Short answer
Because Allah set it in the Quran (2:234). Scholars mention several wisdoms: honouring the marriage and the husband's memory, giving the widow time to grieve, and making certain there is no pregnancy, which shows clearly within that time. It applies even when pregnancy is impossible, which shows it is also an act of worship.
The widow's iddah is fard, set by the text of the Quran, and mourning (hidad) during it is wajib. Its length does not change with age, menopause or whether the marriage was consummated; a pregnant widow's iddah ends with the birth.
The four schools agree on this.
Explanation
The iddah of a widow is not a custom or a punishment. It is a command of Allah: those who die and leave wives, their wives wait four months and ten days. Muslims follow it first because Allah commanded it. Scholars have also reflected on its wisdom, and the four schools agree on its length.
- Honouring the marriage. Marriage is a sacred bond. Its end by death is marked with a period of loyalty and mourning for the husband.
- Time to grieve. The widow is spared new proposals and social pressure while she mourns and settles her affairs.
- Clarity about pregnancy. The hadith describes the child forming in the womb in stages of forty days before the soul is breathed in. By four months a pregnancy is usually clear, and the extra ten days add certainty. This protects the child's lineage and inheritance.
- Worship. The iddah applies to an elderly widow, and to one whose marriage was never consummated, where pregnancy is impossible. So it is not only about the womb; it is submission to Allah's command.
Why only women? The rules of pregnancy and lineage concern the woman, and the iddah protects her rights and her child's. Men have their own waiting rules in some cases; see do men have an iddah.
For what a widow may do, see what a widow can do during iddah. For the end date, use the iddah calculator. For the basics, see what is iddah.
The evidence
Quran, Surah Al-Baqara 2:234
وَٱلَّذِينَ يُتَوَفَّوْنَ مِنكُمْ وَيَذَرُونَ أَزْوَٰجًۭا يَتَرَبَّصْنَ بِأَنفُسِهِنَّ أَرْبَعَةَ أَشْهُرٍۢ وَعَشْرًۭا ۖ فَإِذَا بَلَغْنَ أَجَلَهُنَّ فَلَا جُنَاحَ عَلَيْكُمْ فِيمَا فَعَلْنَ فِىٓ أَنفُسِهِنَّ بِٱلْمَعْرُوفِ ۗ وَٱللَّهُ بِمَا تَعْمَلُونَ خَبِيرٌۭ ﴿٢٣٤﴾
And those who are taken in death among you and leave wives behind - they, [the wives, shall] wait four months and ten [days]. And when they have fulfilled their term, then there is no blame upon you for what they do with themselves in an acceptable manner. And Allah is [fully] Acquainted with what you do.
The widow's iddah is set by this ayah.
Sahih al-Bukhari 1280
Sahih... إلا على زوج أربعة أشهر وعشرا ...
"Except for a husband: four months and ten days."
The widow mourns for the whole iddah.
Sahih al-Bukhari 3208
Sahih... إن أحدكم يجمع خلقه في بطن أمه أربعين يوما ...
"The creation of each of you is gathered in his mother's womb for forty days."
The stages of the womb are part of the wisdom scholars mention.
Where the ruling is recorded
- al-Hidaya, Kitab al-Talaq, Bab al-'Iddah. The widow's iddah applies whether or not the marriage was consummated.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Talaq, Fasl fi al-Hidad. Mourning as wajib on the widow.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.