Birth doula Susannah Birch told Trimester Talk that abdominal muscles holding the baby bump become tired as the day goes on, causing them to loosen up. This allows the bump to expand and sit out further, thus appearing bigger.
The muscles recuperate during sleep, and become refreshed and better able to maintain the bump's position in the morning.
Levels of the hormone relaxin, which causes abdominal muscle stretching during pregnancy, was also found to be highest at night, according to a study published in the Handbook of Biologically Active Peptides.
Women's Health reports that pregnant bellies have also been known to "pop" seemingly overnight, which is caused by the uterus expanding due to the growing fetus, and rising above the pubic bone. For most women, this tends to happen at around 10 to 12 weeks.