If the pregnant woman fears for her fetus that it may be aborted if she observes fasting, then there is no disagreement between the Muslim jurists that she is entitled to the concession of not observing the fasting in Ramadan, provided that she has to make up the days she misses in other days. Almighty Allah says: “But whoever among you is sick or on a journey, then (he shall fast) a (like) number of other days” [al Baqarah 2:184]
The Prophet (peace and blessings be upon him) is reported to have given concession for pregnant and breast-feeding women to break their fast in Ramadan. But after the pregnant woman delivers her baby and the breast-feeding one weans her child, they have to make up for the days they missed.
Moreover, the same ruling applies if the pregnant woman fears that she will suffer health problems or be in hardship if she observes fasting. In this case the ruling of the sick person mentioned in the above quoted verse applies to her situation.
Of course, not all pregnant women fear abortion or fear for themselves. A trustworthy Muslim doctor should be consulted in this regard.