[Spontaneous and manual embryo reduction in mares: a twin prevention programme].

Data from the literature and own data for 67 twin pregnancies were used to establish the factors essential to the decision on how to treat twins at different gestational ages. Spontaneous (natural) reduction was compared with manual embryo reduction. Manual embryo reduction is always indicated when a twin is diagnosed before day 16 after ovulation. Thereafter, the type of fixation is the main determinant. Manual embryo reduction is always first choice for bilateral and unilateral non adjacent embryos and must be applied as early as possible. The change of natural reduction up to day 30, is higher for unilateral adjacent twins than is the chance of success after manual reduction. From day 30 onwards manual embryo reduction is preferable. The results are summarized in a twin prevention programme which describes how twin pregnancies can be avoided after the development of endometrial cups.