I have found this on the web:
Day-specific probabilities of clinical pregnancy
The finding that the estimated peak of fecundability is on the day before ovulation differs from results previously reported (Wilcox et al., 1995WILCOX-ETAL-1995WILCOX-ETAL-1995) showing fecundability peaking on the day of ovulation. The earlier analysis included both early losses and clinical pregnancies, while we use only clinical pregnancies. If intercourse occurs on the day of ovulation then the egg may have aged at the time of fertilization. This has been suggested as an explanation for the apparently high probability of early loss found for conceptions resulting from intercourse on the day of ovulation
(Wilcox et al., 1998WILCOX-ETAL-1998WILCOX-ETAL-1998), a possibility that could explain the difference between the reported patterns.
Couples having difficulty conceiving often try to time their intercourse to optimize their chances. Given that the highest conception rates occur on the 2 days prior to ovulation
, it is important to use a signal that allows couples to time intercourse for the several days of fertility before ovulation. The basal body temperature shift comes too late. Urinary LH kits only identify the short time from the start of the urinary LH surge to ovulation (Collins et al., 1983COLLINS-ETAL-1983COLLINS-ETAL-1983). Cervical mucus change provides an earlier and more useful cue. Mucus receptivity begins several days before ovulation (Katz et al., 1997KATZ-ETAL-1997KATZ-ETAL-1997) so couples who have frequent intercourse after this cue will tend to have intercourse on those days with the highest probabilities of clinical pregnancy.