January Jones and Sienna Miller share pics on Bump Day

It's that time of year again: Bump Day.

The social media tradition, now in its third year, moms and moms-to-be share images of their baby bumps, spreading messages of philanthropy, maternal awareness and empowerment.

This year's offerings included three prominent actresses in Amanda Seyfried, Sienna Miller and January Jones, who shared a tasteful shot of her naked body while underwater.

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Making a splash! January Jones, 39, who shared a tasteful shot of her naked body while underwater during her pregnancy with son Xander Dane Jones, who turned six on Wednesday

Making a splash! January Jones, 39, who shared a tasteful shot of her naked body while underwater during her pregnancy with son Xander Dane Jones, who turned six on Wednesday

'Happy day of birth to my son, my whole heart,' the Mad Men actress, 39, wrote alongside the submerged shot in celebration of her son Xander Dane Jones, who turned six on Wednesday.

January, who's famously remained mum on the identity of Xander's father, told Red magazine earlier this year that she's empowered as a single mom, as her boy has a plethora of male and female role models in his life.

'Xander has a lot of bro time with the neighbor dads and my dad, who is super young,' she said. 'It's good to have strong women around a man to teach him to respect women. He doesn't have a male person saying "Don't cry" or "You throw like a girl" ... all those s------ things that dads accidentally do.'

In the chat, the mom-of-one, added, 'I just don't feel I need a partner. Do I want one? Maybe. But I don't feel unhappy or lonely.'

Classic: Sienna Miller, 35, shared this shot from when she was expecting daughter Marlowe

Classic: Sienna Miller, 35, shared this shot from when she was expecting daughter Marlowe

Life's a beach: Amanda Seyfried, 31, shared this flashback shot on the special day

Life's a beach: Amanda Seyfried, 31, shared this flashback shot on the special day

Elsewhere, Amanda Seyfried, who

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