Because they're adorable... and because it's not the same woman in both photos.
In January 2014, Vanessa Iris was pregnant with their son, Jax, and in January 2015, Melanie was pregnant with their daughter, Ero — you can see Jax at their feet in the righthand photo.
"It's crazy to see that people were referring to my family as an inspiration. We are still in complete shock," Melanie told the Huffington Post.
She and Vanessa hope these photos can inspire other lesbian couples to start families. They also hope these photos will enable women to celebrate the transformation their bodies undergo during pregnancy, rather than feel anxiety over the changes.
"The woman's body is incredible," Melanie added. "The way it creates and grows another human being is amazing. We hope that our picture is that sign that some women may need to encourage them to carry a child."
In addition to the viral photo, Melanie has documented much of the family's experience on Instagram. It's a pretty adorable collection.