بسم الله الرحمن الرحيم
Keep on reciting the following dua extensively:
فَاللّهُ خَيْرٌ حَافِظاً وَهُوَ أَرْحَمُ الرَّاحِمِينَ .
There is no number or time fixed for it, you may read it when you wish and you may read it as many times as you wish. In-shaAllah, you shall have a safe pregnancy.
Allah knows Best!
Darul Uloom Deoband