Iddah

In Islam, a woman must observe the period of waiting, known as ’iddah or iddat (; "period of waiting") following the death of her husband or a divorce, during which she is prohibited from getting married to another man.  Eliminating any uncertainty regarding the paternity of a child born after the previous husband's divorce or death is one of its primary goals.

The duration of 'iddah varies depending on several factors.  A divorced woman's 'iddah is typically three lunar months, or roughly 89 days; however, if the marriage was not consummated, there is no 'iddah.  Whether or not the marriage was consummated, a woman's 'iddah is four lunar months and ten days (about 128 days) following her husband's passing.  The 'iddah continues until the woman gives birth if she is pregnant at the time of her divorce or widowhood.

According to Islamic scholars, this directive strikes a compromise between allowing the widow to grieve her husband's passing and shielding her from potential criticism if she remarries too soon after.Amin Ahsan Islahi, Tadabbur-i-Quran, 2nd ed., vol. 1, (Lahore: Faran Foundation, 1986), p. 546 As four and a half months is half the duration of a typical pregnancy, this is often used to determine if a woman is pregnant or not.Shehzad Saleem. The Social Directives of Islam: Distinctive Aspects of Ghamidi’s Interpretation , Renaissance. March, 2004.
Iddah for widows
Scriptural basis
The Quran (Al-Baqarah 2:234–235) gives the following instructions about a widow's waiting period:

Main directives
The primary instructions (for widows) found in these passages are summarised as follows:

 A widow's waiting period (Iddah) is four months and ten days;
 The woman is not allowed to wed another man during this time;
 A person may announce his plans to marry the widow during this time in a way that is socially acceptable or he may keep them to himself, but he should not make a covert marriage vow to the widow; and
 Only after the four months and ten days have passed should the time and location of the marriage contract be decided upon and committed to.
These are the fundamental Sharia guidelines for a widow's waiting time. Another Quranic command makes it very evident that the woman should not be forced to leave her home during this waiting period.Surah Talaaq 65:1

Iddah for divorced women
Scriptural basis
The Quran (Al-Baqarah 2:228) gives the following instructions about a divorcee's waiting period:

The surah Al-Ahzab states:

The surah At-Talaq Verse 65:4, in the Quran states:

Main directives
The primary instructions (for divorces) found in these verses are summarised as follows:
A lady who is menstruation must wait three months between cycles.
The waiting period for a pre-menstrual girl is, three lunar months
For a lady without monthly courses, the waiting period is three months.
If the husband wishes to make amends, he has a greater right to take her back during this time.  But only in the event of a first or second divorce is this true.
There is no ’iddah if a Muslim man marries a Muslim lady and then divorces her before any physical relationship.

Commentaries

According to Imam Suyuti's commentary, Tafsir al-Jalalayn, "And as for those of your women who read allā’ī or allā‘i in both instances no longer expect to menstruate, if you have any doubts about their waiting period, their prescribed waiting period shall be three months. Additionally, for those who have not yet menstruated due to their young age, their period shall also be three months." These situations apply to everyone except those whose spouses have passed away, in which the verse specifies that they must wait four months and ten days on their own.

Tafsir ibn Kathir, comments thus of this verse, Her Iddah is three months instead of the three monthly cycles for those who menstruate, which is based upon the Ayah in (Surat) Al-Baqarah. (see 2:228) The same for the young, who have not reached the years of menstruation. Their `Iddah is three months like those in menopause. The Tafsir al Jalalyn co-authored by the scholar, Imam Suyuti comments thus about this verseAnd [as for] those of your women who (read allā’ī or allā’i in both instances) no longer expect to menstruate, if you have any doubts, about their waiting period, their prescribed [waiting] period shall be three months, and [also for] those who have not yet menstruated, because of their young age, their period shall [also] be three months

Iddah for pregnant women
Scriptural basis

Main directive
Pregnant widows and pregnant divorced women are not allowed to be married again until after giving birth. Until there is no longer any uncertainty over any current pregnancy, remarriage is not permitted.

 References