All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The most preponderant opinion of the scholars, may, Allaah have mercy on them, is that a pregnant woman does not have menses, as Allaah considered menses as a sign that a woman is not pregnant, and it is for this reason that a waiting period is counted by having menses. The blood that a pregnant woman has is not menses, so she has to pray and fast in this condition, and her husband is permitted to have sexual intercourse with her. She is not obliged to perform Ghusl when she has this blood, but she must perform ablution as it is considered as the blood that a woman has between two menses, and the Prophet ordered a woman who has blood between menses to perform ablution at every prayer as reported by Al-Bukhari and narrated by ‘Aa'ishah .
Allaah Knows best.