When's the best time to take a pregnancy test and can you get a positive reading if taken a day before expected period?

Depends: The sensitivity of the pregnancy tests available in market is different.But if you are pregnant , the test will be positive at this time.Best thing is to wait for 2 wks to do a test after missed period.

See below: Conception occurs within day or so of the sex. It takes about 3 weeks for the embryo to make enough HCG for 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.