The word ghilah signifies both “sexual intercourse during the period of suckling” and “breastfeeding during pregnancy”. Ghilah, therefore, is permissible as long as there is no harm on the suckling baby. It goes without saying that trustworthy doctors are those who can decide whether or not sexual intercourse during pregnancy or breastfeeding during pregnancy could harm the suckling baby.
If there is a possibility that being pregnant during the suckling period will affect the milk, then it will be better for the spouses to use a suitable and safe contraceptive means during the period of suckling. If the wife conceives, and it is thought that milk will cause harm to the child, she should stop breastfeeding him.
The Kuwaiti Encyclopedia of Jurisprudence states the following:
“Linguistically speaking, the word ghilah indicates deception. For example, when it is said that someone was killed by means of ghilah, this means that he was deceived and taken to a place where he was murdered. In Arabic, the word also implies killing by a surprise attack. According to Shari`ah, the word ghilah indicates having sexual intercourse with one’s wife during the suckling period, and it also signifies the woman’s breastfeeding of her child while she is pregnant.
Early Arabs disliked having intercourse with their wives during the suckling period and [the wife] breastfeeding during pregnancy, and thus they abandoned these acts [intercourse and breastfeeding during pregnancy], for they believed this would spoil the milk and be a cause of ailment and weakness for the child’s body. If this claim were a matter of fact, the Prophet (peace and blessings be upon him) would have forbidden doing it [ghilah]. The Prophet (peace and blessings be upon him) said, “I intended to prohibit ghilah, but I considered the Byzantines and Persians, and knew that they do ghilah and this does not cause any harm to their children.” The hadith points out that if sexual intercourse during the suckling period and breastfeeding during pregnancy were harmful, then they would have harmed the Byzantine or the Persian children. They had so many physicians who would have forbidden them from doing so if it had been injurious. That was why the Prophet (peace and blessings be upon him) did not prohibit doing such acts. Moreover, Sa`d ibn Abi Waqqas (may Allah be pleased with him) narrated: A man came to Allah’s Messenger (peace and blessings be upon him) and said, “I do coitus interruptus while having sexual intercourse with my wife.” The Prophet (peace and blessings be upon him) asked, “Why do you do that?” The man replied, “I fear for her child.” Thereupon, Allah’s Messenger (peace and blessings be upon him) said, “If that were harmful, it would harm the Persians and the Byzantines.”
Relying on the former hadith and the latter one narrated by Sa`d ibn Abi Waqqas, Muslim jurists deduced that it is allowable to have intercourse with one’s wife if she is suckling a child and for a woman to breastfeed a baby during pregnancy, since there is no harm in such acts. Had it led to harm, the Prophet (peace and blessings be upon him) would have guided us not to do so, for he is merciful for the believers.”