61:6 If a person prays about something that has already happened, he prays in vain. For example, if one hears a scream coming from the city, it is useless to pray, "Please, G-d, let it not have come from my house" because either it did or it didn't. Similarly, if one's wife is expecting, it is pointless to pray for a particular gender after it has already been decided. One may prayer for a particular gender within forty days of conception; after that he may pray for such things as that the baby be healthy.
61:7 One who is about to measure his harvest should recite a prayer to the effect of, ''May it be Your will, Hashem my G-d, that You send a blessing on this mound of grain.'' Once he has begun measuring, he says: ''Blessed be the One Who sends blessing on this mound of grain." This prayer is said without mentioning G-d's Name or His Kingship (i.e., it is not an actual bracha). If he measured and then prayed, it would be a prayer said in vain, for such blessing can only come on something that has not yet been revealed.