There have been numerous studies showing that women who have morning sickness during the first trimester have a lower odds of miscarriage and other negative pregnancy outcomes.
The exact reason for the link is not known, but one possible explanation is that non-viable pregnancies, such as those affected by chromosomal abnormalities, have lower hCG levels and this might lead to fewer pregnancy symptoms.
If you don't have morning sickness or if your morning sickness has disappeared, however, don't panic. Nausea is not a prerequisite for having a healthy pregnancy -- plenty of women never have morning sickness at all.
In addition, having morning sickness is unfortunately not a guarantee that you will not have a miscarriage; it's possible to miscarry even if you have noticeable pregnancy symptoms.
Sources:
Furneaux, Edwina, Alison Langley-Evans, and Simon C. Langley-Evans, "Nausea and vomiting of pregnancy: Endocrine basis and contribution to pregnancy outcome." Obstetrical & Gynecological Survey 2001. Accessed 21 Nov 2008.
Weigel, Ronald M. and M. Margaret Weigel, "Nausea and vomiting of early pregnancy and pregnancy outcome. A meta-analytical review." BJOG 1989. Accessed 21 Nov 2008.
