I think that the recommendation is more to avoid the rare rare risk of placental abruption (placenta separating from uterine wall) than to help it with the migration. (thepregnantscientist.com)
After estrus, progesterone levels remain high for 8 - 10 weeks causing thickened uterine lining and decreased ability of uterine walls to contract. (illianaveterinaryhospital.com)
Thinner uterine walls mean the muscle may not stretch correctly or be strong enough to carry a baby. (abcnews.go.com)