Now is ok for a pregnant sister who is diabetes to fast during Ramadan, as long as she is told that she needs 3 to 4 meals a day.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

A patient (diabetic or otherwise) who finds it harmful to fast may break his fast and make up for the missing days if his sickness is curable. But if his sickness is incurable, then he has to feed needy people, as Allah Says (what means): {So whoever among you is ill or on a journey [during them] - then an equal number of days [are to be made up]. And upon those who are able [to fast, but with hardship] - a ransom [as substitute] of feeding a poor person [each day].} [Quran 2:184]

Accordingly, if that woman is unable to fast and if trusted doctors affirmed that fasting is harmful for her and that she has to have three or four meals a day, and that these meals cannot be delayed to the night, then in such cases she may break her fast. If her sickness is curable, then she has to make up for the missing days. But if otherwise her sickness is incurable, then she has to feed a poor person for every day that she broke the fast.

For more benefit on expiation for breaking the fast, please refer to Fataawa 113006, 113207, 113713, 114215, 131133, 216094.

Allah knows best.