For many women planning pregnancy raises the question: when you can do a pregnancy test to see two cherished stripes? Manufacturers of tests, doctors, and friends are advised not to do the tests before the delay. Let's look at why and try to calculate the probability of a positive pregnancy test, depending on the DPO (days after ovulation).

For our calculations we take as a basis for a pregnancy test with a sensitivity of 25 mUI. The test reacts to the presence of the pregnancy hormone hCG in the blood of women. When the sensitivity of 25 mUI test can show positive results if the hormone in the urine of women of hCG exceeded this level, ie is above the level of 25 mUI. You should know that the hormone hCG appears immediately after the implantation of the embryo in the uterine wall first in the blood, and later in the urine of women and, in turn, implantation occurs 7-10 days after ovulation. But at the same time, be aware that hormone levels do not jump immediately after implantation to 25 mUI, but rises gradually, increasing every 24-48 hours in 2 times.

Suppose that implantation has occurred on the 7th day after ovulation and start from the fact that in this day hCG "jumped" to 2 mUI. The following day, hCG levels doubled and amounts to "whole" 4 mUI. At 9 DPO again doubling and 8 mUI. Although the pregnancy has already occurred, but for the test with a sensitivity of 25 mUI hormone levels are still very low, so the test will still be negative. Only on the 11th day after ovulation hCG levels for the first time crosses the border in 25 mUI and pregnancy test can be the first time to show a weak second band. Thus in our calculations, we ignore the fact that the content of hCG in the urine may be much lower than in blood, and every woman has this level of an individual.

Now imagine that the implantation did not occur at the 7th SPE, but only on 10th (also the norm!). This means that we estimate first stripes can be seen only at 14 DPO. Here we are getting to the definition of delay, and answers to frequently asked question: "How many days can do a pregnancy test?". Normally, the length of the second phase is about 14 days and this is why manufacturers recommend tests to test for pregnancy only after a delay, ie, the earliest at 15 DPO.