I went through some medical examinations and vowed to fast two consecutive days if the results would be good. Thanks to Allaah, the results were good, and I am now pregnant. Having suffered two miscarriages in the past, I am now afraid that the immediate fulfillment of my vow will cause harm to my pregnancy. Keeping in mind that I intended to fast even if I would do that two days after the day of the result, what should I do now? I take medicine twice a day. Am I permitted to delay the fulfillment of the vow until I get my pregnancy stable? Am I required to offer an expiation if I delay the fulfillment of the vow?

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.

If the results of the medical examinations that you underwent prove that you are in good health, then you are required to fulfill the vow and fast two consecutive days. A conditional vow must be fulfilled once the condition is achieved. However, immediate fulfillment of a vow is not due unless it was specified with a certain time. If you specified a certain time for the vow, then you are required to fulfill it at the specified time. Your statement "I intended to fast even if I would do that two days after the day of the result," does not signify specification; rather, it just indicates your intention to hasten to fast after that date.

Consequently, it is permissible for you to delay the vowed fast due the fact that you take medicine, and you are required to fulfill the vow whenever you are able to do so. You are not required to do anything concerning delaying the fulfillment of the vow. 

Allaah Knows best.