Mary's pulse is racing. Not enough time. Too soon. He'll be here in an hour, and she's not going to be able to hide this. No way, no how. She can feel her heart pounding in her chest, harder than it's ever beat before.
It's not as much nervousness that's causing her heart to pump harder, as it soon becomes obvious. With a creaking sound, Mary begins to progress even further. Her chest broadens, and deepens, becoming far more barrel-like. Likewise, her arms get longer as well. It's a rush, like a swoop of air that hits her, and she begins to feel slightly dizzy. Her neck is about a foot longer.
Which is why her heart was beating harder, because it had nearly doubled in size, having to pump blood a significantly longer distance up to the brain.