LORI.equine

Unusual placentation in a twin pregnancy. 

Keywords: placenta, equine, mare, inverted, twin, complication

Companion to an entry on twinning in mares in LORI.

There is still some uncertainty as to how and why twin equine pregnancies result in abortion. Sharing the available endometrial area is certainly a significant factor. Although typical inflammatory reactions at placental interfaces do not characterize twin abortions, immunological rejection of one co-twin by another may also be an important factor. Figure 1 shows one of numerous permutations of twin placentas apposed to one another within the uterus; placental sharing of the endometrial surface may be almost equal. In others, sharing is dramatically unequal. Figure is a schematic representation of such a case. Indeed, the situation in this pregnancy.

In Figure 1 red arrows indicate an area of apposition between the two chorionic surfaces. In these placentas there was little if any, macroscopic reaction between the two conceptuses despite the intimacy of placental apposition. Approximately 40% of the chorionic surface from the smaller co-twin was invaginated into the placenta of the larger twin.

Figure 1: Apposition of twin placentas and the path of delivery for both foals. Size available: 1626 x 2172px

The green arrows in Figure 1 show where the chorioallantois of the larger twin ruptured; in the region of the cervical star. The chorioallantois containing the smaller foal (yellow arrows) appeared within the allantoic cavity of the larger foal, invaginated within a pocket of the larger foal's placenta. The smaller foal was then delivered through a rupture of two layers of chorioallantois; that of the larger foal and that of its own placenta. The lower part of Figure 1 shows the path (grey arrow) that had to be taken by the smaller foal to be born; through the placenta of the larger foal.

Figure 2. A schematic representation of the placentation in this case. In some cases, the smaller co-twin may die and become a mummified attachment to the placenta of a normal foal. Image size available: 2553 x 1886px

Often, the placentas of twin foals will lie side by side within the uterus and each foal will be born in a conventional fashion, through the rupture of independent chorioallantoic membranes adjacent to the cervix.