Pathological processes.
In an ectopic pregnancy, the implantation of the fertilized egg occurs not in the uterus, but outside it. In this case, hCG is produced, but its concentration is much lower, and the increase is slower. Therefore, with this pathology, the test can give either a negative or a weakly positive result (the second strip is very pale).
Another reason is a frozen pregnancy. The period of 3–4 weeks from the date of the last menstruation is considered the most dangerous in terms of the likelihood of stopping the development of the embryo. In this case, a frozen pregnancy occurs. When the embryo dies, hCG ceases to be produced completely or is produced in minimal concentration for another 2-3 days. The hormone level remains in one place and then begins to decline. In this case, the test may not show a second strip if the fetus died before the hCG concentration reached the sensitivity threshold of the pregnancy test. If the hormone was produced in sufficient quantities before the death of the embryo, the test shows a weak second line.