The actress, 46, will give birth to the tot in the summer, giving her five year-old daughter Nahla a longed for sibling.
Delighted Halle said: ‘I feel fantastic. This has been the biggest surprise of my life to tell you the truth. I thought I was kind of past the point where this could be a reality for me. So it’s been a big surprise and the most wonderful.’
Halle and Olivier – who once dated Kylie Minogue – have been engaged for 15 months. Pals say: ‘It’s a very exciting time for them.’
To see how the Duchess of Cambridge does pregnancy chic, click here