Transuterine embryonal migration in sheep, anteroposterior orientation of pig and sheep fetuses and presentation of piglets at birth.

Little information is available on transuterine embryonal migration in monotocous animals and anteroposterior orientation of fetuses in the uterus. This study was designed to determine the frequency of transuterine embryonal migration in sheep and anteroposterior orientation of sheep fetuses, pig fetuses and piglets at birth. The migration frequency of preimplantation sheep embryos was 4.0%. The proportion of sheep fetuses in the anterior presentation position increased from less than 65% early in pregnancy to greater than 90% at 120 days of gestation. Twin fetuses appeared to assume the anterior presentation position earlier in gestation than did singles. Sixty-two percent of piglets were presented anteriorly at birth. Seventy percent of the piglets born in the last half of each litter and 54% of the piglets born in the first half of each litter were presented anteriorly. A greater proportion of piglets was born anteriorly from sows with larger litters than from gilts with smaller litters. In both small and large litters, the proportion of piglets born in the anterior presentation position increased as parturition advanced. Fifty-eight percent of the three fetuses in the segments of each uterine horn nearest the ovaries in grits killed from 30 to 110 days of gestation were in the anterior presentation position. The proportion for the remaining fetuses near the cervix was 48%. Likewise, in unilaterally ovariectomized gilts, more fetuses in the ovarian segments than in the cervical segments of both uterine horns were in the anterior presentation position. Apparently the distance traveled by the embryo from the ovary during transuterine migration has no effect on fetal anteroposterior orientation in the pig.