Royal pregnancy: Duchess of Cambridge appears in public
The Duchess of Cambridge has made her first appearance since announcing her second pregnancy, joining her husband to welcome the president of Singapore.
The duchess, who is about 12 weeks pregnant, is suffering from extreme morning sickness which has forced her to miss recent engagements.
However, her condition has improved and Catherine said she "had been looking forward to getting out of the house".
Later she attended London's Wildlife Photographer of the Year awards.
At the scene
Peter Hunt, BBC royal correspondent
On a blustery but dry Horse Guards were two couples who represent the present and the future of an ancient institution.
For the Queen, this was the 108th time she's welcomed a visiting head of state. She's greeted presidents, monarchs and a grand duke.
For William and Kate, it was the first time they've been so involved in such an occasion. It was a novelty. It was on the job royal training. It will one day become routine.
Inevitably, the focus was less on Anglo-Singaporean relations and more on a pregnant duchess. In the coming days, weeks and months aspects of this pregnancy will be lived out on a very public stage.
There'll be speculation about the due date and the sex of the child. The fact this baby won't, as things stand, become a king or queen , is unlikely to dampen the international fascination.
The duchess last appeared in public at the start of August, when she visited the World War One poppy installation at the Tower of London.
Her pregnancy was announced in September and the April due date was announced on Monday.
Prince William and Catherine's second baby will become fourth in line to the throne, behind older brother Prince George, who was born in July last year.
The couple met Singapore's President Tony Tan and his wife in the presidential suite of London's Royal Garden Hotel.
Catherine wore a grey Alexander McQueen coat and matching coloured hat by Jane Taylor.
The president commented on the view of Kensington Palace from the hotel and the duke joked: "You can probably see into our bedroom window which is a bit worrying - I wouldn't look too closely."
The four previously met in 2012 when the duke and duchess's Diamond Jubilee tour took them to Singapore.
The two couples then travelled to a ceremonial welcome on Horse Guards Parade in Whitehall, where a royal salute of 41 guns was fired in nearby Green Park upon their arrival.
The Queen, Duke of Edinburgh, Prime Minister David Cameron, Foreign Secretary Philip Hammond, Home Secretary Theresa May and other dignitaries attended the high-security event.
The welcome was the first major ceremonial event to take place in London since the terror threat level was raised to "severe" at the end of August.
The Duchess of Cambridge also attended the Wildlife Photographer of the Year awards gala at the National History Museum on Tuesday evening.
The duchess, who is a keen photographer and patron of the museum, had changed outfits for the occasion into a powder blue gown by Jenny Packham.
Wildlife documentary maker Sir David Attenborough was at the event to present prizes to the winners.