All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Working in family planning is of two types:
The first type: Working in the field of permanent birth control, or preventing birth, due to the fear of poverty or other unacceptable ends: Such a job is not permissible and its earnings are ill-gotten [forbidden]; this is because Islam urges us to multiply our progeny and not to decrease it. Ma’qil Ibn Yasaar narrated that the Prophet said: “Marry the loving and fertile women, as I hope to outnumber [the other nations] by you [by your abundant number]." [Abu Daawood and An-Nasaa’i]
Moreover, the Islamic Fiqh (Jurisprudence) Committee, the Permanent Committee of Iftaa’ (issuing Fatwa] and the Council of the Prominent and Senior Scholars [of Iftaa’] issued a Fatwa to the same effect.
Also, it is forbidden to eat from the wealth of a person whose earnings are entirely ill-gotten; however, if his money is a mixture of lawful money and ill-gotten money, then it is disliked to eat from his money but it is not forbidden.
The second type: Working in the field of birth control for women without opting for permanent birth control but simply leaving a period between one child and another in order to preserve the health of the mother and children and help in the good nurturing of the child, then this is a praiseworthy act and the salary from such a job is lawful.
For more benefit on the ruling on birth control, please refer to Fataawa 89833, 82154 and 81459, and to Fatwa 91257 on consuming ill-gotten money.
Allaah Knows best.