In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Imaam Abdul Azeez Bin Baaz [rahimahullaah] was asked about a husband and a wife who agreed not to have children and whether this is allowed:
The Shaikh [rahimahullaah] responded saying that this agreement of theirs is not permissible. As long as the woman is able [to bear children], then it is not permissible for them to do this because the Islamic legislation requires that the people give concern to bearing children and make the Ummah numerous. The Prophet [sallal-laahu-alayhi-wasallam] said: Marry the childbearing, loving woman for I shall outnumber the peoples by you on the Day of Resurrection. In another wording [of this narration the Prophet said]: Outnumber [i.e. the followers of the other] Prophets on the Day of Judgement.” That is because by way of this [i.e. having many children] those who worship Allaah among the Muslims will be numerous, the Ummah will be numerous and strong in opposing their enemies.
It is not permissible for a man to abandon having children out of fear of either tiredness in seeking after a livelihood or due to difficulty, or due to expenditure, or due to a desire to enjoy one’s wife and other than that. It also not permissible for a woman to do this, rather it is obligated on both of them to pursue the means of having offspring -be eager to seek the means to having children so that the Ummah becomes numerous and to fulfil what the Prophet [sallal-laahu-alayhi-wasallam] stated.
However, if the woman either suffers a lot of pain due to an illness of the womb or she cannot give birth except by way of surgical operation, then this is an excuse to not have children. If the surgical operation [or caesarean] will harm her- it is feared that something will happen to her, and also if the children are many- born at close intervals and bringing them up becomes difficult upon her, then there is nothing to prevent her from taking some pills [or contraception] or some preventative measures for a year or two – the period of breastfeeding- so that she becomes strong enough to nurture the children and able to nurture the other newly born.
Paraphrased…see link with transcript and audio http://www.binbaz.org.sa/noor/11853
NB: This is a general Fatwa, therefore married should ask the scholars about their specific circumstances, in order to receive specific ruling related to their situation.