The hashtag was started by Milli Hill, founder of the Positive Birth Movement, a group that spreads positivity about childbirth and runs free to attend antenatal groups and workshops.
"In our culture, there is this idea that giving birth is always a terribly traumatic experience, which you have to endure and which leaves you feeling ill, but these images challenge that idea by showing women who have just given birth looking radiant and happy," Milli tells HuffPost UK Lifestyle.
Milli felt compelled to encourage mums to share their post-birth selfies after seeing photographer Jenny Young's One Day Young photo series, which captured the joy of new mums and their one-day-old babies.
"The fact that birth can be positive is something that's usually kept quite hidden from women," says Milli. "If the only time you've seen someone giving birth was on One Born Every Minute, then you'd think it was absolutely horrific, no wonder people are terrified, but birth doesn't have to be like that.
"Obviously you can’t control birth to the nth degree, just as you can’t control anything completely, but you can maximise your chances of doing things differently. Hopefully, the #BirthJustHappened photos will give women different expectations about how birth can be."
Melissa Thomas from Nottingham, was one of the women who shared a 'post-birth selfie' with Millie.
"I was initially hesitant when I considered sharing such an intimate picture of my son and I," Melissa explains. "Although there was some thought regarding the nudity, my biggest concern was presenting a static image of such vulnerability, along with how others could potentially interpret its visual representation.
"This photograph not staged. It was captured by my husband, moments after I caught our son, our third baby and my most challenging birth experience of the three. In the dim light of our bathroom, I was unaware he had taken pictures and it was 24 hours after the birth when I first saw the photo.
"At first glance, I briefly criticised the imperfections I saw but those immediate reactions to the truth of my body's image fell away as absolute strength, power and knowing shone through. I felt this was important to share with others as we often have a limited view of how birth and the female body are represented.
"Media images are frequently clinical, polished and without depth, prescribing to culturally predefined stereotypes. This image is a story, it engages the viewer in dialogue and curiousty surrounding the circumstances. Within it there is pain, there is pleasure, there is vulnerability and there is beauty. It is not idealised, it is real, it comes with a history and it depicts a potential we all have within us.
"By removing a hierarchy of desire to attain the unrealistic perpetuated by mass media, it encompasses a self acceptance for our subjective experiences."
See more beautiful #BirthJustHappened photos below: