Abstract
The level of prenatal losses influences the number of littermates especially. The major part of these losses create the embryonic mortality till 35. day of pregnancy, which maximum is till 25. day after ovulation.
If all embryos die, the sow returns to oestrus. We consider returning to oestrus 10% from served sows, irregular returning to oestrus till 5% (from returned sows about 20-35%), and embryonic mortality about 20% to be standard.
The control of returning to oestrus interval and of frequency litters with small number of piglets born should be used for analysis of results of reproduction.