Ernstad EG, et al. - Researchers performed a population-based retrospective registry study including all singletons born after frozen embryo transfer in Sweden from 2005 to 2015 in order to assess the correlation between the protocol used and obstetric outcome after frozen embryo transfer. In addition, they made a comparison for frozen vs fresh transfer and for frozen transfer vs spontaneous conception. After frozen embryo transfer (natural cycles, n=6297, stimulated cycles, n=1983, programmed cycles, n=1446), 9726 singletons were born, after fresh transfer 24,365 singletons were born and after spontaneous conception 1,127,566 singletons were born. The protocols led to no significantly different outcomes regarding preterm birth and low birth weight. However, programmed cycles were noted to be correlated with greater rates of hypertensive disorders in pregnancy, postpartum hemorrhage, postterm birth, and macrosomia. Outcomes of stimulated cycles were similar to that of natural cycles. Given the increasing use of frozen cycles and the new policy of freeze-all cycles in IVF, the findings are important. As per results, the absence of corpus luteum seems to be correlated with adverse obstetric outcomes.