Very light spotting at 18 days past start of last period for 1 day. have been trying to conceive. pregnancy test negative at cycle day 19. confused...

Tested too soon: Conception, as defined by the generation of a fertilized ovum, occurs within day or so of the sex. However, it takes about 2-3 weeks for the embryo to make enough HCG to produce a positive pregnancy test. Blood test may be done a day after a missed period. Home pregnancy tests should be done 4-5 days after the missed period. If the result is negative repeat the test in 4-5 days. Use first morning urine and follow instructions for the test carefully.

Too early: just because you had possible implantation bleeding doesn't mean that there would be enough pregnancy hormone to be picked up by a urine test. Definitely wait until after your first missed period before trying again