Your Pregnancy Matters
How early can home pregnancy tests show positive results?
August 28, 2018
In the Aerosmith song “Sweet Emotion,” Steven Tyler’s lyric “the rabbit done died” refers to the original way a pregnancy test was performed – one that literally killed rabbits. Urine from a woman who thought she might be pregnant was injected into a rabbit, which would then be surgically opened so its ovaries could be inspected. If the woman was pregnant, the rabbit’s ovaries would undergo noticeable changes due to the hormone human chorionic gonadotropin (hCG) that develops early in gestation. The rabbit unfortunately didn’t survive.
Thankfully, pregnancy tests have evolved substantially since the 20th century. Women now have access to advanced tests that can detect a pregnancy days before a missed period. In fact, a CBS News article outlined the best home pregnancy tests according to research from Consumer Reports, based on ease of use, ease of reading results, and effectiveness. Many tests today are marketed as simple and effective, but getting accurate results comes down to when the test is taken.
When is the best time to take a pregnancy test?
Ovulation generally occurs around day 15 of a 28-day cycle. In a normal pregnancy, an egg is fertilized in the fallopian tube and then travels into the uterus, where it implants itself in the uterine wall. A woman’s body begins to produce hCG from cells in the developing placenta (tissue that nourishes a growing fetus) soon after implantation of a fertilized egg inside the uterus. Around eight days after ovulation, trace levels of hCG can be detected from an early pregnancy. That means a woman could get positive results several days before she expects her period to start.
The tricky part of timing when to take a test is that the duration of the first half of a menstrual cycle is more variable than the second. The length of time from the first day of a period to ovulation can vary by several days from month to month. Sexual activity around ovulation leads to the possibility of fertilization of the released egg by sperm. But even then, the time frame for a fertilized egg to implant can vary. And hCG isn’t produced until after implantation has occurred.
For the most accurate results, we recommend that women test in the morning on the day they expect their period to start. This allows for variability in the timing of ovulation, fertilization, and implantation. Testing in the morning provides a urine sample that is concentrated.
“Thankfully, pregnancy tests have evolved substantially since the 20th century. Women now have access to advanced tests that can detect a pregnancy days before a missed period.”
What can cause pregnancy tests to be wrong?
Each type of pregnancy test is designed to detect a set minimum level of hCG. This is why pregnancy test manufacturers advertise the way they do: “Know four days sooner,” or “Accurate up to six days before a missed period.” However, while home pregnancy tests are quite accurate – many boast a 99 percent detection rate based on laboratory testing results – marketing claims can be misleading. For example, a 99 percent detection rate means a test can detect hCG 99 percent of the time on the day a woman misses her period. These statistics also assume that women are testing at the optimal time, following all instructions, and fully understanding how to interpret the results.
In reality, it’s not that easy. Some studies suggest that up to 1 in 4 tests could be misinterpreted, which has led some companies to switch from using the presence of lines or colors to indicate a positive result to using actual words such as “yes,” “no,” “pregnant,” or “not pregnant” for clarity.
Many home pregnancy tests available today are midstream urine dip tests, which are the traditional “pee-on-a-stick” tests and are fairly easy to use. There also are tests in which a woman pees into a cup and dips a test strip into the urine or uses a tiny dropper to transfer urine from the collection cup into a small cassette. Research has shown that average test results when women used the dip and cassette tests matched lab test results just 70 percent of the time, whereas 99 percent of women typically can get an accurate reading from a midstream test.
When a test result is inaccurate, it’s either a false negative (the test says the woman is not pregnant, but she is) or a false positive (the test says she’s pregnant, but she isn’t). Several factors can cause false negative results:
• Having too dilute urine after drinking a lot of water or testing later in the day.
• Testing too soon when there has not been enough time for an early pregnancy to implant and start producing hCG.
• Using a pregnancy test that doesn’t detect lower levels of hCG. You can check the package insert for more information about the level at which the test will be positive.
False-positive results can be particularly upsetting. Sometimes an egg will implant briefly but will not be sustained. This is also called a chemical pregnancy – hCG is produced, but the pregnancy doesn’t persist after implantation. A test performed several days before an expected period can show a positive result, but the woman still gets her period soon or on time. Rarely, certain tumors can produce hCG, causing a pregnancy test to be positive even when we know a woman is not pregnant.
When should I see a doctor for a clinical pregnancy test?
Most women who have a positive pregnancy test should wait a week or two before calling the Ob/Gyn office for a pregnancy blood test. We suggest waiting because the rate of early pregnancy loss is high, and it’s possible that it might have been a chemical pregnancy.
However, women with a history of ectopic pregnancy or current abdominal pain should call the doctor right away to be examined. Ectopic pregnancies implant outside of the uterus and produce hCG, resulting in positive pregnancy tests. However, these pregnancies can cause abdominal bleeding and prompt medical care is necessary. Women who have congenital heart defects or other chronic physical or mental health conditions should also see a doctor right away because treatment plans or medications might need to be altered to protect the woman and the fetus.
Advances in home pregnancy tests give women greater insight into what’s going on in their bodies sooner than ever before. No matter what a woman’s childbearing plans might be, it’s essential to get quick, accurate results at home so she can plan her next steps accordingly.
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