Life is a game. From the moment sperm fertilizes egg, you are either winning or losing. Using game theory--the study of competition and conflict--researchers can quantify the inherent tug-of-war between mother and fetus.
In zero-sum games, what one gains the other loses: when the baby gains nutrients, the mother loses nutrients. The player that wins is rewarded and the loser is punished; the reward may be full-term birth, while the punishment is premature birth.