Of course i'm not disagreeing that you should try to reduce your stress... I think everyone should.
But mother nature is no fool. Pregnancy in and of itself is stressful. Don't expect yourself to not be stressed, to not get upset, etc. In fact, you're likely to be MORE stressed and more frequently get upset than when you're not pregnant. Hormones are a very real thing.
The studies involving low birth rate are about constant life-altering stress, such as the death of a spouse, sibling, best friend etc. Something that really brings your entire life down for the majority of the ongoing pregnancy. They are not about having rough days, weeks or even a bad month or so!!
That being said, I am a big advocate of meditation in general, and especially during pregnancy, as it can help you de stress.
But I'm also a believer that worrying about being too stressed, just makes you more stressed!!! And again, mother nature is not stupid. Pregnancy is a physical stress on your body and a hormonal stress on your emotions. This would not be the case if it was detrimental to our pregnancies.
Crying and being upset will not hurt your baby. Actually sometimes crying can be a great stress reliever!!!! I like to shower during/after a good cry. The running water can be peaceful, and stepping out of the water all clean tends to make the cry more therapeutic and naturally puts me in the mindset of leaving the bad parts behind me and walking out refreshed.