The mum-to-be, who opted out of joining her husband Prince Albert at the D-Day commemorations on 6 June and on a state visit to the Netherlands because of her condition, made an appearance at Fontvieille Park and the Princess Grace Rose Garden on Saturday.
Charlene looked elegant in a sky blue dress, which carefully concealed any hint of a baby bump, as she re-opened the garden with Prince Albert and his sister Princess Caroline.
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The garden has been closed for eight months while it underwent regeneration and is now open to the public again to coincide with its 30th anniversary.
Prince Albert was tasked with cutting the ceremonial ribbon at the entrance to the grounds while his wife unveiled a plaque commemorating the occasion.
Later the 36-year-old princess held a glass of champagne aloft, not for herself, but to 'water' a bouquet of roses taken from the garden.
Since the announcement of Charlene's pregnancy rumours have swirled that the former Olympic swimmer is in fact expecting twins.
Derek Watts, a close friend of Charlene's father Mike Wittstock, appeared to unintentionally break the exciting news when he tweeted that Mike had called him to tell him the princess is expecting not one, but two new arrivals.
"My old Bulawayo schoolmate Mike Wittstock has phoned to say his daughter is pregnant with twins," wrote the South African TV presenter. "Congrats Princess Charlene and Albert!"
Shortly after posting the message, however, Derek deleted it, suggesting that he had been told that the news was to stay under wraps for a little while longer.
Is Princess Charlene of Monaco pregnant with twins?
The Monégasque royal family have remained quiet on the issue, only releasing a statement saying "T.S.H the Prince and the Princess of Monaco are delighted to announce the pregnancy of H.S.H. Princess Charlene.
"The birth is expected at the end of the year."