All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The Iddah (Waiting period observed by a woman after death of her husband or after a divorce, during which she may not marry another man) of the woman whose husband has died isn`t longer, in all situations, than that of the divorced woman:
The Iddah of the pregnant woman when she is widowed or divorced lasts until she gives birth since Allah the Almighty Says {What means}: "for those who carry (life within their wombs), their period is until they deliver their burdens: " {At-Talaq, 4}.
Similarly, the purity period between two menstruations for some women is longer-since there is no limit for the maximum period of purity-in the sense that their Iddah in terms of monthly periods exceeds that of the woman whose husband has died.
Anyway, Allah has a wisdom behind everything, whether His servants realize it or not, and it is an obligation upon us to abide by the rulings of Sharia out of obedience to Allah and His Messenger (PBUH), even if we didn`t figure out the wisdom behind such legislation.
Some contemporary scholars wrote medical papers in an attempt to discover the scientific wisdom behind specifying the Iddah after death of the husband-in some cases-as four months and ten days. Allah the Almighty Says {What means}: "but over all endued with knowledge is one, the All-Knowing" {Yousef, 76}. And Allah the Almighty Knows Best.