Rav Shternbuch address this question in his tshuvos (http://hebrewbooks.org/pdfpager.aspx?req=19963&st=&pgnum=130) and explains the position of the Brisker Rav that a pregnant woman is essentially a choleh sh'ein bo sakanah, in which case she can eat in amounts less than a shiur if she feels she cannot fast. This holding is also consistent with the Brisker Rav's view that a choleh sh'yesh bo sakanah can eat normally, even on Yom Kippur and is not constrained by eating less than a shiur. R. Shternbuch does caution that he is merely explaining the position of the Brisker Rav but he leaves the psak up to the person's regular rabbi.

Rav Shternbuch address this question in his tshuvos (http://hebrewbooks.org/pdfpager.aspx?req=19963&st=&pgnum=130) and explains the position of the Brisker Rav that a pregnant woman is essentially a choleh sh'ein bo sakanah, in which case she can eat in amounts less than a shiur if she feels she cannot fast. This holding is also consistent with the Brisker Rav's view that a choleh sh'yesh bo sakanah can eat normally, even on Yom Kippur and is not constrained by eating less than a shiur. R. Shternbuch does caution that he is merely explaining the position of the Brisker Rav but he leaves the psak up to the person's regular rabbi.