Investigation of the reproductive tracts of 78 pregnant pronghorns (Antilocapra americana Ord) revealed that even though twin births were the rule, three-seven ova were commonly ovulated, fertilized, and developed into expanded blastocysts. Some mortality occurred in the thread-stage because of knotting and tangling of the blastocysts. When more than one embryo per uterine horn survived the thread-stage, the one distal to the corpus uteri was displaced or its membranes were pierced by the necrotic tip of the proximal embryo. Thus, excess blastocysts and embryos were eliminated during the thread-stage or at the time of implantation.

The pronghorn uterus was bicornuate with an average of 92 caruncles which increased approximately 250 times in volume during gestation. The placenta was of the epitheliochorial type. Transuterine migration of ova occurred. When the embryos were 50–75 mm in length, their membranes met in the corpus uteri. The cervix had 4 or 5 muscular valves with papillae projecting caudally. During late gestation a cervical plug formed in the posterior valve.