Bioscientifica Proceedings

Summary. Pig embryos enter the uterus from the oviduct about 48 h after ovulation at about the 4-cell stage. They remain near the tip of the uterine horn until about Day 6 when they migrate towards the body of the uterus. By Day 9 some embryos have entered the horn opposite the one of origin and continue migrating until Day 12. At Day 12 embryos can no longer successfully move to a different site. The critical signal for recognition of pregnancy occurs at Day 12 at the same time that embryos are elongating at a rapid rate. The distribution of embryos throughout the length of the horns has occurred by this time and spacing accompanied by uterine growth takes place. The speed of migration is not influenced by the number of embryos but may be affected by secretions from the embryos acting on the uterus. A significant section of unoccupied uterus at Day 12 will prevent continuance of pregnancy regardless of the number of embryos present in the occupied section. When the number of embryos is so few as to not occupy the uterus fully, pregnancy will not continue. Removal of embryos from a significant section of the uterus after Day 14 does not stop an existing pregnancy. Embryos are spaced nearly equidistant from each other with the absolute space between fetuses dependent on the total uterine space available. The position of the fetus in the uterus affects the spacing between fetuses; fetuses at the tip of the horn before Day 25 have the greatest space with the space decreasing from the tip to the body. Due to resorptions near the body, the space per live fetus is greater and roughly equal near the body and the tip from Day 35 and later. Fetuses in the middle of the horn are then most likely to have limited space and be smaller at birth than their litter mates.

© 1985 Journals of Reproduction & Fertility Ltd