One of the unusual features of meiosis in women is that the oocytes are arrested, for most of the woman's life, in prophase of the first meiotic division. They finish the second meiotic division only after the oocyte has been released from a mature follicle and has been penetrated by a sperm. This paially explains why maternal age has a stronger impact on the risk of trisomies (such as Down's syndrome) than does paternal age, since more things can happen over a 40 year period to chromosomes that are already undergoing meiosis than those that are relatively protected in interphase. Anaphase is the stage during which the chromosomes begin to pull apa from the equatorial plate. Interphase is the stage during which cells are not actively dividing. Metaphase is the stage during which the chromosomes are lined up along the equatoria.