As for the event, this is the first time pregnant Meghan shows up in public since her and Harry's joint tour.
Meghan eventually ended up wearing a diamond and platinum tiara from the Queen's collection.
The Queen reportedly warned Prince Harry over Meghan Markle's behaviour, following a row over a tiara.
Bride-to-be Markle wished to wear a tiara with emeralds but was refused after its origins could not be established.
However, when Meghan and Harry announced their engagement in November 2017, they described the moment she first met the Queen and won over her beloved corgis in their interview.
To which William added: "I know, I've got serious OCD on light switches now, which is bad".
News of the departure comes as more details emerged about the run up to the wedding in Royal biographer Robert Jobson's new book Charles at 70.
Speaking to William - who is married to Catherine, Duchess of Cambridge - Harry said: "Which is insane because actually you know, I don't know whether your wife doesn't, my wife certainly goes, "Well why turn the lights off?"
"And that was the one that, I think, as we tried them on, stood out".
Speaking about the type of grandfather he is, as reported by Express UK, Camilla said: "He will get down on his knees and crawl about with them for hours, you know making amusing noises and laughing and my grandchildren adore him, absolutely adore him".
"Since very early on she made it clear that should Meghan ever need something, or just want to chat, she shouldn't hesitate to get in touch", a source told E!
Despite the rules, the expecting mom is "determined to do things her way", said the source.