STUDY QUESTION: Do different oxygen levels during human IVF embryo culture affect embryo utilization, cumulative IVF success rates per cycle and neonatal birthweight?
SUMMARY ANSWER: After 2 days of culture, a lower oxygen level (5%) leads to more good-quality embryos and more embryos that can be cryopreserved, and thereby to a higher cumulative live birth rate per cycle when compared to embryo culture in 20% oxygen, while birthweights are similar.
WHAT IS KNOWN ALREADY: Several studies have compared IVF outcome parameters after embryo culture in a more physiological level of 5% oxygen and the atmospheric level of 20%. Although there is consensus that embryo development improves in 5% oxygen, effects on pregnancy and live birth rates are mainly seen in blastocyst, but not cleavage-stage transfers. A major drawback of these studies is that only fresh embryo transfers were included, not taking additional frozen-thawed transfers from these cycles into account. This might have underestimated the effects of oxygen level, especially in cleavage-stage embryo transfers. Furthermore, little is known about the effect of oxygen level during culture on birthweight.
STUDY DESIGN SIZE DURATION: This is a cohort study in 871 consecutive patients who had an IVF cycle between January 2012 and December 2013, and 5-7 years follow-up to allow transfer of frozen-thawed embryos. Based on daily availability of positions in the incubators, all oocytes and embryos of one cycle were allocated to one of the three incubators with traditional ambient oxygen levels (6% CO2 and 20% O2 in air), or to a fourth incubator that was adjusted to have low oxygen levels of 5% (6% CO2, 5% O2 and 89% N2). Embryos were cultured under 5 or 20% oxygen until Day 2 or 3, when embryos were transferred or cryopreserved, respectively. Clinical and other laboratory procedures were similar in both groups.
PARTICIPANTS/MATERIALS SETTING METHODS: To compare embryo characteristics and (cumulative) pregnancy outcomes between the two oxygen groups, for each patient only the first cycle in the study period was included in the analysis, resulting in 195 cycles in the 5% group (1627 oocytes) and 676 in the 20% oxygen group (5448 oocytes). Embryo characteristics were analysed per cycle and per embryo and were corrected for maternal age, cycle rank order, fertilization method (IVF or ICSI) and cause of subfertility. Perinatal data from the resulting singletons (n = 124 after fresh and 45 after frozen-thawed embryo transfer) were collected from delivery reports from the hospitals or midwife practices.
MAIN RESULTS AND THE ROLE OF CHANCE: In the 5% oxygen group, there were significantly more embryos of good quality (45.8 versus 30.9% in the 20% group, adjusted odds ratio (OR) [95% CI] = 1.9 [1.6-2.4]). This did not result in higher live birth rates per cycle, but after fresh transfers more good-quality spare embryos could be cryopreserved (46.1 versus 29.7%, adjusted OR [95% CI] = 2.0 [1.7-2.5]).After a follow-up period of 5-7 years, in which 82.4% of the cryopreserved embryos from the 5% oxygen group and 85.4% from the 20% oxygen group were thawed, the percentage of patients with at least one live birth resulting from the study cycle was significantly higher in the low oxygen group (adjusted OR [95% CI] = 1.5 [1.01-2.2]). In 124 live born singletons from fresh embryo transfers and in 45 from transfers of cryopreserved embryos, birthweight was similar in both oxygen groups after correction for confounding factors.
LIMITATIONS REASONS FOR CAUTION: This is a retrospective study, and treatment allocation was not randomised. The study was not powered for a predefined birthweight difference. With the number of live births in our study, small differences in birthweight might not have been detected. The selection of embryos to be cryopreserved was based on embryo morphology criteria that might be different in other clinics.
WIDER IMPLICATIONS OF THE FINDINGS: Improved embryo utilization by more cryopreservation leading to higher cumulative live birth rates per cycle favours the use of 5% instead of 20% oxygen during human IVF embryo culture. This study also demonstrates that for comparison of different IVF treatment regimens, the cumulative outcome, including transfers of fresh and frozen-thawed embryos, is to be preferred instead of analysis of fresh embryo transfers only.
STUDY FUNDING/COMPETING INTERESTS: No external funding was received for this study. None of the authors has a conflict of interest to declare.
TRIAL REGISTRATION NUMBER: NA.