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.
Peter: So how many are there? Is it bad? Olivia: Did you eat? Peter: Yeah. Olivia: Well, that's unfortunate.