Firstly, every Muslim must know, male and female, that pregnancy on its own is not a reason for someone to break their fast. Rather, the pregnant woman has also been addressed to fast the month of Ramadan. For it is only one month in the year and a pillar in Islam.
Secondly, if the pregnant woman sees some sickness (due to her fasting) such that the fasting becomes difficult while pregnant, for example, she vomits or she gets headaches, in this situation she can break her fast and fulfill the fast afterwards.
Thirdly, If the pregnancy affects her unborn child such that she knows this by way of a trustworthy Muslim doctor man or woman then she may break her fast also.
If she cannot find a Muslim doctor then a non Muslim doctor who is known for his skill in this field.