Lady and former pro-photographer here. I could write a book on the anatomy of a pose, the hows, whys, and whens a pose makes a photo more than just an image of a person. Accentuation is a major factor, Men don't tend to lift legs in poses as this makes them seem more sturdy, ragged and solid. While when a woman lifts a leg, it invokes more elegance, appeal, and softness. This is hardwired into our sexual brains, one parallel people tend to not notice in real life or the movies is during a standing kiss you see some women lift their leg and the men don't. Body language speaks on many levels.

(October 3, 2019, 9:18 am)LSeymour I'm not a woman and have never been pregnant (obviously), but I do know that raising a knee like that changes the center of gravity in a human body and reduces the stress on lower back muscles and (some) supporting muscles in the mid-back.  People who have back injuries are given the advice to raise the leg like that if they have to stand for a period of time.  If this is a common pose for pregnant women, I suspect it is somewhat intuitive / instinctive to do that if they are standing up, especially if their bellies cause a lot of stress on the lower back.

I think he's right. It does relieve stress on the lower back. Think of every bar you've ever been in. They all have a brass rail. You belly up to the bar and put your foot up on the rail. Same thing...

I think we need more examples of this majestic power stance

Lol, it's certainly not to help support the extra weight in any way. It doesn't really alter the center of gravity much either. The bent leg isn't doing much of anything, and most or all of the weight is being supported by the straight leg. Bending one leg does cause the pelvis to tilt and push outwards over the supporting straight leg and can help compliment some curves. Though I'm gonna have to agree with Mordona. I think it's kinda just second nature to most women. I also think a person's stance has a lot to do with it. Women have a much narrower stance than men. Stand with your feet shoulder width apart and try bending one knee. It doesn't really work or feel natural. Now stand with your feet together. One of my knees almost always bends automatically. Try it!

It's not just an Instagram thing, and it has nothing to do with pregnancy. This has been seen as a particularly beautiful, feminine pose in Western society for centuries, dating back to classical sculpture. You see it a lot in Renaissance works, it simply creates a more dynamic and interesting silhouette of a human body, accentuating the curves and overall form. For example:

Everyone saying it's related to pregnancy or relieving the weight of the belly... take one look at today's Instagram starlets and you will see it has absolutely no exclusive relationship with pregnancy whatsoever. It is something women do -- pregnant or not.

Two things:
1) Speaking strictly for myself personally, one of my legs is slightly longer than the other. So, I adopt the cocked-leg stance naturally and for the whole of my life.
2) Tomo, when I see casually produced posts like the one above, I have visceral, sometimes painful speculations about the nature and scope of your library. Thank God for Dixie Cups...

'Nuff said.

(October 5, 2019, 7:58 am)jfingers 2) Tomo, when I see casually produced posts like the one above, I have visceral, sometimes painful speculations about the nature and scope of your library. Thank God for Dixie Cups...

'Nuff said.

Not nuff said. Wtf is up with Dixie Cups?
I can't quite tell if you enjoyed my post or it caused you discomfort but heres some more examples I legit found today

Please kindly end this all speculations.
Now i speak as a photographer, and also a portrait painter.
With 20 years of experience in visual art, expecially portrait, here is why :

1. Eesthetics. We don't shoot or drawing/paint with straight pose. Unless it's for your ID card, lol
   A slight bend of knee give a more curvy looks & variation in poses. And for maternity photography, the recommended knee to bend is the front leg (the one facing right to camera)
2. Body language. Whenever one's relaxing & convidence. Nothing more.
Btw, i like your posts. Any more from the last pic of post #18 ??

Thanks to everyone for posting and resolving this mystery for me!! I really appreciate everyone's input!~