Epigenetic modifications caused by extended culture leading to the death of more female embryos has been theorised as the reason why blastocyst transfer leads to a higher male sex ratio; however, adding retinoic acid to the culture can bring this ratio back to normal. A second theory is that the male-biased sex ratio may due to a higher rate of selection of male embryos. Male embryos develop faster in vitro, and thus may appear more viable for transfer.
epIGeNetiC modificatioNs CaUsED by eXteNdeD CulTURe LeAding to tHe deaTh oF moRe FemaLe emBrYos hAs Been TheOrIsEd AS tHe reasOn WHY blaSTocYst tranSfer LeADs to a hiGhER maLe seX RAtio; hOWeVer, AdDing retiNoic acId To tHe CulturE can bring this ratIO baCk to NOrmAl. a seCONd tHEoRy is That tHe male-biasEd sEX ratIO mAy Due To a higHEr rATe oF sElectiOn Of MAle embryOs. male embryos dEvELoP FasteR in vitrO, And Thus may appeAR mOre VIAble FOr TranSfer.