She has to fast for two months but she broke the fast for the sake of her foetus; does this affect the continuity of her fast?

Firstly: 

The expiation for accidental killing is to free a believing slave. If there are no slaves or one cannot afford that, then one has fast for two consecutive months. It is not permissible to move to the option of fasting except in the case of one who cannot find a slave or cannot afford the price of one. 

It says in al-Mawsoo‘ah al-Fiqhiyyah (10/129): Fasting is the second option, if one is not able to free a slave, as Allah, may He be exalted, says (interpretation of the meaning):

“…and whosoever kills a believer by mistake, (it is ordained that) he must set free a believing slave and a compensation (blood money, i.e. Diya) be given to the deceased’s family unless they remit it. …. And whoso finds this (the penance of freeing a slave) beyond his means, he must fast for two consecutive months in order to seek repentance from Allaah. And Allaah is Ever All‑Knowing, All‑Wise.”

[al-Nisa’ 4:92]. End quote. 

Secondly: 

If a person has fasted some days of the expiation, then something happens that is a legitimate excuse such as sickness or travel, or if a woman menstruates or bleeds following childbirth (nifaas) and stops fasting, that does not interrupt the continuity. Then when he recovers from his sickness or the woman becomes pure after menstruating, he or she should complete the fast and they do not have to start all over again.  

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: If he breaks the fast for a reason that makes it permissible to break the fast, such as sickness travel, this does not interrupt the continuity. If it so happens that this man who started the fast for two consecutive months – whether that was expiation for zihaar (a jaahili form of divorce in which the man says to his wife “you are to me like my mother’s back”) or expiation for having intercourse during the day in Ramadan or expiation for manslaughter – travels and breaks his fast, that does not interrupt the continuity because travelling makes it permissible not to fast. But if he used travelling as a trick to break the fast, we say to him: it is not permissible for you and you have to refrain from eating and drinking, because obligatory duties cannot be waived by means of a trick, and if you do not do that then you have to start all over again. End quote from al-Sharh al-Mumti‘, 13/273 

It says in Fataawa al-Lajnah al-Daa’imah, 21/320: 

The basic principle with regard to the way in which expiation for accidental killing is to be done is that the fasts should be consecutive, and interruptions in the fast that do not affect its continuity are matters that a person has no choice about, such as sickness with which one cannot fast, or menses in the case of a woman. This does not interrupt the continuity; rather one should resume from where one left off… End quote. 

But is a woman’s pregnancy on its own regarded as a legitimate excuse which permits the woman to break the fast without interrupting the continuity thereof? 

The answer is that pregnancy on its own does not make it permissible for the woman to break the fast unless it involves hardship or she fears for her baby; in that case it is an excuse. 

Based on that, it is essential to consult a doctor. If he says that you should not fast, then in that case you are excused, then when the excuse no longer applies, you should continue from where you left off. 

But if the doctor says that fasting will not affect the pregnancy, then you have to complete the fast whilst you are pregnant. We hope that there will be no blame on you for breaking the fast because you thought that fasting would affect the pregnancy and you do not have to repeat the days you already fasted; rather it is sufficient for you to complete what you had started. 

And Allah knows best.