Why would all of our piglets be stillborn?

We had all of our piglets die several times from four different pigs. They were bred by a different boar, and each litter took four days to be born. Now we have a new boar, and yesterday the new mama had six piglets, in a matter of a few hours, and they were all born dead. This sow was a sister to the other sows with the stillborn piglets. What would cause this, and what can be done to prevent this?