I think for many women the egg white cervical mucus (EWCM) stops before ovulation. Sperm can live for a few days in EWCM, and they want to be there ready and waiting when the egg pops out. Closer to O, maybe you don't need the EWCM because the sperm will go to work right away and not need to hang around for a while.
Married to a wonderful woman since 2010. Baby boy C arrived in June 2013!
Check out our User Agreement.