In general what is the phase of the menstrual cycle when copulation may lead to fecundation?

Although this is not a rule, to be effective fecundation in general must happen within about 24 hours after ovulation (that happens around the 14th day of the menstrual cycle). Fecundation might be occur even if copulation took place up to 3 days before ovulation since the male gametes remain viable for about 72 hours within the female reproductive system.

The fertile period of the women though is considered the period from 7 days before ovulation to 7 days after ovulation.