These theorists believe that morning sickness prevented the mother from eating spoiled meat or toxic plants, which could damage the developing embryo.

Evolutionary psychologists

The evolutionary psychologists believe that pregnancy sickness should occur when fetus is most vulnerable (first trimester) and that's what prevents the mother to eat something spoiled, like meat, that would be toxic for the baby.
 After the first trimester the sickness should reduce or disappear.
They also believe that if the woman doesn't have the sickness is more probable that she will suffer a abortion.