The World Federation One Stop Fiqh

Ruling 2510

If a woman’s husband marries a girl who is being breast- fed and his [first] wife breastfeeds the girl, then based on obligatory precaution, the woman becomes forever unlawful for him, and as a precautionary measure, he must divorce the woman and never marry her again. If the milk is related to him, the girl who is being breastfed also becomes forever unlawful for him. If the milk is related to the woman’s previous husband, then the marriage contract is invalid based on obligatory precaution.