Meghan posed in a sleeveless white double-breasted dress by Wales Bonner that featured a tie-waist and brown buttons.
Wales Bonner graduated from Central St Martins in 2014, shortly before launching her eponymous brand.
In 2015, the fashion designer was awarded the Emerging Menswear Designer accolade at the British Fashion Awards, having originally launched her label as a menswear brand.
Meghan paired the dress with Manolo Blahnik’s Sandy Beige Pointed Toe Pumps and wore her hair loose.
The newborn was swaddled in a white blanket by G.H. Hurt & Son and wearing a white hat.
Speaking in St George's Hall, Harry held their two-day-old child as Meghan said: “It's magic, it's pretty amazing. I have the two best guys in the world so I'm really happy.
"He has the sweetest temperament, he's really calm."
As they both laughed, Harry said: "I don't know who he gets that from."
Meghan added: "He's just been the dream so it's been a special couple of days."
Royal fans have praised the new mother for emphasising her post-baby bump, with one person tweeting: "I LOVE the fact Meghan Markle still has her post-partum bump going on."
Another added: "I know nothing about being pregnant but I enjoy the fact that Meghan Markle is showing off her post-baby bump so prominently with that belted dress."
The dress Meghan wore for the photocall with her newborn is similar to one she wore for a visit to the Nelson Mandela Centenary Exhibition in London in July 2018. For the engagement, the duchess wore a blush pink sleeveless dress by Canadian brand House of Nonie.
The duchess finished the look with matching stiletto heels and the Mulberry England Clifton Shoulder Bag in blush calfskin.
The last time Meghan was seen in public was in March when she visited the New Zealand High Commission in London to sign the Book of Condolence following terrorist attacks in Christchurch.
The duchess wore an oversized black coat designed by Gucci and cut-out Aquazzura heels for the occasion.
Her last official appearance was at Commonwealth Day service with other members of the royal family.
The duchess wore a cream Victoria Beckham dress and a matching pillbox hat for the occasion, which was held at Westminster Abbey.
Meghan gave birth to her first child early on the morning of Monday 6 May.
The couple announced the news on Instagram (under their @sussexroyal account), saying: “We are pleased to announce that Their Royal Highnesses The Duke and Duchess of Sussex welcomed their firstborn child in the early morning on May 6th, 2019. Their Royal Highnesses’ son weighs 7lbs. 3oz."
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