Human blastocyst formation in an embryo culture medium mimicking human uterine pH and fluid composition

Study questionTo study the development of human embryos in a culture medium with pH 6.8 and composition based on previous in vivo human uterine measurements. Summary answerThe blastocyst formation rate of donated surplus human preimplantation embryos in embryo culture medium with pH 6.8 and uterine composition was similar to control medium. What is known alreadyIn vitro culture conditions can affect human preimplantation embryo development and fertility treatment success rates. The current IVF culture conditions are derived from using animal models, laboratory experience and limited human in vivo evidence. Therefore, we previously measured human in vivo uterine conditions on the third day following a positive LH test or ovum pick-up. Strikingly, the uterine pH at this time of the menstrual cycle (pH 6.8) appeared lower than the pH used for human preimplantation embryo culture in IVF laboratories worldwide (pH 7.2-7.4). Mimicking in vivo conditions may potentially improve in vitro embryo development, implantation rates, and overall fertility treatment and safety outcomes. Study design, size, durationIn this preclinical pilot study, 404 donated surplus human day 3 and day 4 embryos were cultured in one of four culture media: ULPC medium (uterine low pH and uterine fluid composition), UC medium (standard pH and uterine fluid composition), ULP medium (uterine pH and standard composition), or control medium (G-2 PLUS medium with standard pH and composition). Blastocyst formation rate on day 5 (after one or two days in culture) was the primary outcome to assess embryo development. Participants/materials, setting, methodsEmbryos were randomly allocated to the different culture conditions, stratified by sibling status and maternal age, before thawing. Day 3 and day 4 embryos were separately randomized, ensuring that each culture group contained an equal amount of both day 3 and day 4 embryos. All procedures and culture conditions were similar between groups, except for pH and medium composition. Embryo morphology was assessed immediately after thawing and on developmental day 5 for the primary outcome. Assessors were blinded to medium allocation. Main results and the role of chanceAll four culture media supported human preimplantation embryo development into blastocysts. Blastocyst formation rates on day five were 54/102 (52.9%) in ULPC medium, 44/104 (42.3%) in UC medium, 48/98 (49.0%) in ULP medium and 47/102 (46.1%) in control medium. Limitations, reasons for cautionThis study is limited by the use of donated surplus human day three and day four embryos. The effect of these new in vitro conditions on embryo development during the first three days of human embryo culture is not yet known. Wider implications of the findingsThe outcomes of this study indicate the need for further research on mimicking the in vivo uterine conditions for in vitro embryo culture using the conditions proposed here, including a lower pH (of 6.8) than currently used in IVF laboratories (pH 7.2-7.4).