THE PURPOSE OF THE IDDAT
There are several reasons why the iddat has been ordained. As regards the iddat that is calculated according to the cycle of menstruation, the purpose is, firstly, to establish the state of the womb, i.e. to ascertain whether there has been conception prior to divorce or not. When menstruation has been fixed as the basis for iddat, it will be a mere formality to establish this, for when the blood of haidh flows it is a sure sign that the womb is empty of a fetus. However, one period of menses is insufficient to provide definite evidence of the 'emptiness' of the womb, so the Shariah prescribes the elapse of another period for added confirmation. After this second haidh it will be conclusively proven that the womb is indeed free of pregnancy. Now that the womb has been certified as free of burden, the third haidh or menstruation period is merely a mark of respect for the bond of nikah and to prolong the termination of the nikah thus increasing the chances of reconciliation The result of all this is an iddat consisting of three haidh periods. As for those ladies who do not experience haidh for reasons already outlined, the straightforward three-month iddat prescribed for them will serve the same purpose as expounded above, beyond any shadow of doubt. Furthermore, the three-month iddat is to differentiate between the iddat of the free woman and the woman in bondage, whose iddat the Shariah has set at two months, or two menstruation periods.
Secondly, a hadith of Rasoolullah (sallallahu alaihi wasallam) provides us with a very strong reason for observation of a period of waiting called iddat. This hadith states:
"Whoever believes in Allah and the Last Day should not allow his water (semen) to inseminate the plantation (womb) of another." (Abu Dawood, Tirmizhi)
The "womb of another" spoken of here means the womb of the divorcee which was probably inseminated by her former husband. We say probably because such an issue is not clear until the iddat has in fact passed thus proving the existence or non-existence of a fetus. If the latter then it is clear that in reality the womb was not inseminated. Hence the only meaning one can deduce from this hadith is that Rasoolullah (sallallahu alaihi wasallam) is prohibiting marriage during iddat, for marriage is obviously a prelude to intercourse. If there was no waiting period before the next marriage, the possibility was very great that the new husband would have intercourse with the wife while she may be expecting the baby of her former husband. This has been described by the hadith as "inseminating the crops of another", i.e. depositing semen into a womb that has already been impregnated by another. This one teaching of Rasoolullah (sallallahu alaihi wasallam) shows the wisdom and knowledge Allah had granted him, for indeed this practice has not only religious complications but is even detrimental for health reasons.
Allah knows best