My sister and her husband have been trying to get pregnant for the past two years with no success. My mom, who is a very religious woman, wants me to pray for God to fulfill his promise for my sister to have a baby. I told her that God doesn't promise every woman that they will conceive, but she gave me several Biblical examples of women who were barren then prayed and became pregnant. Are there any Biblical examples of women who did not get pregnant though they strongly desired it?
I would say that a few examples are not the same as a divine promise. While I can't think of any women who prayed to get pregnant but did not, they MUST have been there. With the importance placed on child bearing and the stigma attached to “barrenness,” some women must have prayed but never have borne children.

I can think of Paul praying for his thorn to be removed. In addition, it seems to me that the answered-prayer pregnancies had purpose other than just answering a prayer (Abraham and Sarah, Hannah, Elizabeth). The best example is Jesus praying that the cup be taken (Matt 26). It wasn’t. It often isn't.