Analysis of human embryos by time-lapse microscopy has shed new light on human embryonic development. The results, reported in this week’s Nature Biotechnology, may help assisted reproduction clinics increase the success rates of in vitro fertilization (IVF) by improving their methods for selecting embryos to be transferred to women.

Renee Reijo Pera and colleagues filmed IVF embryos for several days after fertilization to search for visual cues that would allow them to predict whether a 2-day-old embryo would develop properly to day 5 or 6 — to a structure known as a blastocyst. They found that embryos with a high likelihood of reaching the blastocyst stage meet three criteria: a first cytokinesis — a stage during cell division — lasting 0-33 minutes, an interval of 7.8-14.3 hours between the first and second mitoses and an interval of 0-5.8 hours between the second and third mitoses. They also discovered that individual cells in a single embryo have different patterns of gene expression, with some cells retaining more transcripts inherited from the mother — via the egg — and other cells having activated embryonic gene expression.

If implemented in clinical IVF programs, the authors’ non-invasive imaging approach could increase the chances of transferring embryos that will lead to successful pregnancies.