Some people have had loved one die or other major tragedys happen and been fine while others it doesn't take much. It's hard to say.
It cannot be measured really, some women have a low threshold for stress, and some women just breeze through it. But as an individual, you know your body, and pregnancy is a very important time to listen to it. So, take it easy, rest when u feel like you should, and make a conscious effort to do the best for you and your Baby.. All the best.. :)