In the 1700's the children were christened very soon after their birth. Later on the time gap widened to several months. If a child was so weak that the parents feared it could die before being christened, they could do the ceremony at home. As a rule we can say that very few children born alive are missing in the church registers. In some cases you may have to search in more than one parish, because children could be christened in another church for some reason.
Marriage is at the same time a religious and a demographic event, at least today.
On the whole very few marriages are missing in the church registers, but you may have to search for a while. A couple could be wed either in his home parish or in her, or in quite another church. You'll soon discover that many deaths are missing in the church registers, especially in older times. There are several reasons for this. If the body couldn't be found, then there was nothing to bury.