The Duchess of Cambridge was as elegant as always as she arrived for the event on Monday afternoon (9 March).
Kate, 33, is expecting her second child in April.
The duchess was joined by husband Prince William, who is just back in the UK from his recent overseas engagements in China.
Prince George, one, was not in attendance.
Today's Commonwealth Day Observance is the UK's largest multi-faith celebration. Dignataries from the UK and around the Commonwealth attend a service at Westminster Abbey alongside the royal family, politicians and schoolchildren.
This is Kate's first public engagement of the week. She will visit Ealing Studios in London on Thursday, 12 March to watch a scene from Downton Abbey being filmed.
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