Why is it that some women have negative hpt, and months later it comes up positive? and still have a healthy pregnancy?

???: Not sure what you mean , but i assume you mean the preg test is neg and later comes up positive, which usually means you were not pg when the first test was done or you were too early for it to be +.

Test method: A likely possibility is the test that gave a negative result was not done correctly. If you miss your period by 4-5 days, do a home pregnancy test. If the result is negative repeat the test 4-5 days later unless you get a period by then. Use first morning urine and follow the instructions for the test carefully. If you do not wish to be pregnant, use contraception all the time, every time.