What should a woman who deliberately aborts her foetus do (to expiate for her sin)?

She has to repent for deliberately aborting her pregnanacy after it had been created, because aborting it is haraam and is not allowed. Once pregnancy is known, it has to be protected and it is haraam for the mother to harm it in any way, because it is a trust which Allaah has placed in her womb, and it has rights. So it is not permitted to treat it badly or to abort it.

Shaykh al-Fawzaan said: if the soul has been breathed into the foetus and it has started to move, then the woman aborts it after that and it dies, then she is considered to have killed a soul and she is obliged to offer the kafaarah (expiation), which is freeing a slave. If she cannot do that, then she has to fast for two consecutive months and repent to Allaah. That is if the pregnancy was over four months, because in that case the soul has been breathed into it. So if she aborts it after that time, then she has to offer kafaarah as we have menioned. This is a serious matter and it is not permissible to take it lightly. And Allaah knows best.

See al-Fataawa al-Jaami’ah li’l-Mar’ah al-Muslimah, 3/1052