Wa alaykum assalam warahmatullah wabarakatuh

In the Name of Allah, the All-Merciful, the Mercy-Giving

All praise is due to Allah. Peace and blessings be upon His Messenger Muhammad.

With regard to the obligation of fasting in Ramadan on a pregnant woman,

If she 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 she fears of any harmful consequences that may affect her or her embryo, then she may (in cases of necessity, she has to) break fasting and then make up the missed days later on.

Almighty Allah knows best.