Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Answering your question, Dr. Muhammad Salama, PhD in Islamic Studies in English, Associate Professor at the Faculty of Islamic Sciences al-Madinah International University (Mediu), states:
If a pregnant woman feels well and strong enough to fast without any negative consequences, neither for herself nor her embryo, then she has to observe fasting, as she has no excuse.
If a pregnant woman fears of any harmful consequences that may affect her or her embryo, then she may, in cases of necessity, break the fasting and then make up the missed days later on.
Allah Almighty knows best.