Twinning - two foals in the uterus during the same pregnancy - is considered to be pathological in mares because of the nature of the equine placenta. This is termed "diffuse" and forms a contact with the entire endometrium, or uterine lining. There is only enough endometrial area in a normal mare to support sufficient placenta for one foal, so two foals will fail to develop to a normal size and level of maturity.
Twins must be detected as early as possible in the pregnancy so that one embryo can be removed to allow the other to remain as a singleton.
The equine embryo enters the uterus about day 5-6 after conception, and from then until day 16 continues to roam around the lumen - a process that enables the mare's reproductive system to recognise and support the pregnancy with the appropriate hormones. On day 16 the foal (or foals) will settle in one spot, usually at the junction of a horn and the uterine body.
If twins are not detected until after day 16, they will have settled in a fixed position - often close together at the same junction. This makes it much more difficult to get rid of one than when both are still mobile on days 14 and 15.
Successful reduction of twin pregnancies to singletons is one of the main reasons we scan all our mares not later than 14 days after ovulation.