The Moonbump hashtag refers to the Moonbump product, a fake pregnant belly made in the UK.
According to the website, Moonbumps are "used extensively for stage and screen performance, advertising and retail and, in many cases, for strengthening emotional bonds as you move forward with a planned surrogacy or adoption".
The strongest believers in the royal conspiracy are convinced the 37-year-old has bought her own Moonbump, either with or without the knowledge of her husband and other royals.
While the reasons for the supposed cover-up vary from person to person, the idea behind them all is the same — that Meghan is lying to the world about a non-existent pregnancy, using a pillow or Moonbump to hoodwink us all.
One of the more popular ideas behind the so-called conspiracy is that it is an elaborate ploy to distract Brits from the Brexit debacle, with many social media users accusing the Windsors of "treason".
Others claim the real Sussex baby is being carried by a secret surrogate, pointing to the "pregnancy contract" Meghan allegedly had with first husband Trevor Engelson.
According to sources, the actor was concerned about gaining weight during pregnancy and snapping back to her pre-baby body after birth, and therefore demanded Mr Engelson pay for a personal trainer and nutritionist if the couple did have a child together.
Those who support the conspiracy have used a number of photos as "proof" — including pictures that supposedly show the "fake bump" sitting unnaturally and slipping.
Others point out that pregnant women are not able to squat and bend over to the same degree that the Duchess has been pictured doing, while others argue she hasn't been spotted visiting any royal doctors or obstetricians.
And other Twitter users claim she's still dying her hair and getting Botox treatments, which means she can't possibly be carrying a child.
In one of the even stranger arguments, one theorist claimed Meghan was a "honeytrap operative" brought in to create a "divide in the UK".
But of course, it's far from the first conspiracy theory regarding a celebrity to run wild on social media.
In 2017, rumours of a "fake Melania" Trump began circulating after a number of photos and videos of the US First Lady looking slightly different to her usual self emerged.
Since then, the claim Ms Trump was using a body double to stand in for her in public has reignited again and again, fuelled by her penchant for sunglasses, clothing and accessories that often cover part of her face.
The theory was strengthened after her mysterious disappearance following a kidney operation last May.
And in 2016, there were claims Hillary Clinton had been replaced by a body double and even a hologram after she collapsed at a 9/11 memorial service in New York after a battle with pneumonia.
Prince Harry and his wife are expected to welcome their baby in April.
They are apparently creating a "gender neutral" nursery in their new home, Frogmore Cottage, using "vegan paint".
