Why does the woman whose husband has died observe an iddah (waiting period) of four months and ten days, while the woman definitively divorced by a major (irrevocable) divorce observes an iddah of three months?

One must believe that everything Allah has ordained has profound wisdom, and our inability to grasp this wisdom does not negate its existence. The waiting period (iddah) has a devotional meaning that must be adhered to. A woman whose husband has died observes a waiting period of four months and ten days due to the wisdom that this period determines the purity of the womb, definitively reveals pregnancy, and shows the fetus, thereby ensuring no mixing of lineages. The four months and ten days is the period during which the soul is breathed into the fetus and its movement becomes clearly apparent. As for divorce, the waiting period is by menstruations (aqra’) because the husband, who holds the right, is present and managing his affairs.