According to Hanafi jurists, purity from menses or post partum bleeding is not a condition for the validity of tawaf and a woman doing tawaf in such cases has to offer a sacrificial animal after Hajj.
However, Imam Ibn Taymiyah and Ibn Al-Qayyim are of the opinion that if she cannot wait until she is purified, she can make tawaf after wearing a sanitary towel without being obliged to offer a sacrificial animal. But if she can wait in Makkah for some time until she is purified, then she has to wait until she is purified to offer tawaf Al-Ifadah.
With the above in mind, we’d like to state that Hajj during the period of pregnancy may expose a woman to dangers. So, it is best if Hajj is delayed to avoid any danger on the woman or the baby and there is no sin on her if her Hajj is delayed because of these circumstances.
Allah Almighty knows best.