http://www.straightdope.com/mailbag/mmorningsickness.html
I thought there was a good deal of research correlating birth control use with “morning sickness,” indicating that it probably is hormonal. (And I’m speaking from experience, here.)
One site says, for instance, “Strong correlations have been noted between nausea/vomiting of pregnancy and nausea brought on by birth control pills, suggesting a common hormonal basis for both phenomena. Some researchers found higher levels of hCG in patients with nausea and vomiting than those who were asymptomatic.”