When a woman is stressed (either from something physical or psychological), there is an increase in the hormone norepinephrine and epinephrine. These help to shunt blood to the most vital organs. Because of this, there is a decrease in the amount of oxygen available to the uterus and thus the baby. Therefore, it is a good idea to take these things into consideration while a woman is in labor.
Obstetrics: Normal and Problem Pregnancies, 5th ed. Gabbe et al.
Fanaroff and Martin's Neonatal-Perinatal Medicine, 8th ed. Martin et al.