If this stress theory were true there would hardly any births. The planet is bursting at the seams with billions of people. Women are already highly stressed out in day to day life. Women are probably even more stressed out in poor countries.
If all women had to do was put themselves into highly stressfull situations (like that show on syfy channel “Scare Tactics”) on purpose to avoid pregnancy and give themselves do-it-yourself miscarriage, there would be no need for birth control.
The reason why there are less pregnancies from rape is because most rapes are just one single act of intercourse. That’s the real reason it makes it less likely.
I’ve been to some pretty poor countries and the people for the most part are not too stressed out.