David, what do you mean by watermark and gray area?

David watermark and mental gray area are two uncertain phenomena for women to verify whether they are pregnant or not.

Watermark: the same imprint as the control line, but without color, just like the imprint on the watermark.

Concept gray: it means that there is no obvious color when measured with pregnancy test paper. However, there is no response after the test, so people don't know whether the final test is the real situation or their own imagination, so it is called mental gray.