25 million: this is the number of pregnancy tests which are sold every year in France. 99% reliable, these devices are used to find out if you are pregnant or not. To indicate this, each pregnancy test has its vocabulary: some work with bars (1 bar = not pregnant, 2 bars = pregnant), others with signs (- = not pregnant, + = pregnant), still others clearly indicate the result (or even the date of conception, for the most advanced).
Patience, a guarantee of reliability
To understand how pregnancy tests work, you have to do a little mental gymnastics.
In women, on average, the menstrual cycle lasts 28 days, which are counted from D-1 to D-28. So, from D-1 to D-7, these are the rules. Then, on D-14, ovulation occurs: one of the two ovaries releases a mature oocyte into the fallopian tubes. For 24 hours (its lifespan), it can be fertilized by a sperm.
If fertilization takes place on D-14, you must however wait until D-20 for the embryo to cling to the walls of the uterus: this is implantation. And it is precisely when the implantation process begins that HCG, or hormone, is secreted. chorionic gonadotropin. The pregnancy test will then be able to detect the presence of HCG, thanks to a chemical reaction between the antibodies (contained in the test) and the molecules which make up the hormone, which are found in the urine.
“Assuming that fertilization takes place on D-14, there is a vague zone between D-14 and D-28: the woman is pregnant, but there is not yet enough HCG in the body, explains Dr. Benoît Gallier. Therefore, during this “black hole” period, the pregnancy test will be negative, because it cannot detect anything.” This is then called a false negative.
How to properly use a pregnancy test ? Dr. Benoît Gallier recommends taking a pregnancy test on D-28, the date on which your period is supposed to arrive. Before, it’s useless! If the test is negative but you still have doubts, it is advisable to repeat the test on D-30 then on D-32. Indeed, not all women have an archi-regular cycle: some ovulate on D-17, for example. Which shifts everything.
“Take your pregnancy test at the start of the day, especially if you want to be determined from D-28, adds the specialist. Indeed, morning urine is more concentrated in HCG. Otherwise, it is also possible to get a prescription a blood test (fully reimbursed by Social Security) from your general practitioner. This, which detects the presence of HCG in the blood, is 100% reliable from D-26.”
Last tip: “Opt for an easy-to-use test: there is no need to buy an ultra-technological tool, since they all work on the same principle. Don’t give in to marketing sirens!” On average, a pregnancy test costs less than €10 (not reimbursed by Social Security), in a pharmacy or supermarket. “After 15 days of late menstruation (D-43), you can also go see the gynecologist to carry out a first ultrasound.” This allows you to know how many babies are on the way!
The false positive, a chemical short circuit
If a pregnancy test may not detect an ongoing pregnancy, the opposite is also possible… Thus, when the test turns out to be positive even though there has been no fertilization, we speak of a false positive.
“These are quite rare cases,” explains Dr. Benoît Gallier. “If the woman has suffered a voluntary termination of pregnancy (abortion) or childbirth or miscarriage 1 month before the pregnancy test (in some cases this delay can extend to 2 months), the test may be falsified. Indeed, HCG does not disappear from the body overnight: it takes a certain time for it to be completely evacuated.
Same difficulty in the context of in vitro fertilization: “to boost the chances of success of IVF, a certain quantity of HCG is artificially injected into the patient. As a result, the pregnancy test can show a positive result while none pregnancy has started. Here again, to dispel doubts, an appointment with the gynecologist is necessary.
Thanks to Dr. Benoît Gallier, gynecologist-obstetrician in Chambray-les-Tours (37).
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