How long should you wait to take a pregnancy test? 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 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. With a sensitivity of 25 mUI test can show positive results if the urine of women in the hormone HCG exceeded this threshold, i.e. is above the level of 25 mUI. Need to 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 implantation, in turn, occurs at 7-10 days after ovulation. But one should bear in mind that hormone levels do not jump immediately after implantation to 25 mUI, and rises gradually increasing every 24-48 hours in 2 times.
To better understand how increasing the hormone HCG in the first days after implantation, look at the graph (growth hormone, HCG is marked green line):
Assume that implantation occurred on the seventh day after ovulation and start from the fact that in this day HCG levels “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 11 days after ovulation, HCG levels for the first time crossed the border in 25 mUI and pregnancy test can be the first time, show a weak second strip. At the same time, 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 on the seventh DPO, but only on the 10th (also the norm!). This means that according to our calculations the first stripes can be seen only at 14 DPO. Here we are getting to a certain delay. Normally, the length of the second phase is about 14 days and this is why manufacturers are advised to conduct tests, pregnancy tests only after a delay, i.e., the earliest at 15 DPO.