Pregnancy or multiple pregnancy is considered the major problem associated to the assisted reproduction techniques. Although these problems are becoming less frequent, we still have a very long way to go and completely solve these problems.
From a medical point of view, multiple pregnancy brings with it several risks for both the mother (hypertension, placental abruption bleeding, urinary tract infection, anaemia, etc.) and for the baby (all the problems associates with prematurity, congenital malformations, neurological disorders, cerebral palsy, etc.), which in extreme cases can lead to death.
Obviously, the more babies are involved, the more problems associated to pregnancy may appear; and, therefore, we should not lose sight of a twin pregnancy, since it involves a number of complications bigger than a single pregnancy.
In Spain, the Law 14/2006 of Assisted Reproduction Techniques limits the number of embryos that can be transferred to only three. Fortunately, such transfers are very rare, and in most cases, because of the improvement of techniques and the protocols used, the number of embryos transferred is 2, though, each time more frequent, is of 1 single embryo.
Thus, as we have embryos of higher quality (more potential implementation) and excellent results as for the freezing/thawing of the embryos, the number of embryos to be transferred can be reduced, but always maintaining the pregnancy rate and, consequently, reducing the number of multiple pregnancy.