Some women are very regular with their periods
and can therefore tell when they might ovulate,
and others are less regular
and so ovulation is more of a guess.
Is this a situation where the girl wants to get pregnant and is trying to time things, or is this a boy who wants to get a girl pregnant? I really don't recommend the girl try to get pregnant on purpose from a boy, unless they are married. It is not very fair to the boy if he has not agreed to the plan.