Blastomere number

The first cleavage may occur as soon as 25h after insemination. The fast cleaving embryos are at the 4-cell stage on day 2 ( 44-46 h post insemination) and at the 8-cell stage on Day 3 (66-68h post insemination). As cleavages of human embryos are asynchronous, embryos at the 3, 5, 6,7 or 9-cell stage are frequently observed.

Blastomere size

From the 2-cell to the 8-cell stage, there are 3 different sizes of blastomeres, as shown in Figure 1. Identical size of blastomeres is observed only at the 2, 4 and 8-cell stages. Embryos presenting blastomeres of different sizes at the 2, 4 and 8-cell stages are considered as uneven embryos. Embryos at the 3-cell stage with one large and two small blastomeres are even embryos.



3 states of nucleation of the blastomeres can be observed: one nucleus per blastomere, no nucleus (M-phase, before cleavage) or several nuclei (multinucleation). Multinucleation, that is highly abnormal, is better observed at the 2-cell stage.

Extracellular fragmentation

The degree of fragmentation may vary considerably and is usually expressed as % of embryo volume. Fragmentation can be separated in 3 categories: < 20%, 20-50% and >50%.

Effects of embryo morphology on implantation

The influence of embryo morphology on implantation is summarized in the following table.

Effect on implantation
Blastomere number Beneficial if 4-5 on Day 2 or 7-9 on Day 3
Uneven embryo Detrimental
Multinucleation Detrimental
Fragmentation Detrimental if severe


  • Rienzi et al. Significance of morphological attributes of the early embryo. RBMonline 10: 669-681, 2005
  • Roux C et al. Morphological classification of human in-vitro fertilization embryos based on the regularity of the asynchronous division process. Human Reprod Update 1: 488-496, 1995.
  • Van Royen et al. Multinucleation in cleavage stage embryos. Human Reprod 18: 1062-1069, 2003