Risks of spontaneously and IVF-conceived singleton and twin pregnancies differ, requiring reassessment of statistical premises favoring elective single embryo transfer (eSET).

A published review of the literature by Dutch investigators in 2004 suggested significant outcome differences between spontaneously - and in vitro fertilization (IVF) - conceived singleton and twin pregnancies. Here we review whether later studies between 2004-2015 confirmed these findings. Though methodologies of here reviewed studies varied, and all were retrospective, they overall confirmed results of the 2004 review, and supported significant outcome variances between spontaneously- and IVF-conceived pregnancies: IVF singletons demonstrate significantly poorer and IVF twins significantly better perinatal outcomes than spontaneously conceived singletons and twins, with differences stable over time, and with overall obstetrical outcomes significantly improved. Exaggerations of severe IVF twin risks are likely in the 50 % range, while exaggerations of milder perinatal risks are approximately in 25 % range. Though elective single embryo transfers (eSET) have been confirmed to reduce pregnancy chances, they are, nevertheless, increasingly utilized. eSET, equally unquestionably, however, reduces twin pregnancies. Because twin pregnancies have been alleged to increase outcome risks in comparison to singleton pregnancies, here reported findings should affect the ongoing discussion whether increased twin risks are factual. With no risk excess, eSET significantly reduces IVF pregnancy chances without compensatory benefits and, therefore, is not advisable in IVF, unless patients do not wish to conceive twins or have medical contraindications to conceiving twins.