Influence of atmospheric versus reduced oxygen concentration on development of human blastocysts in vitro:: a prospective study on sibling oocytes

Numerous studies show the beneficial effect of reduced oxygen on the culture of annual embryos in vitro. However. few similar studies have been carried Out in humans, and the conclusions fi-om these were contradictory. Using sibling human oocytes, a prospective study was carried out to analyse the effect of 5 and 20% oxygen on prolonged development of embryos. The outcomes measured were fertilization rate and proportion of morphologically optimal embryos, blastocysts and optimal blastocysts developing on day 5. The results were analysed separately for the group of IVF (n = 988 oocytes) and ICS1 (11 = 928 oocytes) cycles. It was found that low oxygen did not influence fertilization, but in comparison with 20% oxygen, it resulted in a si.-nificantly higher proportion of embryos bein.- optimal on day 3 after IVF (59 versus 43.2%: P < 0.001) as well as after ICS1 cycles (51.2 versus 28.5%, P < 0.001). In both methods. the lower oxygen concentration improved tile blaStUlation rate (73.2 versus 63.1 %; P < 0.05 and 67.4 versus 54.7%; P < 0.00 1) and increased the proportion of embryos reaching the stageof expanded blastocyst with normal innercell rnassonday.5 (31.1 versus 14.6%; P< 0.001 and 18.9 versus 11.4%; P < 0.0 1). The ratio of successful embryo development to optimal blastocyst stage on day 5 of culture, calculated for two oxygen concentrations, was 2.1 for IVIF and 1.7 for ICSI, in favOUr of lower oxygen tension.