In Islam, iddah or iddat (Arabic: العدة; period of waiting) is the period a woman must observe after the death of her husband or after a divorce, during which she may not marry another man. Its purpose is to ensure that the male parent of any offspring produced after the cessation of a nikah (marriage) would be known. The length of iddah varies according to a number of circumstances.
You need to wait till the 3rd starts and as soon as it starts the Iddah period is over.
The Iddah ( waiting period) for divorced woman is three menstrual periods or three months. If she is pregnant, her Iddah will last till the delivery of the child.
If the woman was divorced before any sexual intercourse took place between her and her husband, then No Iddah on her because marriage was never consummated.
If the woman is old and she has no menstrual period , then there no Iddah of divorce on her.
1. Just a letter from the husband or his attorney does not mean real Islamic Talaq unless Talaq has been pronounced properly in front of two pious witnesses along with all other conditions of Talaq ( Husband should not be forced to divorce, wife should not be passing her menstrual period, etc)
2. Iddah of Talaq is obligatory on the wife for three periods of menses or three months after she knows about Talaq, no matter how much time they were separated.
3. If the wife is old enough that she has no menstrual period at all, then she has no Iddah after Talaq.
4. If the marriage was only been recited but never consummated, then no Iddah after Talaq.
Iddah has different types:
1. Iddah after divorce (عدة الطلاق) which is must after a woman is divorced from her husband. It takes three months of less depending on three menstrual periods if she is not pregnant and if she is pregnant, then her Iddah will continue till she delivers her child. Iddah of Talaq must be observed in the matrimonial house. It is not allowed for the husband to drive her away, nor she is allowed to leave matrimonial home.
(O Prophet, when you divorce women, divorce them according to their Iddah, and count the Iddah, and fear Allah your Lord. Do not turn them out of the homes of their homes, not shall they leave) (Sura Al-Talaq, verse 1).
2. Iddah after husband's demise (Iddah Wafat) which is four months and ten days during which she must observe mourning and not take part in any celebration or happy event, nor wear colorful dress.
She can observe this Iddah of Wafat in her home or any other safe place where she observes full Hijab with mourning.
Iddah is the waiting period which must be observed after termination of marriage whether permanent or temporary , or death of the husband. During Iddah she will not be allowed to even talk about marriage or receive proposals from other men.
Iddah is two types:
1. Iddah of Talaq or after termination of temporary marriage provides confirmation that the woman does not have in her body from her husband, not only pregnancy but any remaining from sexual intercourse. This Iddah is three menstrual periods after Talaq, and two periods after temporary marriage. If the wife is pregnant, her Iddah will continue till the end of the pregnancy. There is no Iddah if the marriage was never consummated ( No sexual intercourse at all) or if the wife is old enough that she has no menses.
2. Iddah of Wafat, which is obligatory after death of the husband in any type of marriage and it goes on till four months and ten day after wife is informed about her husband’s death. If the death news reached the wife late, her Iddah time starts from that time not from the real date of death. Iddah of Wafat shows the respect which every wife should show to her husband, that respect which makes her more respected in the society. In some ignorant societies, they look to the woman who lost her husband as vulnerably low standard person. Allah ( SWT) wants her to be always respected
The observation of the 'iddah period is not only to ascertain pregnancy, or not, but rather for many issues, and most importantly it is a divine ruling placed by Almighty Allah.
As long as the marriage has been consummated, even though ejaculation did not occur, when the husband divorces his wife, she will still need to observe 'iddah.
There must be a "cooling off" period for the purpose of emotional detachment, recovering from previous relationship, transitioning back to another life, society realising their marital status, and more reasons that would support the importance of 'iddah for divorce, or 'iddah for passing away of husband.
And Allah knows best.
Being sterilized or using any other means of contraception don't have anything to do with observing Iddah period.
In fact preventing Ikhtilate Miah* is one of the reasons of putting this law and not the entire reason.
It is said that the respect of the previous marriage is among the reasons.
Unless she has reached the age of menopause, so in this case there is no need for observing Iddah period.
*It means the mixture of semens and to avoid uncertainty in whose child the foetus will be, is intended by it.