Monozygotic (identical) twins most commonly form when a zygote splits during the early:

1. Monozygotic twins arise when a single zygote divides into two separate embryos during early development. 2. This splitting can technically happen as early as the two-cell stage, but that is not the most common timing. 3. Splitting most commonly occurs during the early blastocyst stage, with roughly 70-100 cells present. 4. Morula stage and Pronuclear stage are both different figures a reader might guess from memory, but neither matches the specific figure given here. 5. Fetal stage is also incorrect for the same reason: it does not match the specific figure given here. 6. This timing determines whether the twins share a placenta (blastocyst-stage split) or have individual placentas (two-cell-stage split). _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 28 "Development and Inheritance", p.1321-1322 §The Zygote_