Single embryo transfer (set) not only leads to a reduction in twinning rates after IVF/ICSI, but also improves obstetrical and perinatal outcome of singletons.

This work consists of two parts. In the first part we investigated the influence of first trimester bleeding on perinatal outcome after assisted reproductive technologies (ART) and we found that first-trimester bleeding not necessarily leads to spontaneous abortion, as in spontaneous pregnancies. On the other hand, first-trimester bleeding clearly is associated with an impaired pregnancy outcome, and as an independent risk factor for obstetrical and perinatal outcome, it should be taken into account in studies analysing pregnancy outcome after ART. The fact that the incidence of first trimester bleeding increases with the number of embryos transferred is another argument to move to single embryo transfer (SET). Next to a very effective reduction in multiple pregnancy rates, SET indeed seems to be beneficial in terms of pregnancy outcome characteristics in singleton pregnancies as well. In the second part of this work we indeed have demonstrated that mean birthweight is significantly higher in SET singletons compared with DET singletons. Also the incidence of preterm birth and low birthweight is clearly reduced when SET is compared with DET. A possible explanation for this outcome can be the 'avoidance' of vanishing twins in IVF/ICSI procedures when SET is applied, but this hypothesis needs further investigation. A more frequent application of SET in assisted reproductive technologies clinics could be an essential step to reduce differences in outcome between spontaneous and assisted reproduction children.