She revealed the news on her website and it seems she knows exactly how her fans think, because she mentioned progress on book six too.
"Let me reassure you that book six remains well on track and fingers crossed of course, I don't see any baby-related interruptions or delays."
The baby will be a brother or sister to daughter Jessica and her little brother David who was born last year.
The title of the sixth book was revealed last month, also on JK's website.
Some wellwishers will get a chance to see the writer next month when she appears at the Edinburgh Book Festival on 15 August.
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