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