A Chicago mom has finally discovered why she couldn't get a clear ultrasound picture while pregnant ... that this position makes Abigail, who was 5 weeks at the time of the video, calm.

Using your LMP to find your due date means that when you get confirmation that you're pregnant, you'll most likely be 4 or 5 ...

Gypsy-Rose Blanchard and her boyfriend Ken Urker posted photos from a 4D ultrasound of her daughter ... stronger and are now visible at 24 weeks +5 days,” Blanchard wrote on a video.