Why Do Women Get Morning Sickness?

There's no obvious explanation for the bouts of nausea that pregnant women get, especially in the first trimester (13 weeks).  The most commonly cited hypothesis is from articles by an American named Margie Profet.  Profet's formal qualifications are in political philosophy and physics which, some claim, do not add credit to her theories.  However, she has attracted support of and worked with a very eminent biochemist (Bruce Ames).

Profet proposes that among the many good things for us in fruit and vegetables are also many slightly toxic chemicals (naturally occuring, no outs if you eat only organic food!).  Normaly we can tolerate these, but at ealry points of pregnancy, when the foetus is very vulnerable to toxins, the pregnant woman becomes highly adverse.... and wants to vomit.  Especially if contronted with a vegetable.

An alternative to Profet's ideas could be socially-motivated and anthropological reasons, the sickness may announce to the group that the women is expecting.  Also, childbirth and rearing children is an enormous burden to a women in a hunger-gatherer society.  Is morning sickness a way of testing whether conditions, such as security of food supply, are good enough to bring a child into the group?