The Duchess was sitting next to Prince Harry during the service, which is attended by the royals every year.
The ring features a large centre diamond from Botswana flanked by two smaller diamonds from his late mother, Princess Diana's collection.
"Sometimes pregnancy can cause your fingers to swell because of oedema - basically water retention that can make hands and often legs and ankles get a little bit puffy", birth expert Milli Hill told Express.
Harry designed Meghan's diamond ring himself. Duchess Kate is wearing a Catherine Walker coat.
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The swelling can become even worse as the days goes by, as the build-up of water increases gravity and draws it downwards.
Aside from the two pairs, at the service were also Queen Elizabeth, Prince Charles and his Duchess of Cornwall wife, Camilla. Meghan donned a dark green coat by Canadian-born, London-based designer Erdem for the engagement.
Commonwealth Service Day was filled with extra memorable moments-Meghan Markle channeled Jackie O while visiting a Canada-themed youth event with Prince Harry and she wore a stunning Victoria Beckham-designed dress for the traditional service held later in the day at Westminster Abbey.
The 2019 Commonwealth marks the 70th anniversary of annual event.
While Victoria Beckham doesn't usually create bespoke pieces, it is believed Meghan's coat alone is worth an estimated £4,000 and the dress £3,000, plus a further £2,000 for the custom bag and shoes - bringing the total to £9,000.