The birth of the microbiome Barriers to embryo research Studying the early development of a human embryo can be difficult due to the limited number available and ethical and legal constraints. The International Consensus and National law for culturing human embryos states that embryos obtained by IVF can be cultured up to 14 days post-fertilization and/or the formation of a primitive streak, whichever is first, the Australian study said. "The applicability of the '14-day rule' to in vitro models of early development that are not derived by fertilization is not clear," the authors wrote. This led the team to be cautious and only culture the blastoids for up to five days.