An embryo produced by ICSI gives, in the vast majority of cases, a single foal. It can nevertheless divide after transfer and give monozygotic twins, that is, twins derived from a single embryo and genetically identical. This is rare, but more frequent than with an embryo recovered by uterine flushing, and these twins escape the usual day-14 check.
What we call an “embryo”
The same word refers to several very different stages.
The embryo transferred at around seven or eight days is a blastocyst. It is a small sphere whose wall carries a denser cluster of cells, the inner cell mass. The foal will develop from this cluster alone; the rest of the sphere will form its membranes.
After transfer, the sphere grows in the uterus. On ultrasound it appears as a round, black vesicle, but with nothing inside during the first two weeks. The embryo proper, derived from the inner cell mass, only appears on ultrasound at around day 21, at the bottom of the vesicle. Its heart beats from about day 25.
Where these twins come from
In the mare, most monozygotic twins described arise from an inner cell mass that has split within a single blastocyst. They then share one vesicle and one chorion, but each develops in its own amnion. More rarely, the embryo separates earlier, before the blastocyst stage, and gives two distinct vesicles.
The mechanism that favours this split in in vitro production is not established. In the main published series, it occurred in 1.6% of pregnancies obtained with in vitro produced embryos (4 of 254), compared with none with in vivo embryos (0 of 413; Dijkstra et al., 2020).
Why they are not seen at the first check
Ordinary twins, from two ovulations, are detected between days 14 and 16: two vesicles are seen, and one can be reduced before they become fixed. Monozygotic twins form a single vesicle. At the first check, nothing distinguishes them from a single pregnancy. They can only be recognised once the embryos are visible, at around day 21, and their two hearts at around day 25 (Dijkstra et al., 2020).
This is why we ask for the recipient of an ICSI embryo to be presented to us at around day 25.
What to do if they are present
Because they share the same chorion, and probably part of their blood circulation, monozygotic twins respond poorly to the reduction techniques used for ordinary twins. At Utrecht University, compression of the thorax of one fetus, attempted in six mares between days 55 and 79, led each time to the loss of both fetuses within twenty-four hours (Arnold et al., 2024). Only one successful reduction with this technique has been published after transfer of an ICSI embryo (Peere et al., 2022). Left to itself, such a pregnancy usually ends with the loss of both fetuses, sometimes late in gestation (Dijkstra et al., 2020); the birth of a live foal remains exceptional (Peere et al., 2024b).
The Ghent University team currently considers terminating the pregnancy before the endometrial cups form, that is before about day 35, to be the most appropriate solution (Peere et al., 2024a). At that stage, the recipient returns to oestrus within a few days and can receive another embryo. If your mare has other frozen embryos, the season is therefore not lost.
We discuss it with you before any decision.
References
- Arnold L.C., Stout T.A.E., Claes A.N.J. Ultrasound-guided fetal thorax compression to reduce post-fixation twins in the mare. Equine Veterinary Journal, 2024, 56, 735-741.
- Dijkstra A., Cuervo-Arango J., Stout T.A.E., Claes A. Monozygotic multiple pregnancies after transfer of single in vitro produced equine embryos. Equine Veterinary Journal, 2020, 52, 258-261.
- Peere S., Papas M., Gerits I., Van den Branden E., Smits K., Govaere J. Management of monozygotic twins in the mare. Journal of Equine Veterinary Science, 2022, 113, 103988.
- Peere S., Van den Branden E., Papas M., Gerits I., Smits K., Govaere J. Twin management in the mare: a review. Equine Veterinary Journal, 2024a, 56, 650-659.
- Peere S., Van den Branden E., Broothaers K., Polfliet E., Smits K., Govaere J. Birth of a healthy monozygotic twin foal with hydrops and a dead co-twin. Veterinary Sciences, 2024b, 11, 649.
- Roberts M.A., London K., Campos-Chillón L.F., Altermatt J.L. Presumed monozygotic twins develop following transfer of an in vitro-produced equine embryo. Journal of Equine Science, 2015, 26, 89-94.