has attached to the uterine wall. This usually occurs within 14 days of fertilization. So, if you wait until after your next period was supposed to start, the blood test will be almost always accurate. Some over-the-counter tests need higher levels of the hormone to be accurate, and they may say you’re not pregnant, even though you are. If you’re concerned about getting a false positive result, you can take another test a few days later to be sure.
Can you get a false positive result?
Although it may seem unbelievable, it is possible for a pregnancy test to show a false positive. If the test is read after the recommended time frame, the urine on the test strip may evaporate and consequently change the color to indicate a positive result. If you believe you may have read the test results too late, retake the test for more accurate results.
If you’re taking fertility hormones, it’s possible that a home pregnancy test may show a false positive result. The hormone hCG (human chorionic gonadotropin) is often included in fertility injections, and if present, will show up on the test and give an inaccurate reading.
In extremely rare instances, a false positive reading could be caused by certain forms of cancer.
What about the rabbit?
Back in your grandmother’s day, there were no over-the-counter pregnancy tests – only a doctor’s visit could tell you for sure whether or not you were expecting. To test for pregnancy, the doctor would take a urine sample from the patient and inject it into a female rabbit. If the rabbit was injected with urine that contained the pregnancy hormone, her ovaries would change in response. However, these changes usually took several days to occur, and by that time the rabbit was too old to live on her own, so they had to put her to sleep.
Fortunately, we’ve come a long way since then and can now test for pregnancy without harming any animals in the process.