It takes about 3 to 7 days for a pregnancy test to change from a weak positive to a strong positive. The exact time varies depending on the brand of the test strip, the individual’s physical condition and many other reasons. Early pregnancy tests determine whether a woman is pregnant by detecting the level of HCG in the urine. A weak positive result indicates that one red line is obvious and the other is lighter in color, probably due to the short duration of pregnancy and insufficient HCG levels in the body. It takes about 3 to 7 days for the HCG level in the body to gradually increase and the test may show a strong positive result. Different brands of test strips show a strong positive result at different times, so it is recommended to buy more test strips of different brands for testing. In addition, ectopic pregnancy can lead to a poor doubling of HCG levels in the body, resulting in a weakly positive test result. If the result is still weakly positive after 3 to 7 days, it is recommended to consult a doctor for a clear diagnosis.