Praise be to Allah.
It is permissible for a man to have intercourse with his wife who is breastfeeding, and there is nothing wrong with it. And there is nothing wrong with a woman breastfeeding when pregnant, because of the report narrated by Muslim (1442) from Judaamah bint Wahb al-Asadiyyah (may Allah be pleased with her), that she heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “I was thinking of forbidding intercourse with a breastfeeding woman, until it occurred to me that the Romans and Persians do that and it does not harm their children.”
But the woman should examine her own case and her ability to put up with that, and she has to weigh up her duty towards her nursing infant and what she herself will need during the period of pregnancy. If she thinks that she may not be able to balance matters, or she thinks that it will harm her, then there is no reason why she should not put off getting pregnant for a while, so that she can finish breastfeeding, then start a new pregnancy.
Ibn Jibreen (may Allah have mercy on him) was asked:
I am a woman with six children, and in most cases I got pregnant before completing the breastfeeding of the previous child -- before the end of the two years -- and I want to use the so-called “coil” [intra-uterine device, a form of contraception that is inserted in the uterus] to prevent pregnancy for two years only. What is the Islamic ruling on that?
There is no reason why you should not do that, if both spouses agree to it, and they are certain that it will not cause any harm to the uterus or prevent having any children in the future and so on. End quote.
Fataawa al-Shaykh Ibn Jibreen, 96/2
See also the answer to question no. 21203.
And Allah knows best.