Purity: Menstruation During Pregnancy and Breastfeeding
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 435:
It is possible for a pregnant woman and a breastfeeding woman to menstruate. The ruling of a pregnant woman and a non-pregnant woman is the same except that if a pregnant woman who has a habit of time[1] experiences bleeding with the attributes of ḥayḍ after the passing of twenty days from the first day of her habit, it is necessary for her, based on obligatory precaution, to do the things that a mustaḥāḍah must do and refrain from doing the things that are unlawful for a ḥāʾiḍ to do.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)
1
This is explained in Ruling 468.