The Queen, the Duke of Edinburgh, the Prince of Wales, the Duchess of Cornwall and Duke and Duchess of Cambridge are said to be “delighted” for the couple.
Doria Ragland, the mother of the duchess, is “very happy about this lovely news” and “looks forward to welcoming her first grandchild”.
Woody Johnson, the US ambassador to the UK, was among the first to respond to the royal baby, tweeting: “Happy news to wake up to on a Monday morning – congratulations to the Duke and Duchess of Sussex!!”
Grant Harrold, a former butler to Charles, William and Harry, also tweeted: “Congratulations to The Duke and Duchess of Sussex who are expecting their first child in the spring.”
Britain's Prime Minister Theresa May said: “My warmest congratulations to the Duke and Duchess of Sussex on the happy news they are expecting a baby in the spring. Wishing them all the best.”
The Archbishop of Canterbury posted on Twitter: “Congratulations to the Duke and Duchess of Sussex on their happy news! Prayers for them in the months ahead.”
The news ends weeks of speculation that Meghan is expecting.
Just minutes after touching down in Australia, Meghan Markle further fuelled rumours she was pregnant.
The Duke and Duchess of Sussex arrived in Sydney on Oct. 15 with much fanfare.
And as soon as the footage began rolling in, the 37-year-old former Suits star had tongues wagging when she chose to walk from the airport to the car wearing a large coat and carrying a number of folders which shielded her stomach from view.
Fashion expert Donny Galella told New Idea, her outfit choice was no coincidence, all but confirming she was expeciting the couple’s first child.
"Meghan has touched down in Sydney in chic black turtleneck knit and berry hue coat," he said.
"Accessorised with matching folders that she held covering her mid section. Of course adding more speculation to the pregnancy rumours."
"I cannot wait to see what she wears and what Aussie designers she selects to wear.”
Meghan and Harry's trip, their first outside the UK and Ireland, will officially begin on Tuesday and will see the couple watch the Invictus Games, visit a Sydney zoo and visit the rural Flying Doctor service.
The couple were driven from the airport to Admiralty House, the Sydney residence of Governor General Peter Cosgrove, the representative of the Queen in the country.
Once there, the couple appeared to order takeaway from Menulog - perhaps a sign that mum-to-be Meghan was experiencing a pregnancy craving.
Hundreds of people gathered to catch a glimpse of the couple and cheered as their car was driven into the property a short distance from the iconic Sydney Harbour Bridge.
The 76 engagements also include visits to schools, greeting members of the public at the Sydney Opera House, and meeting young leaders and community members at Government House.
The couple's arrival in Sydney came just three days after Princess Eugenie's royal wedding on Oct. 12.
Meghan also wore a large navy coat which concealed her stomach on that occasion.
Upon arriving at the ceremony, Meghan and Harry were ushered through a side entrance to the chapel – avoiding photographers.
"Something about Meghan's outfit choice makes me think she's pregnant and hiding a baby bump. It's so conservative," wrote one fan - who has since been proven correct - on social media.
Another added: "I have a feeling that Meghan is pregnant...she's glowing and her face looks fuller to me…can't wait for them to announce it."
They didn't have to wait long.