Praise be to Allaah.
Using the coil is permissible subject to two conditions:
The first condition is that it should not cause harm to the woman.
The second condition is that her husband should give his permission for that.
We would like to remind women that a woman should not do things to prevent pregnancy, because this goes against the aims of sharee’ah. It is better for her to remain as Allaah created her, bearing lots of children, because having lots of children serves a great purpose and does not harm a person in his provision, in raising them or in health.
But if a woman is physically weak and is very sick, such that having a child every year would harm her, in this case she is excused if she takes something to prevent pregnancy. But she has to have her husband’s permission to use that which will prevent pregnancy, and using it should not cause any harm.
Hence it is prescribed in Islam for a man to marry a woman who is loving and fertile, i.e., she should be one of the women who are known to have many children, so that the Prophet (peace and blessings of Allaah be upon him) may feel proud of the great numbers of his ummah, and the numbers of Muslims may be increased thereby.
Summarized from a fatwa of Shaykh Ibn ‘Uthaymeen. See Fataawa Manaar al-Islam, vol. 3, p. 784
Sheikh Muhammed Salih Al-Munajjid