Praise be to Allah.
According to the Islamic Shari`ah, the iddah is a waiting period with certain exigencies required of a woman when the appropriate condition exists; either divorce or death of her husband.
The ‘iddah for one who still menstruates, is three menstrual cycles, whereas for one who is either too young or too old to menstruate, it is three months.
As for the widow whose husband has passed away, if she is pregnant then her ‘iddah is until she delivers her child; otherwise, it is four months and ten days.
And Allah knows best.