You are probably observing a vaginal prolapse, which is different from a uterine prolapse. A uterine prolapse occurs after calving and is the entire uterus turned inside out.

Vaginal prolapses occur before calving, during late gestation, and are the wall of the vagina pushing out through the vulva. Initially, this happens while the cow is lying down. Unfortunately, vaginal prolapses tend to get worse with time in the pregnancy (as tissues stretch). Eventually, the prolapse may stay out and may even entrap the urinary bladder. When the vaginal prolapse stays out, the cow strains because of the pain and irritation, which causes even more of the vagina to prolapse. When the vagina prolapses, the cow may not be able to give birth normally because the cervix doesn't dilate.

Veterinarians use a variety of surgical procedures to hold the vagina in place; however, none of these methods are consistently successful. Typically, the circumstances get worse each year. Consider culling this cow after this pregnancy because there seems to be a genetic link, and you don’t want to keep heifers from these females as replacements.