But if anyone thought pregnancy would dim the tennis star's unstoppable drive, they were wrong.
The 35-year-old shared a sweet note to her unborn baby on Monday morning - in which she wrote of her delight at reclaiming the world number one spot.
'My Dearest Baby,' she wrote alongside a picture of her cradling her bump in a sports bra.
'You gave me the strength I didn’t know I had. You taught me the true meaning of serenity and peace. I can't wait to meet you. I can't wait for you to join the players box next year.'