Theoretically, for that to happen, the following conditions have to be met:
– ovulation takes place relatively early in the cycle, and
– sperm survive from the end of menstruation until ovulation.
Even if this sounds highly unlikely, it is not impossible. If the first half of the cycle is shorter, the time of ovulation may shift forward accordingly, and take place earlier in the cycle. In that case, if sperm cells survive in the uterus or the fallopian tubes until the day of ovulation, fertilisation and subsequent implantation of the ovum are possible, albeit all in all, quite improbable. However, it is incorrect to claim that it is absolutely impossible for a woman to be infertile during menstruation.