Because stress can affect conception and embryo survival, it's advisable to avoid mixing sows or gilts before and immediately after breeding. That recommendation comes from Todd See, swine specialist at North Carolina State University.
To avoid stress, he says, it is preferable to mix gilts and sows into new groups only at weaning time. This also applies to moving one or more bred animals to a different location.
See explains, "After fertilization occurs in the oviduct, pig embryos descend into the uterus within 24 to 48 hours. However, implementation does not occur until the 13th day. Any unnecessary stress during this period – such as moving or mixing – can cause embryos to become detached and die.
He also cautions about changing feeding levels within the first 30 days after a sow or gilt is bred. "Don't raise or lower feeding levels,
expecting to improve reproductive performance. It just does not work," See notes. "Just provide proper nutrition at a level rate during early gestation."