There are a huge number of home pregnancy tests (HPTs) available now, but that wasn’t always the case: HPTs were not readily available until the late 1970’s. In 1976 the FDA granted approval for the first home pregnancy testing kit, the “e.p.t.” Since that time HPTs have become one of the symbols of women taking control of their fertility. From the public health perspective, studies have shown that use of HPTs has shortened the time it takes for women to seek care for either a pregnancy or a pregnancy termination.
How do HPTs work?
After ovulation it can take several days for fertilization to occur. The rapidly dividing mass of cells has to go through the Fallopian tube and find a comfortable spot in the lining of your uterus. Once implanted the “pregnancy hormone” hCG (human chorionic gonadotropin) is produced and enters the blood stream. The average time from ovulation to implantation is about 10 days (range 6-12 days). Initially hCG is present in small amounts, but in a thriving pregnancy hCG levels should double about every 48 hours.
HPTs work by testing for the presence of hCG. Thus, if a woman has ovulated later, has had a longer time to implantation, or is using a test that cannot measure very small amounts of hCG, she might end up with a negative HPT result, even though she’s in the very early stages of pregnancy.
When should I take the HPT to get the most accurate result?
Current HPTs are advertised as being sensitive on the first day of a missed period—or perhaps a few days before. The most recent review of accuracy of HPTs, published in August of 2011, tested these claims. One brand, First Response, detected 97% of pregnancies on the first day of a missed period. The EPT manual (54%) and digital (67%) and the Clear Blue manual (64%) and digital (54%) tests all detected fewer actual pregnancies. These results may reflect that the First Response could detect as little as 5.5 mIU/ml of hCG in the urine while the other two brands could detect 22 mIU/ml. Generally speaking, a reliable pregnancy test in a doctor’s office should be able to detect a level of 22 units of hCG or higher.
Remember, it takes an implantation to produce hCG to trigger a test. In a study where the day of implantation was documented with a sensitive blood test, it was determined that 10% of viable pregnancies were not yet implanted by the first day of the missed period!
Having read all this you can now understand why a GYN may recommend using a HPT twice, if the first instance is at the time of a missed period. About 97% of viable pregnancies will have implanted by seven days after the first day of missed menses. Taking a second reading can also enhance accuracy where your HPT is not one of the super-sensitive brands.
Are there things that can interfere with accurate results?
There are two general areas where false answers can be generated: medical conditions and user-linked problems.
Medical conditions. Here at WebMD we get frequent questions asking if birth control pills, progesterone, recreational drugs, antibiotics, or blood in the urine from a bladder infection can influence HPT results. The short answer is no. Only drugs that contain hCG can give a positive result. Such medications (e.g., Novarel or Pregnyl) are usually given by injection. Other fertility medications (e.g., Clomid, Pergonal) do not influence hCG levels. If you are taking injectable hCG your GYN should have given you guidelines about when you can test with a HPT. It can take 7-14 days, depending upon the dose given, for the injected hCG to disappear from your body.
There are a few, uncommon, medical conditions which might yield a false positive result. These can include: high levels of LH, and some immunoglobulin antibodies.
The most common medical condition to produce false positive results is a pregnancy that will be lost to very early miscarriage. It is believed that up to 25% of pregnancies are lost in very early miscarriages. Before the advent of super-sensitive hCG tests (and women testing even before a missed period), the loss of pregnancy which did not have an embryo or gestational sac could look like a slightly late period. There might be just enough hCG to trigger a very sensitive HPT, but levels would fall shortly after. This is why we say that it is difficult to judge if a slightly late period was a miscarriage or a late period — one would have had to have a positive hCG test around the time of the bleed to know for sure.
User-linked problems. A test can be very reliable but if instructions are not clear, or if its results are not easy to interpret, one can get a wrong result. In 2009, when 16 HPTs were reviewed for readability, the investigators found that the reading levels ranged from 7th to 10th grade on user instructions, and 11th to college level on the question and answer section. The recommended level for medical instructions is 6th grade reading level. Thus, it may not be surprising that another survey found only 32% of women totally complied with the written instructions on the HPT.
In another experiment, volunteers testing urine in a lab had more accurate results than when actual patients performed tests on their own urine samples. To get around some of these sources of inaccurate results (including evaporation lines and confusing interpretations) manufacturers of HPTs are shifting from manual to digital tests with a built-in monitor reading the results, then giving a simple yes or no.
There is an abundance of written material available on the topic “What to Expect When You Are Expecting.” If you would like more detailed information on what to expect when you are testing for expecting, you might want to check both these sites: