Polygamy
Polygamy has existed all over the world as an aspect of culture and/or religion, including in the Hindu, Jewish, Christian, and Islamic faiths. For Muslims, according to the Qur'an, a man may have up to four wives at any one time; the restriction on the number was not customary before the advent of Islam in Arabia. There are two conditions to polygamous marriage in Islam: first, additional wives may only be taken when orphans and widows cannot be maintained by society as a whole and would better benefit from maintenance by a single man; second, the husband is required to treat all wives equally. If a man fears that he will not be able to meet these conditions then he is not allowed more than one wife.
"If ye fear that ye shall not be able to deal justly with the orphans, marry women of your choice, two, or three, or four; but if ye fear that ye shall not be able to deal justly (with them), then only one, or that which your right hands possess. That will be more suitable, to prevent you from doing injustice." (Qur'an 4:3) Yusuf Ali translation.
A bride-to-be may stipulate in the marriage contract her conditions such as monogamy. A woman may not have more than one husband, which is regarded as unacceptable because it could create difficulty in determination of paternity and hence responsibility of upbringing of children and inheritance. In addition, each husband would need to wait for his turn to have a child with his wife, as she cannot carry more than one man's baby at a single time. The Qur'an also recommends a woman nurses her child for two years before having another, thus adding to the wait time of each husband to have his own child.
The mothers shall give suck to their offspring for two whole years... (Quran 2:233) Yusuf Ali translation.
Read more about this topic: Marriage In Islam