There was never a time when I did not exist, nor you, nor any being. Nor is there any future in which we shall cease to be. That which is non-existent can never come into being. That which is can never cease to be. That reality which pervades the universe is indestructible. No-one has the power to change the changeless. Bhagavad Gita
Here is a photo of a 5 month pregnant ashtanga yoga student at Nancy's recent London workshops.
She had a beautiful fluid practice and was taking appropriate modifications for pregnancy-I would not recommend you take up ashtanga yoga in pregnancy- but as you can see, if you already have a practice you can continue once you have passed the 3 month period ( Sharath Jois is recommending you do not practice ashtanga in the first three months). Talk to your teacher for advise on modifications, as not all poses are suitable during pregnancy