Does single embryo transfer improve the obstetric and neonatal outcome of singleton pregnancy?

Singleton pregnancies following in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) are associated with increased risks of abnormal placentation, pre-eclampsia and preterm birth. These risks might partly be a consequence of the number of transferred embryos. In this commentary we summarize the results of three observational studies and one randomized study with 1052 pregnancies following single embryo transfer (SET). An increased age- and parity-adjusted risk of gestational hypertension and placenta previa in the SET pregnancies as compared to the spontaneously conceiving controls was reported in one of the studies. Preterm births occurred in 6-12% of SET pregnancies and 4-6% of the newborns were of low birth weight (LBW). The frequency of preterm birth and LBW after SET were considered either lower than or similar to those after double embryo transfer (DET) but higher than after spontaneous conception. Comparison of SET and DET pregnancies thus shows heterogeneous results. Still, it seems that not even a transfer of one good-quality embryo at a time diminishes all the previously identified increased risks of certain obstetric complications and preterm births in singleton pregnancies following IVF or ICSI.