I read somewhere she's 5 months so she would be able to still but she wouldn't be able to explain it to everyone why she did it so yeah that could be another reason.

However I understand her letting go - Its easier to let go than pull herself up over the bannister - especially using only her arms and with fainting she probably doesn't have that much strength. If she lets go then probably she knows Lucas/Amelia/Sean will be home soon.